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Upcoming Events

Event Date and Location Summary
Shixiong Zhang, Indiana University Mon. March 18th, 2019
12:45 pm-1:45 pm

host: Xuan Gao

Vladimir Dobrosavljevic, Florida State University Mon. April 8th, 2019
12:45 pm-1:45 pm

host: Xuan Gao

Horacio Castillo, Ohio University Mon. April 15th, 2019
12:45 am-1:45 am
at Rockefeller 221 (Les Foldy Room)

Host: Philip Taylor


External website: https://www.ohio.edu/cas/physastro/faculty/profiles.cfm?profile=castillh
Aldo Romero, West Virginia University, Mon. April 22nd, 2019
12:45 am-1:45 am
at Rockefeller 221 (Les Foldy Room)

host Walter Lambrecht


External website: https://physics.wvu.edu/faculty-and-staff/faculty/aldo-romero

Past Events

Event Date Summary
Samo Kralj,Josef Stefan Institute in Ljubljana and University of Maribor, Slovenia, Topological defects in nematic liquid crystals: playground of fundamental physics Mon. December 3rd, 2018
12:45 pm-1:45 pm

Topological defects in nematic liquid crystals: playground of fundamental physics

Samo Kralj

 

1Faculty of Natural Sciences and Mathematics, University of Maribor, Maribor, Slovenia

2Solid State Department, “Jožef Stefan” Institute, Jamova 39, Ljubljana, Slovenia

 

       Topological defects (TDs) are an unavoidable consequence of continuous symmetry breaking phase transitions [1]. They appear at all scales of physical systems, including particle physics, condensed matter and cosmology. Due to their topological origin they display several universalities that are independent of the systems’ microscopic details.

Maryam Ghazisaeidi, Ohio State University, High entropy alloys: mechanical properties and phase stability Mon. November 19th, 2018
12:45 pm-1:45 pm

High entropy alloys: mechanical properties and phase stability
Maryam Ghazisaeidi, Department of Materials Science and Engineering, Ohio State University

The term “High entropy” alloys (HEA) refers to a relatively new class of multicomponent—usually
five or more—metallic alloys in equal or near equal atomic concentrations. Instead of ordered
intermetallics, expected from classical physical metallurgy, some HEA systems strikingly crystalize
as single phase solid solutions with simple crystal structures. The complex compositions of these
alloys, and their derivatives, lead to unique properties. They also encourage new ways of viewing
fundamentals of physical metallurgy,

Joe Trodahl, Victoria University of Wellington, Rare-earth nitrides; semiconductors, spin/orbit magnetism, tunnelling MRAM, superconductivity Mon. November 12th, 2018
12:45 pm-1:45 pm

Rare-earth nitrides; semiconductors, spin/orbit magnetism, tunnelling MRAM, superconductivity

Joe Trodahl

 MacDiarmid Institute for Advanced Materials and Nanotechnology

Victoria University of Wellington New Zealand

Controlling the flow of electronic spin in addition to the charge promises speed and power demand advantages. However, there are as yet few “spintronic” devices on the market, in part due to a lack of intrinsic ferromagnetic semiconductors that would permit full exploitation of the coupled spin/charge technology. To date the only full series of such materials are the mononitrides of the lanthanides, the 14 rare-earth elements.

F. De Angelis, Istituto Italiano di Tecnologia, Genoa, Italy, 3D plasmonic nanostructures for biology and medicine Mon. November 5th, 2018
12:45 pm-1:45 pm

3D plasmonic nanostructures for biology and medicine

Francesco De Angelis

Istituto Italiano di Tecnologia, Genoa, Italy

In this talk we will show our last achievements and future perspectives of distinct class of plasmonic devices devoted to biological and medical applications. Among them, we will introduce the concept of meta-electrodes, namely a nanostructured surface that can work as electrode, a broad band plasmonic antenna, and optimal cellular interface (see Figure 1). We show that meta-electrodes combined with commercial CMOS technology enable high quality intracellular electrical signals on the large network scale of human neuron and cardiomyocytes .

Ezekiel Johnston-Halperin, The Ohio State University, Quantum Magnonics in V[TCNE]2 Mon. October 29th, 2018
12:45 pm-1:45 pm

Quantum Magnonics in V[TCNE]2

 

The study of quantum coherent magnonic interactions relies implicitly on the ability to excite and exploit long lived spin wave excitations in a magnetic material. That requirement has led to the nearly universal reliance on yittrium iron garnet (YIG), which for half a century has reigned as the unchallenged leader in high-Q, low loss magnetic resonance, and more recently in the exploration of coherent quantum coupling between magnonic and spin [1] or superconducting [2] degrees of freedom. Surprisingly, the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]2) has recently emerged as a compelling alternative to YIG.

Mike Martens (CWRU Physics) Thu. October 25th, 2018
4:00 pm-5:00 pm

Conserving Helium: A story of MgB2 superconducting wire and MRI magnets

The fabrication of MgB2 superconducting wire has enabled the development of novel magnet designs for MRI systems. Compared to MRI magnets in use today, which are submerged in a bath of liquid helium, the higher critical temperature (39K) of the MgB2 facilitates conduction cooling which reduces the use of liquid helium by a factor of 100 or more. In collaboration with Hyper Tech Research, a world leader in the manufacture of MgB2 wire, and the Center for Superconducting and Magnetic Materials at the Ohio State University,
Sergey Kravchenko, Northeastern University, The latest developments in the field of the metal-insulator transition in 2D Mon. October 15th, 2018
12:45 pm-1:45 pm

Sergey Kravchenko,

Northeastern University

The latest developments in the field of the metal-insulator transition in 2D

Abstract:
Ignited by the discovery of the metal-insulator transition, the behavior of low-disorder two-dimensional (2D) electron systems is currently the focus of a great deal of attention. In the strongly-interacting limit, electrons are expected to crystallize into a quantum Wigner crystal (Wigner, 1934), but no definitive evidence for this effect has been obtained despite much experimental effort over the years. Now we have found two-threshold voltage-current characteristics with a dramatic increase in noise between the two threshold voltages.

Tim Linden (Ohio State University) Mon. October 1st, 2018
12:45 pm-1:45 pm

Michelson Postdoctoral Prize Lecture 1

Astrophysical Signatures of Dark Matter Accumulation in Neutron Stars

Over the past few decades, terrestrial experiments have placed increasingly strong limits on the dark matter-nucleon scattering cross-section. However, a significant portion of the standard dark matter parameter space remains beyond our reach. Due to their extreme density and huge gravitational fields, neutron stars stand as optimal targets to probe dark matter-nucleon interactions. For example, over the last few years, the mere existence of Gyr-age neutron stars has placed strong limits on models of asymmetric dark matter. In this talk,

Alkan Kabakcioglu, Koc University, DNA folding thermo/dynamics with a twist Mon. September 24th, 2018
12:45 pm-1:45 pm

 DNA folding thermo/dynamics with a twist

Alkan Kabakcioglu, Koc University, Istanbul

DNA denaturation is possibly one of the earliest problems in biophysics that grabbed the attention of statistical physicists. The nature of the folding/melting transition has been subject to debate since 60’s until a breakthrough in the past decade mostly settled the question. We recently readdressed the problem for circular DNA (which has a topologically imposed, fixed linking number due to helicity) and found that the melting behavior is qualitatively different from that of the unconstrained DNA with freely dangling ends.

no seminar/faculty meeting Mon. September 17th, 2018
12:45 pm-2:00 pm
Wei-Cheng Lee, Binghamton University-SUNY, Orbital Selective Mott Transition in Thin Film VO2 Mon. April 23rd, 2018
12:45 pm-1:45 pm

Orbital Selective Mott Transition in Thin Film VO2

Wei-Cheng Lee

Department of Physics, Applied Physics, and Astronomy, Binghamton University – SUNY

In this talk, evidences of strain-induced modulation of electron correlation effects in the rutile phase of epitaxial VO2/TiO2 will be presented. The strain is engineered by different growth orientations (001), (100), and (110). We find that the hard x-ray photoelectron spectroscopy (HAXPES) reveals significant suppression of the density of states at the Fermi energy in (100) and (110) samples at a temperature well above the metal-insulator transition temperature, but not in the (001) sample.

Fac. meeting Mon. April 16th, 2018
12:45 pm-1:45 pm
Susan Fullerton, University of Pittsburgh, Using Ions to Control Transport in 2D Materials Mon. April 2nd, 2018
12:45 pm-1:45 pm

Using Ions to Control Transport in 2D Materials

Susan Fullerton, University of Pittsburgh

 

Electrostatic gating of two-dimensional (2D) materials with ions is an effective method to achieve high carrier density (10^13 – 10^14 cm^-2) and excellent gate control by creating an electric double layer (EDL) with large capacitance density (>2 μF/cm^2). I will review our use of EDL gating to investigate transport properties of 2D materials including MoTe2, MoS2 and WSe2, and introduce new device concepts that employ EDL gating as an active device component. These include a monolayer electrolyte for application in flash memory,

Olle Heinonen, Argonne National Laboratories, Quantum Monte Carlo modeling of real materials Fri. March 30th, 2018
3:30 pm-4:30 pm

Quantum Monte Carlo modeling of real materials

Olle Heinonen, Argonne National Laboratory

 

Because of recent advances in algorithms and hardware, it is now possible to do quantum Monte Carol simulations of real materials systems, such as correlated oxides, for which standard density functional theory methods have well-known problems. I will here briefly introduce variational and diffusion Monte Carlo methods, and then discuss some results for correlated oxides as well as for some chemical systems. I will end with discussing on-going developments and an outlook towards the future.

 

 

Benjamin Fregoso, Dept of Physics, Kent State University, Nonlinear photocurrents in two-dimensional ferroelectrics and beyond Wed. March 28th, 2018
12:45 pm-1:45 pm

 Nonlinear photocurrents in two-dimensional ferroelectrics and beyond

Benjamin Fregoso, Dept. of Physics, Kent State University

Abstract:

In recent years, it has become clear the need for efficiently harvesting solar energy. Unfortunately, silicon-based solar cells with high efficiency are very costly. These devices rely on pn-junctions to separate positive and negative charge carries. I this talk, I explore a less known (but very interesting) nonlinear optical effect, so-called `shift current’, to generate large photocurrent beyond the pn-junction paradigm. I will describe the shift-current mechanism in insulators and ferroelectrics and its relation to spontaneous electric polarization.

Katy Keenan Applied Physics Division, Physical Measurement Lab National Institute of Standards and Technology Quantitative MRI for Precision Medicine Thu. March 22nd, 2018
2:00 pm-3:00 pm

IMAGING PHYSICS SEMINAR

Katy Keenan
Applied Physics Division, Physical Measurement Lab
National Institute of Standards and Technology

Quantitative MRI for Precision Medicine

The ability of MRI to measure real, physical parameters of interest requires reference standards to ensure accuracy and reproducibility of data. Currently, variability exists across MRI systems, manufacturers, models, software versions, and analysis packages, which impedes comparison of data across patients, centers, and time. To move towards precision medicine, we must be able to determine the threshold of normal compared to disease state with a diagnostically useful uncertainty.

Debra McGivney, Dept. Radiology CWRU, Inverse Problems in Medical Imaging Tue. March 20th, 2018
1:00 pm-2:00 pm

IMAGING PHYSICS SEMINAR
Debra McGivney
Research Scientist, Department of Radiology
Case Western Reserve University

Inverse Problems in Medical Imaging

Mathematical inverse problems are used to model a wide variety of practical problems, including problems in medical imaging. Here, the unknown of interest is an image of the inside of the human body, which is not directly observable, but must be reconstructed given measurements made outside of the body. Oftentimes, reconstruction problems in imaging are ill-posed, which can result in errors in the reconstructed solution. Medical imaging plays a vital role in the diagnosis,

Yuan-Ming Lu, The Ohio State University, Tunable Surface States of Topological Materials Mon. March 19th, 2018
12:45 pm-1:45 pm

Tunable Surface States of Topological Materials

Yuan-Ming Lu, The Ohio State University

 

The discovery of topological insulators revealed a large class of topological materials, which exhibit novel surface states with unusual properties. I will discuss some recent progress in engineering surface states of topological materials, focusing on two different systems. The 1st class of materials is three-dimensional Dirac semimetals including Na3Bi and Cd3As2, whose topological surface states can be deformed in these materials by either doping or applying mechanical strain. The 2nd class of materials are spin-orbit coupled quantum magnets,

Alexey Tonyushkin University of Massachusetts Boston, Breaking the Rules in Magnetic Particle Imaging and Ultra-High Field MRI Thu. March 15th, 2018
12:30 pm-1:30 pm

IMAGING PHYSICS SEMINAR
Alexey Tonyushkin
University of Massachusetts Boston
Breaking the Rules in Magnetic Particle Imaging
and Ultra-High Field MRI

Magnetic Particle Imaging (MPI) is a new tomographic imaging modality that offers high spatial and temporal resolution. Compared to the other imaging modalities such as MRI/CT/PET, MPI is non-toxic, more sensitive, and fully quantitative technique. To date a few small-bore MPI systems were developed, however, human-size MPI scanner has yet to be built. The major challenge of scaling up of MPI is in high power consumption that is associated with the traditional approach to designing the scanner.

Spring break ( no seminar) Mon. March 12th, 2018
12:45 pm-1:45 pm
Michael Boss, NIST, Quantitative MRI: from Bench to Bedside Mon. March 12th, 2018
4:30 pm-5:30 pm

IMAGING PHYSICS SEMINAR
Michael Boss
National Institute of Standards and Technology
Quantitative MRI: from Bench to Bedside

Quantitative MRI: from Bench to Bedside Magnetic Resonance Imaging (MRI) is an exquisite tool for probing the anatomical structure of the human body. It is also capable of measuring physical parameters such as relaxation times, diffusion and temperature, known as quantitative imaging biomarkers (QIBs). When acquired using methods with known limits of bias and reproducibility, these QIBs allow for comparison of scan data across patients, imaging sites, and time, turning into a powerful tool for clinical trials and patient care to evaluate disease state and treatment response.

APS March Meeting ( no seminars) Mon. March 5th, 2018
12:45 pm-1:45 pm
APS March Meeting preview: student practice talks Mon. February 26th, 2018
12:30 pm-2:00 pm

Shuhao Liu:  A Temperature Driven Hole-phonon Coupling Enhancement Effect in a Strongly Correlated 2D Hole System.

Kasun V. M. N. G. Premasiri:  Tuning Rashba Spin-orbit Coupling in Few-layer InSe.

Kyle Crowley: Doping and Field Effect in Novel 2D Layered Oxides

Santosh Kumar Radha: Distortion modes in inorganic halide perovskites: to twist or to stretch.

Narasak Pandech: First-principles Investigation of The Role of Organic Molecules Inside The α-phase of Hybrid Halide Perovskite CH3NH3BX3 (B= Pb,

Andrew Stephens, Northwestern U., Separating the role of chromatin from lamins in mechanics and morphology of the cell nucleus Thu. February 22nd, 2018
4:30 pm-5:30 pm

Separating the role of chromatin from lamins in mechanics and morphology of the cell nucleus

Andrew Stephens, Northwestern U.

The nucleus is the 10 µm ellipse compartment in the cell which must properly transduce or resist biophysical
forces to dictate the spatial organization of the 2 meters of genome inside of it. Organization and
mechanotransduction determine the expression profile of genome which dictates cell function. Previous studies
revealed that the two major contributors to nuclear mechanics are lamins, protein intermediate filaments lining
the inner nuclear envelope, and chromatin, the DNA genome and its associated proteins,

Fac. meeting Mon. February 19th, 2018
12:45 pm-1:45 pm

No seminar physics fac. meeting

Lydia Kisley, Univ. Illinois at Urbana-Champaign, Proteins in nanoporous hydrogels: adsorption, diffusion, and folding Mon. February 19th, 2018
4:30 pm-5:30 pm

Proteins in nanoporous hydrogels: adsorption, diffusion, and folding

Lydia Kisley
Beckman Institute, University of Illinois at Urbana-Champaign
Abstract:  Proteins within nanoporous hydrogels have important biotechnological applications in
pharmaceutical purification, tissue engineering, water treatment, biosensors, and medical
implants. Yet, oftentimes proteins that are functional in solution lose activity when in contact
with soft nanostructured materials due to perturbations in the folded state, conformation,
diffusion, and adsorption dynamics of the protein by the material. We have developed several
unique nanoscale fluorescent spectroscopies to image the heterogeneity of protein dynamics
within hydrogels.
Maxim Dzero, Kent State University, Spins & Knots: The rise of Topology in f-orbital materials Mon. January 29th, 2018
12:45 pm-1:45 pm

Spins & Knots: The rise of Topology in f-orbital materials

Maxim Dzero

Kent State University

In my talk I will review the key recent theoretical and experimental works on a new class of topological material systems – topological Kondo insulators, which appear as a result of interplay between strong correlations and spin-orbit interactions. I will discuss the history of Kondo insulators is along with the theoretical models used to describe these heavy fermion compounds. The Fu-Kane method of topological classification of insulators is used to show that hybridization between the conduction electrons and localized f-electrons in these systems gives rise to interaction- induced topological insulating behavior.

Elshad Allahyarov, Duisburg-Essen University and CWRU, Smectic monolayer confined on a sphere: topology at the particle scale Mon. January 22nd, 2018
12:45 pm-1:45 pm

Prof. Dr. Elshad Allahyarov, 

Duisburg-Essen University, Germany, and  Physics Department  CWRU

Smectic monolayer confined on a sphere: topology at the particle scale

The impact of topology on the structure of a smectic monolayer confined to a sphere is explored by particle-resolved computer simulations of hard rods. The orientations of the particles are either free or restricted to a prescribed director field with a latitude or longitude orderings. Depending on the imprinted topology, a wealth of different states are found including equatorial smectic with isotropic poles, equatorial smectic with empty poles,

Samo Kralj, University of Maribor, Impact of intrinsic and extrinsic curvature on membrane shapes Wed. November 29th, 2017
12:30 pm-1:30 pm

Prof. Samo Kralj

University of Maribor, Maribor & Jožef Stefan Institute, Ljubljana, Slovenia

 

 Impact of intrinsic and extrinsic curvature on membrane shapes

Red blood cells (erythrocytes) are present in almost all vertebrates and their main function is the transport of oxygen to the body tissues. Their shape dominantly influences their functionality. In almost all mammals in normal conditions erythrocytes adopt a disk-like (discocyte) shape which optimizes their flow properties in large vessels and capillaries. Experimentally measured values  of  the  relative volume v of stable discocyte shapes  range in a relatively broad window.

No seminar, Faculty meeting Mon. November 27th, 2017
12:45 pm-2:00 pm
Farida Selim, Bowling Green State University, Positron Annihilation Spectroscopy and Measurements of Origin of Novel Electronic Phenomena in Semiconductors and Oxides Mon. November 20th, 2017
12:45 pm-1:45 pm

Positron Annihilation Spectroscopy and Measurements of Origin of Novel Electronic Phenomena in Semiconductors and Oxides    

Farida A. Selim, Department of Physics and Astronomy, Bowling Green State University  

Center for Photochemical Sciences, Bowling Green State University

 Positron Annihilation Spectroscopy (PAS) has been established as an effective tool to probe electron states and measure atomic scale defects in solids. However, when combined with other techniques, PAS becomes also a powerful tool for revealing and explaining many interesting electronic phenomena. In our laboratory, we combined PAS with structural and transport measurements as well as with infrared,

Vincent Sokalski, Carnegie Mellon University, A New Kind of Magnetism – The Dzyaloshinskii-Moriya Interaction Mon. November 13th, 2017
12:45 pm-1:45 pm

A New Kind of Magnetism – The Dzyaloshinskii-Moriya Interaction

Vincent Sokalski, Dept. of Materials Science and Engineering, Carnegie Mellon University

Magnetism has had a profound effect on our everyday lives from compass needles in ancient times to the modern hard disc drive in today’s computers.  The existence of magnetic materials is rooted in the Heisenberg exchange interaction energy, , which favors parallel (or anti-parallel) alignment of neighboring spin vectors and their associated magnetic dipole moments as found, for example, in Fe, Ni, and Co.  In the past decade a different type of magnetic exchange came to the forefront of modern physics called the Dzyaloshinskii-Moriya Interaction (DMI) given by ,

Jeanie Lau, The Ohio State University, Spin, Charge and Heat Transport in Low-Dimensional Materials Mon. November 6th, 2017
12:45 pm-1:45 pm

Spin, Charge and Heat Transport in Low-Dimensional Materials

Chun Ning (Jeanie) Lau

Department of Physics, The Ohio State University, Columbus, OH 43210, USA

 

Low dimensional materials constitute an exciting and unusually tunable platform for investigation of both fundamental phenomena and electronic applications. Here I will present our results on transport measurements of high quality few-layer phosphorene devices, and the unprecedented current carrying capacity of carbon nanotube “hot dogs”. In the second half of the talk, I will present our recent observation of robust long distance spin transport through the antiferromagnetic state in graphene.

Eric Stinaff, Ohio University, Opto-electronic studies of novel self-contacted 2D materials based devices Mon. October 16th, 2017
12:45 pm-1:45 pm

Opto-electronic studies of novel self-contacted 2D materials based devices

Eric Stinaff

Department of Physics and Astronomy, Ohio University

Interest in two-dimensional crystals has grown exponentially over the last decade, a testament to their vast technological and scientific potential. In addition to properties such as high mobilities, semiconducting and superconducting behavior, and excellent thermal properties, many of these materials have the potential for novel opto-electronic applications, with large absorption, strong room-temperature emission, non-linear response, and optical control of spin and valley degrees of freedom. In this presentation, we will discuss an experimental investigation of mono-to-few-layer sheets of MoS2 and WS2 employing femtosecond transient absorption spectroscopy (FTAS) and microscopy.

Michael Fisch, Kent State University, X-ray Experiments in Liquid Crystal Science and Technology Mon. October 9th, 2017
12:45 pm-1:45 pm

X-ray Experiments in Liquid Crystal Science and Technology

Michael Fisch

Kent State University

 

The use of X-rays to study liquid crystals has a long history, and is still of continuing interest.  A brief review of liquid crystals and X-ray diffraction from common liquid crystalline phases will be presented.  Interpretation of the resulting diffraction patterns will be discussed, and some of our current experiments in bent-core molecules and “organic salts will be discussed.  The relationship of these studies to current problems in liquid crystal science and technology will be briefly explored,

No seminar, faculty meeting Mon. October 2nd, 2017
12:45 pm-2:00 pm
Maosheng Miao, California State University Northridge, Automatic search versus chemical rules in materials structure study Mon. September 25th, 2017
12:45 pm-1:45 pm

Automatic search versus chemical rules in materials structure study
Maosheng Miao
Department of Chemistry and Biochemistry, California State University Northridge CA,
USA; Beijing Computational Science Research Center, Beijing, China

The increase of the computer power in the past decades not only allow us to calculate
larger systems with higher accuracy in materials studies, but also provide the opportunity
to explore large configuration spaces such as structures and compositions. Automatic
structure searches have been very successful in predicting structures of bulk materials. It
seems out of question whether the automatic search is advantageous over traditional
structure design based on chemical knowledge and intuition.

Liang Wu, UC Berkeley, MPPL3, Antiferromagnetic resonance and in-gap terahertz continuum in Kitaev Honeycomb magnet α−RuCl3 Fri. September 15th, 2017
12:45 pm-1:45 pm

Antiferromagnetic resonance and in-gap terahertz continuum in Kitaev Honeycone magnet α−RuCl3

Spin-1/2 moments in the antiferromagnetic Mott insulator α-RuCl3 are coupled by strongly anisotropic bond-dependent exchange interactions on a honeycomb lattice. Intense study of α- RuCl3 by inelastic scattering has been driven by the proposal that its low energy excitations may be adiabatically connected to the Majorana quasiparticles that emerge in the exact solution of the Kitaev spin liquid model. In my talk, I will present optical absorption measurements using time- domain terahertz spectroscopy in the range 0.3 to 10 meV that reveal several new features of the low-energy spectrum of α-RuCl3 [1].

Liang Wu, University California Berkeley, MPPL2,Giant nonlinear optical responses in Weyl semimetals Tue. September 12th, 2017
11:30 pm-12:30 pm

Giant nonlinear optical responses in Weyl semimetals

Recently Weyl quasi-particles have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable properties. The TMMPs are polar metals, a rare subset of inversion- breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures, most notably second-order nonlinear optical polarizability, leading to phenomena such as second-harmonic generation (SHG).

Liang Wu, University California Berkeley, MPPL1, Low-energy Electrodynamics of 3D Topological Insulators Mon. September 11th, 2017
12:45 pm-1:45 pm

Low-energy Electrodynamics of 3D Topological Insulators

 

Topological insulators (TIs) are a recently discovered state of matter characterized by an “inverted” band structure driven by strong spin-orbit coupling. One of their most touted properties is the existence of robust “topologically protected” surface states.  I will discuss what topological protection means for transport experiments and how it can be probed using the technique of time- domain THz spectroscopy applied to 3D TI thin films of Bi2Se3.  By measuring the low frequency optical response, we can follow their transport lifetimes as we drive these materials via chemical substitution through a quantum phase transition into a topologically trivial regime [1].

Condensed Matter Seminar: Jie Gao, Missouri University of Science and Technology (University of Missouri – Rolla) Thu. May 11th, 2017
11:30 am-12:30 pm

Jie Gao

Missouri University of Science and Technology (University of Missouri – Rolla)

Tailoring light-matter interaction with metamaterials and metasurfaces

Metamaterials and metasurfaces with designed subwavelength nanostructures exhibit intriguing electromagnetic phenomena, such as negative refraction, invisible cloaking, sub-diffraction imaging, near-zero permittivity and hyperbolic dispersion. In this talk, I will present our recent work on tailoring light-matter interaction with metamaterials and metasurfaces, including the realization of enhanced spontaneous emission, ultrasensitive molecule detection, strong plasmon-phonon interaction, optical vortex generation and full-color metasurface hologram. These results present opportunities and challenges in understanding new physics of light-matter interaction in those artificially structured optical materials and realizing many unprecedented applications in nanophotonics.

CANCELED: Maosheng Miao, California State University Northridge,Simulate to discover: from new chemistry under high pressure to novel two-dimensional materials Mon. April 24th, 2017
12:45 am-1:45 am

CANCELED. Will be rescheduled.

Simulate to discover: from new chemistry under high pressure to novel two-dimensional materials

 

Maosheng Miao

Department of Chemistry and Biochemistry

California State University Northridge, California 91330, USA

 

The periodicity of the elements and the non-reactivity of the inner-shell electrons are two related principles of chemistry, rooted in the atomic shell structure. Within compounds, Group I elements, for example, invariably assume the +1 oxidation state, and their chemical properties differ completely from those of the p-block elements.

David Pace, General Atomics, San Diego, The Fast and the Furious: Energetic Ion Transport in Magnetic Fusion Devices Wed. April 19th, 2017
12:45 am-1:45 am

The Fast and the Furious: Energetic Ion Transport in Magnetic Fusion Devices

D.C. Pace and the DIII-D National Fusion Facility Team

General Atomics, P.O. Box 85608, San Diego, CA 92186-5608, USA

David Pace

Nuclear fusion has the potential to be an energy source that powers society without generating greenhouse gases or high-level radioactive waste. The tokamak approach to controlled nuclear fusion employs a toroidally-shaped magnetic field configuration to confine plasmas at temperatures beyond 200 million K (20 keV). Future reactors aim to utilize the deuterium-tritium fusion reaction due to its favorable cross-section,
Louis F. Piper, Binghamton University, Shining new light on old problems in lithium ion batteries Mon. April 17th, 2017
12:45 am-1:45 am

Shining new light on old problems in lithium ion batteries

 

Louis Piper

Binghamton University, State University of New York

 

Improving the energy storage and release of lithium ion battery is largely limited to the cathode (positive electrode).  Commercial high capacity LIBs employ Ni-rich layered oxides (derived from LiCoO2) as cathodes.  In these systems, the reversible energy storage capacity is limited to 1 Li+ per transition metal (i.e. Co3+/4+ redox couple).  However, only 2/3 of Li+ per redox couple are typically intercalated due to capacity retention issues with fast cycling and high voltages. 

Nandini Trivedi, The Ohio State University, Novel magnetic phases in spin-orbit coupled oxides Mon. April 10th, 2017
12:45 pm-1:45 pm
Novel magnetic phases in spin-orbit coupled oxides
Nandini Trivedi,
 
Department of Physics, The Ohio State University
 

Abstract: I will discuss puzzles about magnetism in some of the simplest oxide materials with a single electron in the d-orbital.  Starting from a microscopic model of a Mott insulator with both spins and orbitals, I will obtain the effective magnetic Hamiltonian and provide insights into the experimental puzzles. 

Nate Stern, Northwestern University, Monolayer Semiconductor Opto-Electronics: Controlling Light and Matter in Two-Dimensional Materials Mon. April 3rd, 2017
12:45 pm-1:45 pm

Monolayer Semiconductor Opto-Electronics: Controlling Light and Matter in Two-Dimensional Materials

Nathaniel Stern

Department of Physics and Astronomy, Northwestern University

The discovery of monolayer two-dimensional semiconductors of atomic-scale thickness presents a new two-dimensional landscape in which to play with the interaction between light and matter. These nanomaterials at the extreme limit of surface-to-volume ratio exhibit rich optical phenomenology such as layer dependent bandgaps and degenerate, but distinct, valley-polarized excitonic states. The unique features of atomically-thin materials suggest that these layered systems can be exploited to achieve new regimes of light-matter interactions.

Paul Kelly, University of Twente, Turning up the heat in first principles Quantum Spin Transport Wed. March 22nd, 2017
12:45 pm-1:45 pm

Turning up the heat in first principles Quantum SpinTransport

 Paul J. Kelly

Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

 

The spin Hall angle (SHA) is a measure of the efficiency with which a transverse spin current is generated from a charge current by the spin-orbit coupling and disorder in the spin Hall effect (SHE). In a study of the SHE for a Pt|Py (Py=Ni80Fe20) bilayer using a first-principles scattering approach,

No Seminar, APS March Meeting and Spring Break Mon. March 13th, 2017
1:00 am-1:00 am
Francesca F. Serra, Johns Hopkins University, Control of liquid crystals through topography for optics and assembly Mon. February 27th, 2017
12:45 pm-1:45 pm
Control of liquid crystals through topography for optics and assembly 
Dr. Francesca Serra 

Physics and Astronomy 

Johns Hopkins University
 
Soft materials are a promising tool to explore controllable energy landscapes. Liquid crystals, in particular, combine reconfigurability, unique optical properties and the possibility of directing their self-assembly via the bounding surfaces. I will show, for example, how smectic-A liquid crystals under different boundary conditions create microlens arrays made of focal conic defects or light guides in an aqueous solution. Focal conic domains act as gradient refractive index lenses that can be assembled and ordered exploiting topographical cues.
Hamza Balci, Kent State University, A Single Molecule Approach to Study Protein, Small Molecule, and G-Quadruplex Mon. February 20th, 2017
12:45 pm-1:45 pm

A Single Molecule Approach to Study Protein, Small Molecule, and  G-Quadruplex Interactions

Hamza Balci

Kent State University, Physics Department, Kent, OH

 

G-quadruplex (GQ) structures are non-canonical nucleic acid secondary structures that form in guanine-rich segments of the genome, most prominently at telomeres. In addition, several hundred thousand potential GQ forming sequences have been identified in human genome, with particularly higher frequency at promoter regions. When GQ structures (GQs) form at telomeres, they cap chromosome ends and are involved in stabilizing these vulnerable regions. Also, GQs have been shown to regulate transcription and translation level gene expression when they form in promoter regions of DNA and 5′-UTR of RNA,

Saw-Wai Hla, Ohio University, Operating Individual Quantum Molecular Machines Mon. February 6th, 2017
12:45 pm-1:45 pm

Operating Individual Quantum Molecular Machines

Saw-Wai Hla

 Department of Physics & Astronomy, Ohio University, OH 45701, USA

and

Nanoscience and Technology Division, Argonne National Laboratory, IL 60439, USA.

E-mail: hla@ohio.edu , URL: www.phy.ohiou.edu/~hla

 

A recent emergent research direction is the development of complex molecular machines suitable to operate on solid surfaces. Biological machines have the sizes from tens of nanometers to a few microns –a range where classical machine concepts hold while artificially designed molecular machines can be in the size range of a few nanometers or less,

Mike Boss, NIST, Physics and Impact of Quantitative Magnetic Resonance Imaging Mon. January 30th, 2017
12:45 pm-1:45 pm

Physics and Impact of Quantitative Magnetic Resonance Imaging

Michael Boss,

Applied Physics Division
National Institute of Standards and Technology, Boulder, CO

Each year, millions of U.S. patients are scanned using Magnetic Resonance Imaging (MRI), costing billions of dollars.  The resultant images are typically qualitative, limiting the ability to compare results across patients, time, and scanners. However, a suite of physical parameters (e.g., relaxation times, diffusion coefficients) are interrogable with magnetic resonance, enabling quantitative imaging biomarkers (QIBs). QIBs can provide threshold values for disease diagnosis, allow meaningful measurement of longitudinal change for evaluating treatment response,

Lucile Savary (MIT) — Michelson Postdoctoral Prize Lecturer Fri. January 27th, 2017
12:45 pm-1:45 pm

Quantum Loop States in Spin-Orbital Models on the Honeycomb and Hyperhoneycomb Lattices

In the quest for quantum spin liquids, the challenges are many: neither is it clear how to look for nor how to describe them, and definitive experimental examples of quantum spin liquids are still missing. In this talk I will show how to devise a realistic model on the honeycomb lattice whose ground state realizes Haldane chains whose physical supports fluctuate, hence naturally providing the hallmark “fractional excitations” of quantum spin liquids. When taken to the three-dimensional hyperhoneycomb lattice, the ground state becomes a full-fledged symmetry-enriched U(1) quantum spin-orbital liquid,

Lucile Savary (MIT) — Michelson Postdoctoral Prize Lecturer Tue. January 24th, 2017
11:00 am-12:00 pm

Quantum Spin Ice

Recent work has highlighted remarkable effects of classical thermal fluctuations in the dipolar spin ice compounds, such as “artificial magnetostatics.” In this talk, I will address the effects of terms which induce quantum dynamics in a range of models close to the classical spin ice point. Specifically, I will focus on Coulombic quantum spin liquid states, in which a highly entangled massive superposition of spin ice states is formed, allowing for dramatic quantum effects: emergent quantum electrodynamics and its associated emergent electric and magnetic monopoles. I will also discuss how random disorder alone may give rise to both a quantum spin liquid and a Griffiths Coulombic liquid–a Bose glass-like phase.

Michael Snure, AFRL, Two dimensional BN an atomically thin insulator, substrate, and encapsulation layer from growth to application Mon. January 23rd, 2017
12:45 pm-1:45 pm

Two dimensional BN an atomically thin insulator, substrate, and encapsulation layer from growth to application

Michael Snure

Air Force Research Laboratory, Sensors Directorate, Wright Patterson AFB, OH

Since free standing graphene was found in 2004, there has been an explosion of research on atomically thin two dimensional (2D) materials based isolated sheets of layered van der Waals solids.  The spectacular electrical and thermal transport properties of graphene generated a great deal of hype making it a heavily researched material for ultra-high-speed electronics; however, strong interaction with conventional 3D substrates and the lack of a band gap has proven to degrade properties and limit its usefulness in these devices. 

Lucile Savary (MIT) — Michelson Postdoctoral Prize Lecturer Mon. January 23rd, 2017
4:15 pm-5:15 pm

A New Type of Quantum Criticality in the Pyrochlore Iridates

The search for truly quantum phases of matter is one of the center pieces of modern research in condensed matter physics. Quantum spin liquids are exemplars of such phases. They may be considered “quantum disordered” ground states of spin systems, in which zero point fluctuations are so strong that they prevent conventional magnetic long range order. More interestingly, quantum spin liquids are prototypical examples of ground states with massive many-body entanglement, of a degree sufficient to render these states distinct phases of matter. Their highly entangled nature imbues quantum spin liquids with unique physical aspects,

Christopher Wolverton, Northwestern University, Accelerating Materials Discovery with Data-Driven Atomistic Computational Tools Mon. December 5th, 2016
12:45 pm-1:45 pm

Accelerating Materials Discovery with Data-Driven Atomistic Computational Tools

Chris Wolverton

Dept. of Materials Science and Eng., Northwestern University, Evanston, IL (USA)

c-wolverton@northwestern.edu

 

Many of the key technological problems associated with alternative energies (e.g., thermoelectrics, advanced batteries, hydrogen storage, etc.) may be traced back to the lack of suitable materials. Both the materials discovery and materials development processes may be greatly aided by the use of computational methods, particular those atomistic methods based on density functional theory (DFT).   Here, we present an overview of our recent work utilizing high-throughput computation and data mining approaches to accelerate materials discovery,

Marie-Charlotte Renoult, Université de Rouen, Free falling jets of a viscoelastic solution Wed. November 23rd, 2016
12:45 pm-1:45 pm

Title: Free falling jets of a viscoelastic solution
Prof. Marie-Charlotte Renoult
Université de Rouen, France

Abstract:

We conducted free falling jet experiments of a Newtonian solution with a polymer additive, i.e., a viscoelastic solution.Viscoelastic jets usually break up with the formation of beads-on-a-string (BOAS) structures, where large beads are connected by thin threads. These structures form when the polymer solution begins to exhibit strain-hardening, i.e., an increase in extensional viscosity with extensional rate. Associated with this viscoelastic property is a characteristic relaxation time.In this presentation, two methods of image analysis will be presented: a shape analysis and a multi-scale analysis that are applied to a large number of free falling jet visualisations performed at different jet velocities.The results obtained demonstrate the power of these two experimental techniques to gain a deeper insight into BOAS formation and to probe complex liquid rheology such as the subtle measurement of the polymer relaxation time.

Keji Lai, Univ of Texas, Austin/Microwave Imaging of Edge States and Electrical Inhomogeneity in 2D Materials Mon. November 21st, 2016
12:45 pm-1:45 pm

The understanding of various types of disorders in 2D materials, including dangling bonds at the edges, defects in the bulk, and charges in the substrate, is of fundamental importance for their applications in electronics and photonics. Because of the imperfections, electrons moving on the 2D plane experience a spatially non-uniform Coulomb environment, whose effect on the charge transport has not been microscopically probed. Using a non-invasive microwave impedance microscope with ~100nm resolution and ~1nS sensitivity, we can visualize the spatial evolution of the insulator-to-metal transition in mono-layer and few-layer MoS2 field-effect transistors. As the transistors are gradually turned on, electrical conduction emerges initially at the edges before appearing in the bulk,

Salah Eddine Boulfelfel, Georgia Institute of Technology, Atomic-Scale Modeling of Activated Processes in the Solid State Mon. November 14th, 2016
12:45 pm-1:45 pm

Atomic-Scale Modeling of Activated Processes in The Solid State

Salah Eddine Boulfelfel

School of Chemical and Biomolecular Engineering

Georgia Institute of Technology

In the practice of solid-state chemistry, processes either thermally-activated or induced by external high-pressure are common events. Often, the simplicity of the material’s structure involved in the activated process is in contrasts with the theoretical and experimental difficulties in assessing its mechanism. Large hysteresis effects, nucleation and growth scenarios, and first-order kinetics require dedicated computational approaches in order to correctly unravel the complex nature of activated process at the atomistic level of details.

Jim Andrews, Youngstown State University, Coherent Perfect Polarization Rotation–Beyond the Anti-Laser Mon. November 7th, 2016
12:45 pm-1:45 pm

We describe the distinguishing characteristics of coherent perfect optical conversion processes using two-beam interference, as compared to single-beam ‘critical coupling’ processes.  We extend the application of two-port coherent conversion processes to magneto-optical (Faraday) rotation in structured systems and present our recent laboratory demonstration of coherent perfect polarization rotation (CPR) which is a conservative, reversible counterpart to coherent perfect absorption (CPA, or the so-called ‘anti­laser’). conclude with a brief summary of theoretical studies suggesting a CPR-based miniaturization of optical isolators and the extension of coherent perfect  phenomena in non-linear optics.

Samo Kralj, University of Maribor, Effective Topological Charge Cancellation Mechanism Mon. October 31st, 2016
1:00 pm-2:00 pm

Effective Topological Charge Cancellation Mechanism

Samo Kralj1,2

1FNM, University of Maribor, Koroška 160, 2000 Maribor, Slovenia

2Jožef Stefan Institute, Jamova 39,1000 Ljubljana, Slovenia

Topological defects (TDs) appear almost unavoidably in continuous symmetry breaking phase transitions [1]. Topological origin makes their key features independent of systems’ microscopic details and therefore TDs display many universalities. In general, TDs have strong impact on material properties and play significant role in several technological applications.

Patrick Woodward, The Ohio State University, The magnetism of double perovskites containing osmium and rhenium Mon. October 24th, 2016
12:45 pm-1:45 pm

 

Patrick M. Woodward

Department of Chemistry and Biochemistry, The Ohio State University

Over the past several years we have been synthesizing and studying the magnetic properties of A2MOsO6 and A2MReO6 (Mg, Zn, Cr, Fe, Co, Ni) double perovskites in a quest to understand how the sign and strength of the superexchange interactions change as a function of the relative filling of the 3d and 5d orbitals, as well as the geometry of the crystal structure. In double perovskites where the 5d ion is the only magnetic ion we find that spin-orbit coupling plays a role,

Nayana Shah, University of Cincinnati, Manifestations of spin-orbit coupling and topology in out-of-equilibrium hybrid superconducting systems Mon. October 10th, 2016
12:45 pm-1:45 pm

Recently there has been a lot of excitement generated by the possibility of realizing and detecting Majorana fermions within the arena of condensed matter physics and its potential implication for topological quantum computing.  Although already at the end of twentieth century emergent Majorana end-states were shown to exist in a theoretical model of spinless p-wave superconductor (Kitaev) chain, it was only a decade later that proposals to experimentally realize such a model emerged. These were motivated by the discovery of topological insulators that ushered a new era of so-called symmetry-protected topological phases but also stemmed from existent studies of hybrid superconductor-ferromagnet systems that form the basis of another highly active area of superconducting spintronics.

Zhaoning Song, University of Toledo,The Formation and Degradation of Metal Halide Perovskites Mon. September 26th, 2016
12:45 pm-1:45 pm

Solar cells based on organic-inorganic metal halide perovskite materials, such as methylammonium lead iodide (CH3NH3PbI3), have been the subject of intense investigation during the past 5 years due to high power conversion efficiencies (>22%) and relatively low manufacturing costs. Never before has the field of photovoltaics (PV) seen such rapid and exciting progress. The results are surprising because various low-temperature, solution-based processing methods have been successful in fabricating high-efficiency devices. Nevertheless, much of the work in this area has focused on device performance optimization and there is a lack of basic understanding of underlying physics and chemistry.

Observation Of Interlayer Phonons in Transition Metal Dichalogenide Atomic Layers and Heterostructures – Rui He Mon. May 2nd, 2016
12:30 pm-1:30 pm

Interlayer phonon modes in atomically thin transition metal dichalcogenide (TMD) heterostructures were observed for the first time. We measured the low-frequency Raman response of MoS2/WSe2 and MoSe2/MoS2 heterobilayers. We discovered a distinctive Raman mode (30 – 35 cm-1) that cannot be found in any individual monolayers (see Fig. 1). By comparing with Raman spectra of bilayer (2L) MoS2, 2L MoSe2 and 2L WSe2, we identified the new Raman mode as the layer breathing mode (LBM) arising from the perpendicular vibration between the two TMD layers. The LBM only emerges in bilayer regions with atomically close layer-layer proximity and clean interface.

The 17 Position Knob: Tuning Interactions With Rare Earths – Paul C. Canfield Mon. April 18th, 2016
12:30 pm-1:30 pm

Physicists see the rare earth group of elements as a powerful tool for tuning the properties of materials. Choice or control of rare earths can be used to modify (i) the size of the unit cell, (ii) the size of the local moment and degree of coupling, (iii) the size and direction of magnetic anisotropy, (iv) the amount of entropy that can be removed at low temperatures, (v) the degree of band filling, and / or (vi) the degree of hybridization. In this seminar I will provide an overview and examples of how this region of the periodic table can be used to guide and inspire research into a wide swath of novel materials and ground states.

Mapping the Phase Diagram of a One-Dimensional Topological Superconductor – Sergey Frolov Mon. April 11th, 2016
12:30 pm-1:30 pm

Download the abstract Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional NbTiN superconductors to obtain additional signatures of Majorana fermions and to explore the magnetic-field driven topological phase transition. With improved device fabrication, namely more transparent contacts to superconductors and stronger coupled gate electrodes, we are mapping out the phase diagram of the topological phase in the space of Zeeman energy and chemical potential, and investigating the apparent closing and re-opening of the superconducting gap.

Nanoscopic Manipulation and Nanoimaging of Liquid Crystals – Charles Rosenblatt Mon. April 4th, 2016
12:30 pm-1:30 pm

Liquid crystals present a remarkable array of fascinating physical phenomena, and are now a >200 billion dollar world-wide industry. As liquid crystals most often are housed in a closed cell or sit atop a substrate, the treatment of the substrate plays a pivotal role. For the past fifteen years we have developed and exploited scanning probe microscope techniques to manipulate the liquid crystal’s orientation and order parameter at a surface on length scales down to a few tens of nanometers, and performed optical imaging with volumetric resolution 1000 times better than confocal microscopy. In this talk I will present our experimental techniques at the nanoscale,

Nanoscopic Manipulation and Nanoimaging of Liquid Crystals – Charles Rosenblatt Mon. April 4th, 2016
12:30 pm-1:30 pm

Liquid crystals present a remarkable array of fascinating physical phenomena, and are now a >200 billion dollar world-wide industry. As liquid crystals most often are housed in a closed cell or sit atop a substrate, the treatment of the substrate plays a pivotal role. For the past fifteen years we have developed and exploited scanning probe microscope techniques to manipulate the liquid crystal’s orientation and order parameter at a surface on length scales down to a few tens of nanometers, and performed optical imaging with volumetric resolution 1000 times better than confocal microscopy. In this talk I will present our experimental techniques at the nanoscale,

Nanomaterials in Liquid Crystal Mediated Interactions – Rajratan Basu Mon. March 28th, 2016
4:00 pm-5:00 pm

In liquid crystals (LC) the effect of nonmesogenic guest-nanoparticles on the LC’s bulk properties often rests on the molecular identification at the nanoscale in order to share and disseminate the `information’ coded into the nanostructure of the nanoparticles. I will present two types of nanomaterials and their intriguing interactions with LCs. Graphene is a twodimensional crystalline carbon allotrope where carbon atoms are densely packed in a regular sp2- bonded atomic-scale hexagonal pattern. This graphene nanostructure can used to enhance the tilted smectic-C order in an LC, giving rise to a faster ferroelectric switching. The presence of graphene can improve the electro-optic response and decrease the rotational viscosity of an LC.

APS March Meeting Mon. March 14th, 2016
12:30 pm-1:30 pm
Preview APS March Meeting Talks – Graduate Students Thu. March 10th, 2016
11:00 am-12:00 pm

Sukrit Sucharitacul, Few-layer III-VI and IV-VI 2D semiconductor transistorsShuhao Liu, Imaging the long diffusion lengths of photo-generated carriers in mixed halide perovskite films

Shuhao Liu, Imaging the long diffusion lengths of photo-generated carriers in mixed halide perovskite films Robert Badea, Magneto-optical mapping of the domain wall pinning potential in ferromagnetic films

Robert Badea, Magneto-optical mapping of the domain wall pinning potential in ferromagnetic films Michael Wolf, Coupling a driven magnetic vortex to individual nitrogen-vacancy spins for fast, nanoscale addressability and coherent manipulation

Michael Wolf, Coupling a driven magnetic vortex to individual nitrogen-vacancy spins for fast,

FMR-Drive Pure Spin Transport in Metals and Magnetic Insulators – Fengyuan Yang Mon. March 7th, 2016
12:30 pm-1:30 pm

Spintronics relies on the generation, transmission, manipulation, and detection of spin current mediated by itinerant charges or magnetic excitations. Ferromagnetic resonance (FMR) spin pumping is a powerful technique in understanding pure spin current. Building on the highquality Y3Fe5O12 (YIG) films grown by our sputtering technique and the large inverse spin Hall effect (ISHE) signals enabled by these films, we have characterized pure spin currents in several classes of materials with different magnetic structures, including: nonmagnetic (NM) metals, ferromagnetic (FM) metals, nonmagnetic insulators, and antiferromagnetic (AF) insulators. The spin Hall angles determined for a series of 3d, 4d, and 5d NM metals show that both atomic number and d-electron count play important roles in spin Hall physics.

Tailored Radiative Processes of Quantum Dots and 2D Materials – Maiken H. Mikkelsen Mon. February 29th, 2016
4:00 pm-5:00 pm

Metal-dielectric nanocavities have the ability to tightly confine light to small mode volumes resulting in strongly increased local density of states. Placing fluorescing molecules or semiconductor materials in this region enables wide control of radiative processes including absorption and spontaneous emission rates, quantum efficiency, and emission directionality. In this talk, I will describe our recent experiments utilizing a tunable plasmonic platform where emitters are sandwiched in a sub-10-nm gap between colloidally synthesized silver nanocubes and a metal film. Utilizing dye molecules with an intrinsic long lifetime reveals spontaneous emission rate enhancements exceeding a factor of 1,000 while maintaining directional emission and high quantum efficiency [Akselrod et al.

Aspects of Photonic Topological Insulators – Mikael Rechtsman Mon. February 22nd, 2016
12:30 pm-1:30 pm

I will present the observation of the topological protection of light – specifically, a photonic Floquet topological insulator. Topological insulators (TIs) are solid-state materials that are insulators in the bulk, but conduct electricity along their surfaces – and are intrinsically robust to disorder. In particular, when a surface electron in a TI encounters a defect, it simply goes around it without scattering, always exhibiting – quite strikingly – perfect transmission. The structure is an array of coupled helical waveguides (the helicity generates a fictitious circularly-polarized electric field that leads to the TI behavior), and light propagating through it is ‘topologically protected’

Combined First-Principles Molecular Dynamics / Density-Functional Theory Study of Ammonia Oxidation on Pt(100) Electrode – Dmitry Skachkov Mon. February 1st, 2016
12:30 pm-1:30 pm

A combined first-principles molecular dynamics/density functional theory study of the electrooxidation of ammonia is conducted to gain an atomic-level understanding of the electrocatalytic processes at the Pt(1 0 0)/alkaline solution interface and to probe the mechanistic details of ammonia electrooxidation on the metal surface. A systematic study of adsorption and relative stability of ammonia and the intermediate species on the Pt(1 0 0) surface as a function of potential is carried out and activation energy profiles for the mechanistic steps in the ammonia oxidation are presented. The reaction mechanism is potential dependent: the modeling study supports the Oswin and Salomon’s mechanism for moderate surface potentials (≥ +0.5 V vs.

Non-adiabatic Transport in Single-Electron Transistors in the Kondo Regime – Andrei Kogan Mon. November 23rd, 2015
12:30 pm-1:30 pm

Magnetic impurities in conductors alter the Fermi sea: A many-body state (A Kondo singlet) is formed that entangles itinerant carriers and the impurity site. This causes a sharp rearrangement of the density of states near the Fermi surface into a hierarchical set governed by a single energy parameter Tk, the Kondo temperature. Equilibrium physics of such electronic “knots” scales with Tk and is highly universal: impurities that differ microscopically from one another yet have similar Kondo temperatures produce Kondo states with similar properties. Recent studies of Kondo physics with voltage-controllable spin traps known as Single-Electron Transistors (SETs) have focused on nonequiibrium Kondo phenomena,

Chip-integrated Nanophotonic Structures for Classical and Quantum Devices – Antonio Badolato Mon. November 16th, 2015
12:30 pm-1:30 pm

Chip-integrated nanophotonics investigates the interaction of light with nanostructures integrated on a chip. Lying at the intersection of condensed matter physics, optics, nanotechnology, and materials science, nanophotonics draws upon expertise from broad areas of physics and engineering, while presenting major opportunities to advance fundamental physics and transformative photonic technologies. In this talk, I will focus on our experimental research in two areas of nanophotonics. First, I will show that nanostructured semiconductors, such as quantum dot heterostructures coupled to photonic crystal nanocavities, can now offer

First, I will show that nanostructured semiconductors, such as quantum dot heterostructures coupled to photonic crystal nanocavities,

Michelson Postdoc Lecture – Michael Hatridge Mon. November 9th, 2015
12:30 pm-1:30 pm
Supercooling-Driven Glass Behaviour in Systems Exhibiting Continuous Symmetry Breaking – Sami Kralj Wed. November 4th, 2015
12:30 pm-1:30 pm

Symmetry breaking is ubiquitous in nature and represents the key mechanism behind rich diversity of patterns exhibited by nature. One commonly introduces an order parameter field to describe onset of qualitatively new ordering in a system on varying a relevant control parameter driving a symmetry breaking transition. In case of continuous symmetry breaking an order parameter consists of two qualitatively different components: an amplitude and gauge field. The latter component enables energy degeneracy and reveals how symmetry is broken. Inherent degeneracy could in general lead to nearby regions exhibiting significantly different gauge fields. Resulting frustrations can nucleate topological defects (TDs) [1].

Photogeneration and Charge Transport in Liquid Crystalline Organic Semiconductors – Sanjoy Paul Mon. October 26th, 2015
12:30 pm-1:30 pm

Organic semiconductors (OSCs) are emerging candidates for the applications in electronic and photonic devices due to material’s low cost and ease of processing. Many materials have been studied to understand the charge generation and transport physics, as well as to develop techniques for facile processing into light emitting diodes, thin film transistors, photovoltaics, and host of other devices. A recurring theme in this effort is the role of disorder in determining critical material parameters, such as mobility and photogeneration efficiency. A particularly useful class of materials in this quest is that of liquid crystalline (LC) OSCs. LCOSCs offer many advantages including facile alignment and the opportunity to study the effects of differing intermolecular geometries on transfer integrals,

Spins in 2D Materials – Roland Kawakami Mon. October 19th, 2015
12:30 pm-1:30 pm

Two-dimensional crystals such as graphene and monolayer transition metal dichalcogenides (TMD) possess unique properties not found in bulk materials. These materials are atomically-thin, yet are strong enough to remain intact as free standing membranes. Because these materials are “all surface”, they tend to be highly surface sensitive and amenable to inducing proximity effects. In this talk, I will discuss our progress of investigating spin-dependent phenomena in graphene and TMD monolayers. We investigate spin transport in graphene utilizing ferromagnetic electrodes to inject and detect

In this talk, I will discuss our progress of investigating spin-dependent phenomena in graphene and TMD monolayers.

Static and Dynamic Flowers in Strained Graphene – Nancy Sandler Mon. October 12th, 2015
12:30 pm-1:30 pm

The coupling of geometrical and electronic properties is a promising venue to engineer conduction properties in graphene. In particular, different regimes can be achieved by manipulating confinement and strain fields, as shown in recent experiments on nanobubbles, drumheads oscillating membranes, and narrow strips deposited on patterned SiC substrates [1].

To investigate strain signatures on graphene systems, we focus on a simple model with a circularly symmetric out-of-plane deformation. Results from numerical tight-binding and Dirac-continuum models for a static deformation reveal intriguing flower-shaped structures in the local density of states with profound consequences for charge transport through the structure [2].

The Status and Challenges of Lead Halide Perovskite Solar Cells – Yanfa Yan Mon. October 5th, 2015
12:30 pm-1:30 pm

Organic-inorganic methylammounium lead halide perovskites, CH3NH3PbX3 (X= Cl, Br, I), have revolutionized the field of thin-film solar cells. Within five years, the efficiency of lead halide perovskite-based thin-film solar cells have increased rapidly from 3.8% in 2009 to 20.1% for a planar CH3NH3PbI3-based thin-film solar cell in 2014. Such rapid progress has never been seen before in the history of solar cell development. In this talk, I will review the history and status of lead halide perovskite thin films solar cells. I will explain why lead halide perovskites exhibit superior photovoltaic properties that conventional solar cell materials such as Si,

Device-compatible Defect Engineering of Rare Earth Doped Nitrides – Volkmar Dierolf Mon. September 28th, 2015
12:30 pm-1:30 pm

LED-lighting is at the verge of replacing conventional incandescent light sources. These white LEDs are based on nitride technology which produces the blue emission, that is subsequently converted in a separate phosphorescent layer to provide the additional required colors. The latter often consists of an insulating material doped with rare earth ions. In order to facilitate further integration, the possibility of introducing rare earth ions directly into the nitride material has been explored, with considerable success. Doping with europium ions (Eu) is of particular interest since they can produce the red color, which remains a challenge for nitride based materials.

Quantum Magnetism in Low Dimensions: An Intriguing Phenomenon Connecting Biology with Physics – Yi-Kuo Yu Mon. September 14th, 2015
12:30 pm-1:30 pm

Magnetism is an important problem in many areas of science including biology, physics and material science. For example, many migratory animals (birds, whales and sea turtles) use magnetism to sense direction for their migrations; computer hard drives store information via magnetism; and so forth. Quantum magnetism in low-dimensional systems plays a particularly important role in biophysical systems within which magnetic moments of different sizes might be useful for different purposes. In this perspective, the role of magnetism with higher magnetic moments is relatively less understood.
To gain a better understanding for magnetism encompassing low and high moments, we studied a quantum mechanical spin lattice system consisting of one-dimensional anti-ferromagnetic Heisenberg chain of spin s embedded in a three dimensional lattice.

Buckling Instabilities and Recoil Dynamics in Free-Standing Liquid Crystal Filaments – Tanya Ostapenko Mon. May 18th, 2015
12:30 pm-1:30 pm

Quasi-one-dimensional free-standing fluid structures are not often found in nature, but may be formed by any material that can overcome capillary instability. Once this instability is suppressed, long filaments, with a length-to-diameter ratio greater than �, may form. Liquid crystals are an extraordinary system that can form free-standing fluid filaments with length-to-diameter ratios exceeding 7000. Buckling instabilities in freestanding liquid crystal filaments formed from bent-core liquid crystals in the B7 phase may be induced in a variety of ways, e.g. by acoustical or electrical vibration. However, this talk will focus on instabilities induced by compressing the filament, as well as those from a mechanical or thermal rupture.

Quantum Phase Transitions in Magnets – Ribhu Kaul Mon. May 11th, 2015
12:30 pm-1:30 pm
Thank You for Flying the ‘Vomit Comet’: Using Parabolic Flights to Examine Quantitatively the Stability of Liquid Bridges Under Varying Total Body Force – Greg DiLisi Mon. April 20th, 2015
12:30 pm-1:30 pm

Liquid bridges were flown aboard a Boeing 727-200 aircraft in a series of parabolic arcs that produced multiple periods of microgravity. During the microgravity portion of each arc, g_eff , the effective total body acceleration due to external forces became negligibly small so that cylindrical liquid bridges could be suspended across two coaxial support posts. Near the bottom of each arc, g_eff slowly increased to a maximum of 1.84g, causing the liquid bridges to deform and in some cases collapse. Although the physics of liquid bridges subject to varying total body force is well-established and has been analyzed extensively both theoretically and experimentally,

V2O5, a Strongly Correlated 2D System with 1D Aspects – Walter Lambrecht Mon. March 30th, 2015
12:30 pm-1:30 pm

V2O5 is a layered material with chains within the layer. I will discuss how this is manifested in its electronic band structure. The quasiparticle self-consistent GW method in this material strongly overestimates the band gap. The main reasons for this are examined and found to be a lattice polarization contribution to the screening of the electron-electron interaction. This is related to the large LO/TO phonon splittings in this material. Changes in band structure and phonons between bulk and monolayer will be discussed.

Predictive First-principles Simulations of Excited Electrons and Ultrafast Electron-ion Dynamics in Complex Materials – Andre Schleife Mon. March 23rd, 2015
12:30 pm-1:30 pm

Rapidly advancing high-performance super computers such as “Blue Waters” allow calculating properties of increasingly complex materials with unprecedented accuracy. In order to fully take advantage of leadership-class machines and to accurately describe modern materials, codes need to scale well on hundreds of thousands of processors. This talk focuses on electronic excitations and their ultrafast attosecond dynamics that are notoriously difficult to capture due to the quantum-mechanical electron-electron interaction. Being omnipresent in electronic and optical materials, an accurate description is a crucial factor for computational design of materials for technological applications.

It will be outlined how cutting-edge first-principles techniques based on many-body perturbation theory accomplish predictive theoretical spectroscopy of electronic excitations e.g.

Graphene on Ir(111), Adsorption and Intercalation of Cs and Eu Atoms – Pedrag Lazic Mon. March 16th, 2015
12:30 pm-1:30 pm

Experimental and theoretical study of Cs and Eu atoms adsorption on graphene on Ir(111) will be presented [1,2]. Graphene on Ir(111) surface is an interesting system because graphene has almost pristine electronic structure in it due to its weak bonding character to iridum surface. The bonding is almost exclusively of the van der Waals type. However adding Cs or Eu atoms graphene gets doped and and nature of binding changes – especially in the case when the atoms intercalate. Density Functional Theory calculations with standard semilocal functionals (GGA) – fail to reproduce experimental findings even qualitatively. Only when the newly developed nonlocal correlation functional is used (vdW-DF) which includes van der Waals interactions,

March Meeting Preview Talks – Graduate Students Mon. February 23rd, 2015
12:30 pm-1:45 pm

APS March Meeting 2015 graduate student talks

Jiayuan Miao: Molecular-dynamics study of the Case-II diffusion of methanol in PMMA

Sukrit Sucharitakul: Field effect vs. Hall mobility in back-gated multilayered InSe FETs

Nicholas J. Goble: Effects of structural phase transitions on the interface of perovskite oxides

Bin Liu: Ultrastrong exciton-photon coupling in single and coupled organic microcavities

Ittipon Fongkaew: Electric field and spin-orbit coupling effects on the band structure of monolayer WSe2

Exploring Soft Matter with DNA – Tomasso Bellini Mon. February 16th, 2015
12:30 pm-1:30 pm

The combination of solubility, coded pairing and adjustable flexibility make DNA a unique polymer for designing highly-controlled self-assembled complex nanostructures and novel materials. The same tools can be exploited to produce DNA-based systems enabling the exploration of challenging topics in soft matter physics. In the talk I will exemplify this approach by describing experiments and results in which DNA assembly was used to study living polymerization, liquid crystal ordering, the templating of chemical reactions, and phase behavior and gelation transition of low-valence colloidal particles.

The Chirality of SiO4 in Materials – David Avnir Wed. February 11th, 2015
12:30 pm-1:30 pm

SiO4 is a common building block of many materials, both crystalline such as quartz, silicates and zeolites, and amorphous, such as silica. Although intuitively one would think that SiO4 is an achiral perfect tetrahedron, in the vast majority of silicon-oxide based materials, that tetrahedron is of lower symmetry, to the degree of being chiral. Discussion of the chirality of SiO4 and its manifestation in crystalline and amorphous materials, will be the main focus of this lecture. Specific topics to be covered include the induction of chirality in silicas; the contribution of randomness to the emergence of chirality; the chirality of zeolites and silicates;

Spin-dependent Scattering in Graphene: Electronic Birefringence and Kondo Transitions – Sergio Ulloa Mon. February 2nd, 2015
12:30 pm-1:30 pm

Graphene, a monoatomic layer of carbon, is perhaps the simplest and most easily available material where electrons behave as massless Dirac particles. Apart from the many promising technological applications, the study of graphene (and other layered materials) has opened a number of interesting theoretical questions: the microscopic crystalline structure requires an additional degree of freedom (the pseudo spin) that gives rise to effects such as the Klein paradox or Veselago electron lenses. The spin-orbit interaction (SOI) in materials arises from intrinsic lack of inversion symmetry in the lattice structure or from external or interfacial fields that break spatial symmetries. Although SOI is weak in natural graphene,

The Break-up of Viscoelastic Jets and Filaments: The Beads-on-a-string Structure – Marie-Charlotte Renoult Mon. December 1st, 2014
12:30 pm-1:30 pm

Capillary pressure can destabilize a thin stream of water and break it up into a succession of small droplets. The addition of a minute quantity (some part per million) of a long, flexible and water-soluble polymer is enough to modify the growth and morphology of this instability and leads, close to breakup, to the development of Beads-on-a-string structures (BOAS) where droplets are connected by thin threads. The BOAS phenomenon is also observed after stretching a bridge of a viscoelastic liquid.

Experiments on jets and stretched bridges of viscoelastic polymeric solutions were conducted to gain more insight into the formation and time evolution of the BOAS in both configurations.

Soft Materials Approaches to Carbon Nanotubes: from Gels to Composites – Mohammed F. Islam Mon. November 10th, 2014
12:30 pm-1:30 pm

Carbon nanotubes combine low density with exceptional mechanical, electrical and optical properties. Unfortunately, these nanoscale properties have not been retained in bulk structures. I will describe surface modification assisted self‐assembly of single wall carbon nanotube into macroscopic nanotube networks  ‐  hydrogels and aerogels. The nanotube networks are ultra‐ lightweight, electrically conducting and thermally insulating. The shapes and sizes of these nanotube networks are readily tunable and is a tremendous strength of our fabrication method. The interesting properties and structure of these nanotube networks make them suitable for diverse applications. For example, we have used these networks as scaffolds to enhance elastic modulus of polymers by 40,000%.

Soft Magnetic Materials for Energy Applications in Extreme Environments – Matthew A. Willard Mon. October 27th, 2014
12:30 pm-1:30 pm

A fundamental transformation of the transportation sector in the United States is underway. In parallel with advances in renewable energy resources for power generation, the rising use of electric and hybrid vehicles is reshaping the future of public transportation. Similar efforts are moving forward for more-electric ships, aircraft, and other military technologies. Due to their prevalence, magnetic materials play an important role in improving the efficiency and performance of devices in electric power generation, conditioning, and conversion. However, significant challenges exist for magnetic materials especially when used for transportation technologies, where enhanced reliability, power density, and overall energy capacity are increasingly important.

On Demand 2D Electron Gas at LaAlO3/SrTiO3 Interfaces – Cheng Cen Mon. October 20th, 2014
12:30 pm-1:30 pm

The development of complex oxides over the past fifteen years has raised the prospect for new classes of electronic devices. In particular, it has been discovered that a high-mobility two-dimensional electron gas (2DEG) can be formed at the interface between two high-k insulators: LaAlO3 and SrTiO3. More interestingly, in samples with 3-unit-cell LaAlO3 (LAO) film grown on SrTiO3 (STO) substrate, a biased conducting atomic force microscope probe can locally and reversibly control the interfacial metal-insulator transition. This method is capable of patterning arbitrary conducting structures at LAO/STO interfaces with a spatial resolution of only a few nanometers. Based on such technique,

“How Big is the Proton Anyway?” – Amar Vutha Mon. October 13th, 2014
12:30 pm-1:30 pm
The proton is a bound state of quarks and gluons, described by the low-energy limit of quantum chromodynamics. Recent measurements using muonic hydrogen have, however, called our understanding of proton physics into question. In this first lecture, I will describe the significant discrepancy that exists between the recent muonic hydrogen measurements and previous measurements on protons — this is the // proton radius puzzle //. In the absence of any feasible theoretical solutions, new experiments might provide the best clues. I shall describe some of the experiments that are attempting to shed light on this puzzle, including our ongoing efforts to measure the proton radius via the Lamb shift in hydrogen atoms.
Spin-charge Conversion in Interfacial Electron Liquids – Giovanni Vignale Mon. October 6th, 2014
12:30 pm-1:30 pm

Semiconductor quantum wells, inter-metallic interfaces, layered oxides, and monolayer materials are all promising platforms for the observation of spincharge conversion due to strong spin-orbit interaction in the quasi two dimensional electron liquid they host. In this talk I focus on two closely related effects that can occur in these materials, namely the conversion of charge current to spin current (spin Hall effect) and the generation of spin polarization from an electric current (Edelstein effect). Together with their inverses (in the sense of Onsager reciprocity relations), these effects constitute a useful set of tools for spintronic applications. The theoretical challenge is to provide a unified treatment of the different mechanisms at work,

Ordered self-assembly of molecules on gold substrates, for activated organic monolayers – Prof. Emmanuelle Lacaze Wed. July 17th, 2013
12:30 pm-1:30 pm

Photochromic molecules are characterized by a functional group whose configuration is modified by absorption of light, in a reversible manner. They could be at the basis of new electronic displays which would be activated by light irradiation. For the formation of ultra-thin electronic displays, researchers now try to adsorb them on metallic substrates. Two main questions are thus asked : Firstly, is it possible to self-assemble this type of molecule on a substrate. Second, when self-assembled in monolayer, do the molecules remain active under irradiation, and most importantly, can they be locally switched under STM tip Ordered self-assemblies have been successfully obtained with azo-benzene based photochromes,

Topological transition of graphene from quantum Hall metal to quantum Hall insulator – Prof. XiangRong Wang Fri. May 17th, 2013
12:30 pm-1:30 pm

In this talk, I will first review the basic electronic properties of graphene. In particular, I will explain why the recently observed insulating phase of graphene at charge neutrality point in high magnetic field quantum Hall (QH) experiments is a big surprising. Then I will present a simple single-particle theory for this intriguing finding. We show that the magnetic field driven Peierls-type lattice distortion (due to the Landau level degeneracy) and random bond fluctuations compete with each other, resulting in a transition from a QH-metal state at relative low field to a QH-insulator state at high enough field at the charge neutrality point.

Quantum Fluids of Light – David Snoke Mon. April 29th, 2013
12:30 pm-1:30 pm

In the past few years a new class of solid state optical systems has been developed in which photons have an effective mass and a repulsive interaction between each other. These renormalized photons are known as “polaritons”. One way of looking at this type of system is as an optical medium with world-record nonlinearity, leading to new possibilities for modulating light. Another way of looking at this type of system is as an analogue of a gas of atoms, which can undergo Bose-Einstein condensation and can become superfluid, allowing us to study superfluidity in a new way. I will review the basic experimental methods and recent results of polariton superfluids.

Semiconductor nanowires : from LEDs to solar cells – Silvija Gradecak Mon. April 22nd, 2013
12:30 pm-1:30 pm

Semiconductor nanowires are quasi-one-dimensional single-crystals that have emerged as promising materials for the development of photonic and electronic devices with enhanced performance. Nanowires offer solutions to some of the current challenges in science and engineering, but realization of their full potential will be ultimately dictated by the ability to control their structure, composition, and size with high accuracy. In this talk, I will discuss our recent results on the controlled growth, doping, and applications of III-V nanowires, as well as advanced electron microscopy techniques for direct correlation of structural and physical properties with high spatial resolution. We have developed a simple,

Mapping spin-orbit effects in semiconductors – Vanessa Sih Mon. April 15th, 2013
12:30 pm-1:30 pm
Spin-orbit coupling is a consequence of relativity but can be observed and used at the device scale to electrically initialize and manipulate electron spin polarization. Understanding how to exploit spin-orbit effects in non-magnetic semiconductors may enable the development of new devices with enhanced functionality and performance, such as spin-based devices that combine logic and storage and fast optical switches for information processing. In this talk, I will describe time- and spatially-resolved measurements of electron spin transport that enable sensitive measurements of the spin-orbit field and its dependence on applied electric fields and mechanical strain. These spin splittings also provide a mechanism for the electrical generation of spin polarization.
Short-range order in nematic liquid crystals formed by reduced symmetry molecules – Sam Sprunt Mon. April 8th, 2013
12:30 pm-1:30 pm

Small molecules constructed from familiar chemical components, but with an unconventional (reduced symmetry) molecular shape, hold promise for developing nematic liquid crystals with macroscopic biaxiality or even polarity. These properties, realized over practical temperature ranges using thermotropic compounds, could open new avenues in technologies including optical displays, mechanical sensors, and low-cost personal power generation. I will report on recent studies of short-range order – a guidance, if not a direct precursor, to rational development of biaxial/polar nematics – in three types of reduced symmetry thermotropic materials: bent-core (V-shaped) liquid crystal compounds and rod-like molecules containing either lateral branches (Y-shaped) or bridges (H-shaped).

Hybrid Quantum Devices with Single Spins in Diamond – Gurudev Dutt Mon. April 1st, 2013
12:30 pm-1:30 pm

Single spins associated with defects in diamond have emerged as a promising and versatile experimental system. They can be used as qubits in optically connected quantum networks, as sensors for magnetic imaging with sub-micron resolution, as readout heads for detecting and engineering quantum states of nano-mechanical oscillators, and even as probes in biological systems. I will discuss some of the key experimental progress and future prospects along these paths.

Point defect studies in ZnO: oxygen vacancy and p-type doping – Walter Lambrecht Mon. March 25th, 2013
12:30 pm-1:30 pm

In the first part of the talk, I will tell you about the controversy about the position of the defect levels for the oxygen vacancy in ZnO and how we tried to resolve it. In the second part, I will discuss the case of nitrogen in ZnO. I will discuss why nitrogen on an oxygen site forms a deep rather than shallow acceptor level. However, it is known that there exists a shallow level related to nitrogen doping. The question is then what defect complex is responsible for this shallow level? I will try to convince you that a N2 molecule located on a Zn-site has all the expected behavior of a shallow acceptor.

Semiconductor nanocrystals for room-temperature coherent electronics: A flexible platform for manipulating spin coherence – Jesse Berezovsky Mon. March 4th, 2013
12:30 pm-1:30 pm

One route towards future electronics is to exploit interactions between coherent electron spin states and photons in semiconductor structures. This will require an understanding of the coherent evolution of spin states, the eventual decoherence of these states, and how these states interact with light, all in a scalable room-temperature system. In this seminar, I will present our work on spins in semiconductor nanocrystal quantum dots (NCQDs). This system provides a platform to study room-temperature coherent spin states and their interactions with light. In an NCQD, a spin optically initialized into a superposition of eigenstates remains coherent for approximately one nanosecond at room temperature.

Shedding some light on liquid crystalline organic semiconductors – Brett Ellman Mon. February 25th, 2013
12:30 pm-1:30 pm

We live in a world whose technology is ruled by a small set of inorganic semiconductors, notably silicon. Research on organic semiconductors (OSCs), molecular materials based on organic compounds, seeks to supplement the reigning paradigm rather than to supplant it. In particular, OSCs may improve photovoltaics, LEDs, sensors, and flexible, cheap electronics. In this talk, I will describe recent work at Kent State on liquid crystalline (LC) semiconductors, a subclass of OSCs that offer distinct advantages and disadvantages relative to more common crystalline or polymeric organics. After a (very) short introduction to the physics of OSCs and LCs, I will discuss how the ability to align the molecules in LC OSCs over macroscopic distances can (profoundly) improve transport characteristics.

Nanostructures in Motion: Probing Surface Science and Fracture Mechanics at Molecular Level – Zenghui Wang Mon. February 18th, 2013
12:30 pm-1:30 pm

Nanomaterials, since their debut, have greatly advanced human knowledge from many aspects. For example, carbon-based nanomaterials, such as carbon nanotube and graphene, have been the subjects of intensive study over the last two decades and greatly improved our understanding of phenomena happening at the nanoscale. On the other hand, microelectromechanical systems, MEMS, research has thrived over the last few decades in the engineering field and brought along many new applications. In this talk, I will illustrate that, by combining nanomaterials with MEMS technology, even more new opportunities and new sciences can be unveiled. I will mostly focus two systems: 1.

Self-Assembly and Packing of Polyhedra into Complex Structures – Michael Engel Mon. February 11th, 2013
12:30 pm-1:30 pm

Isolating the role of building block shape for self-assembly and packing provides insight into the ordering of molecules and the crystallization of colloids, nanoparticles, proteins, and viruses. We investigated a large group of polyhedra whose phase behavior arises solely from their anisotropic shape. At intermediate packing density, our results demonstrate a remarkably high propensity for thermodynamic self-assembly and structural diversity. We show that from simple measures of particle shape and local order in the fluid, the assembly of a given shape into a liquid crystal, plastic crystal, or crystal can be predicted. Towards higher density, packing considerations dominate. Good packings can often be distinct from what is observed to assemble from the disordered state.

Routing Light with Spatial Solitons: Light Localization and Steering in Liquid Crystals – Antonio DeLuca Mon. February 4th, 2013
12:30 pm-1:30 pm

Nematic Liquid Crystals (NLCs) support strong nonlinear effects, most of them due to the high birefringence and non-local response. Light self-confinement via reorientational nonlinearity and nonlocality, yields to the creation of robust light filaments named ‘optical spatial solitons’, which can trap, switch and route optical signals. In the last ten years, the attention to NLC systems, due to their large and polarization dependent nonlinearity, allowed the observation of self-focusing and spatial solitons with a significant attention devoted to reduce thermal contributions to the nonlinear phenomena and lower the required optical power. In all the observed cases, self-confinement was observed over short distances (hundreds of micrometers) and with non-negligible thermo-optic effects.

Terahertz plasmons and magnetoplasmons in graphene – Hugen Yan Mon. December 3rd, 2012
12:30 pm-1:30 pm

Plasmons in metal surfaces and clusters have been extensively studied due to their potential applications in sensing, imaging, light harvesting and optical metamaterials. Graphene is a semimetal with tunable conductivity and hence can support plasmons as well. In addition to the tunability, graphene plasmons have relatively weak damping due to the high carrier mobility. In this talk, I will present our recent progress on the plasmon excitations in graphene micro-structures and their behavior in an external high magnetic field. We demonstrated graphene plasmonic terahertz filters and polarizers with graphene/insulator stacks and revealed the unique properties of Dirac plasmons with and without a magnetic field.

Quantum Dots and Magnetic Quantum Dots for Biomedical Imaging and Separations – Jessica Winter Mon. November 26th, 2012
12:30 pm-1:30 pm

Quantum dots, semiconductor nanocrystals, have unique optical properties, including narrow emission bandwidths, broad excitation spectra, and remarkable photostability, which have made them excellent candidates for biological imaging. Since their introduction into the biological milieu in 1998, they have been applied for in vitro and in vivo imaging, diagnostic testing, and multiplexing. As researchers have appreciated the benefits of quantum dots for imaging, emphasis has shifted to fabricating nanocomposites containing quantum dots, and among these magnetic quantum dots have attracted significant attention. Here, we describe our efforts to fabricate quantum dots and magnetic quantum dots. Highlighting our most recent efforts in this area,

Quench dynamics in one-dimensional systems – Aditi Mitra Mon. November 19th, 2012
12:30 pm-1:30 pm

How an interacting many-particle system which is initially out of equilibrium evolves in time, is a challenging question, especially for large system sizes where numerical simulations are difficult. The most puzzling issue is understanding the onset of thermalization, a process in which the system completely looses memory of its initial state, with the long time behavior characterized by only one or two parameters. Understanding this issue is important as ideal, thermally isolated systems, and their time-evolution can now be routinely studied in experiments. Using a novel time-dependent renormalization group approach I will show how a reduced part of a strongly interacting system can look effectively classical (or thermal) by being characterized by a dissipation and a noise,

Playing with monomolecular layers: model biological systems and liquid crystal alignment layers – Elizabeth Mann Mon. November 12th, 2012
12:30 pm-1:30 pm

Self-assembly within biological membranes controls structure, from the nano- to the microscale. The same physical processes also apply to synthetic systems. Here, I survey two different model systems for structure and dynamics within molecularly thin films.

Half Metallic Ferromagnetism in Complex Oxides and Implications for Spintronics – Nandini Trivedi Mon. November 5th, 2012
12:30 pm-1:30 pm

I will discuss the mechanism behind the remarkable properties of double perovskites like Sr2FeMoO6 that show half-metallic ground states with 100% polarization and a ferromagnetic Tc above room temperature. I will conclude with a broad overview of other remarkable properties that can be achieved by changing the transition metal atoms. Reference: O. Erten, et. al Phys. Rev. Lett. 107, 257201 (2011).

FUNCTIONAL FILMS AND CERAMICS – Alp Sehirlioglu Mon. October 29th, 2012
12:30 pm-1:30 pm

The presentation summarizes our recent efforts in developing new functional materials with a focus on operation in extreme environments. Discussion will include both fundamental aspects of behavior and the path to next generation of devices and applications. Two main topics will be discussed: (i) Oxide based heterointerfaces: Formation of a two dimensional conducting interface between two perovskite insulators (i.e., LaAlO3 on SrTiO3) was first reported in 2004. In 2006 it was reported for the first time that the conductivity of the hetero-interface could be switched between two states by application of an external field (analogous to gate voltage). This technologically significant but still infant discovery holds great potential for next-generation extreme environment electronics that can have both (i) higher information density and (ii) larger operation domain.

Qubit-Coupled Mechanics – Matt LaHaye Mon. October 22nd, 2012
12:30 pm-1:30 pm

There is a rapidly growing effort to integrate quantum technologies with mechanical structures in order to manipulate and measure quantum states of mechanics for applications ranging from quantum computing to sensing of weak forces to fundamental explorations of quantum mechanics at massive scales. A central focus of this effort, informally dubbed quantum electromechanical systems, has been the integration of superconducting electronics as control and measurement elements in nano and microelectromechanical systems (NEMS and MEMS). In fact, in just the last few years, spectacular advancements have been made in this area, providing researchers with a suite of tools for preparing, manipulating and measuring NEMS and MEMS near and even in the quantum domain.

Michelson Postdoc Prize talk 3:Many-body interactions in two-dimensional crystals – KinFai Mak Fri. October 19th, 2012
12:30 pm-1:30 pm

The problem of electrons in 2D is one of the most important topics in contemporary condensed matter physics. Coulomb interactions between charge carriers in 2D are dramatically enhanced with the much-reduced dielectric screening compared to their bulk counterpart. Recent advances in the development of atomically thin layers of materials have opened up new opportunities for the study of many-body effects in 2D. In the last talk, we will discuss the observations of strong excitonic effects in graphene and in a valley Hall semiconductor through optical spectroscopy. We will demonstrate the control of Coulomb interactions in such atomic membranes by tuning their dielectric screening through an electrostatic gate.

Michelson Postdoc Prize talk 2:Optics with Dirac electrons – KinFai Mak Tue. October 16th, 2012
12:30 pm-1:30 pm

Optical spectroscopy provides an excellent means of understanding the distinctive properties of electrons in the two-dimensional system of graphene. Within the simplest picture, one has a zero-gap semiconductor with direct transitions between the well-known conical bands. This picture gives rise to a predicted frequency-independent absorption of \pi\alpha = 2.3%, where \alpha is the fine-structure constant. We will demonstrate that this relation is indeed satisfied in an appropriate spectral range in the near infrared, but that at higher photon energies electron-hole interactions significantly modify this result through the formation of saddle-point excitons. Optical spectroscopy also permits a detailed analysis of how the linear bands of graphene,

Michelson Postdoc Prize talk 1:Novel two-dimensional systems: graphene and beyond – KinFai Mak Mon. October 15th, 2012
12:30 pm-1:30 pm

The past few years have witnessed a surge of activities in the study of graphene, a stable sheet comprised of just a single atomic layer of carbon atoms in a honeycomb lattice structure. Indeed, 2010 Nobel Physics Prize recognized two researchers for their pioneering contributions to this field. In this talk we will describe the development of the field and some of the reasons for the intense interest in this new material system, highlighting its unusual electronic dispersion and its distinctive mechanical and chemical properties. We will also discuss recent advances in the fabrication and investigation of other 2D atomic membranes.

Multiferroic vortices in hexagonal manganites – Weida Wu Mon. October 8th, 2012
12:30 pm-1:30 pm

Topological defects are pervasive in complex matter such as superfluids, liquid crystals, and early universe. They have been fruitful playgrounds for many emergent phenomena. Recently, vortex-like topological defects with six interlocked structural antiphase and ferroelectric domains merging into a vortex core were revealed in multiferroic hexagonal manganites. Numerous vortices are found to form an intriguing self-organized network, and may be used to test Kibble-Zurek model of early universe. Furthermore, emergent conduction and piezoelectric properties were observed in charged ferroelectric domain walls protected by topological defects. More excitingly, unprecedented alternating uncompensated magnetic moments were discovered at coupled antiferromagnetic-ferroelectric domain walls in hexagonal manganites,

Into the flat land: Transport studies of ultra-dilute GaAs two-dimensional hole systems in zero field – Jian Huang Mon. October 1st, 2012
12:30 pm-1:30 pm

Low temperature charge transport studies of high purity electron systems encompass fundamental subjects of disorder and electron-electron interaction. 50 years after Anderson’s theory of localization for non-interacting electrons, the question on whether and how electron-electron interaction qualitatively alters the picture is still unsettled. Fascinating subjects on interaction-driven phenomena such as Wigner crystallization of electrons (for the quantum scenario) have never been demonstrated. Experimentally, high-purity semiconductor bulk materials offer a desirable tunability of charge density down to ultra-dilute limits where both new frontiers of physics and important applications such as quantum information technologies can be explored. However, such a transition is often overshadowed by the substantial disorder which competes with or even dominates over interaction by rendering the system into an Anderson insulator.

Valley-Electronics in 2D Crystals – Di Xiao Mon. September 24th, 2012
12:30 pm-1:30 pm

In many crystals the Bloch bands have inequivalent and well separated energy extrema in the momentum space, known as valleys. The valley index constitutes a well-defined discrete degree of freedom for low-energy carriers that may be used to encode information. This has led to the concept of valleytronics, a new type of electronics based on manipulating the valley index of carriers. In the first part of the talk, I will describe a general scheme based on inversion symmetry breaking to control the valley index, using graphene and monolayers of MoS2 as an example. In particularly, the valley Hall effect and valley-dependent optical selection will be discussed.

Novel Ferroelectric Polymers as High Energy Density and Low Loss Dielectrics – Lei Zhu Mon. September 17th, 2012
12:30 pm-1:30 pm

The state-of-the-art polymer dielectrics have been limited to nonpolar polymers with relatively low energy density and ultra low dielectric losses for the past decades. With the fast development of power electronics in pulsed power and power conditioning applications, there is a need for next generation dielectric capacitors in areas of high energy density/low loss and/or high temperature/low loss polymer dielectrics. Given limitations in further enhancing atomic and electronic polarizations for polymers, this perspective article focuses on a fundamental question: Can orientational polarization in polar polymers be utilized for high energy density and low loss dielectrics? Existing experimental and theoretical results have suggested the following perspectives.

Interfacial Charge Transfer in Nanomaterial Based Light Harvesting Devices – Mat Sfire Mon. September 10th, 2012
12:30 pm-1:30 pm

We purposefully design and study “molecular-like” interfacial interactions between the multidimensional nanometer-scale building blocks that compose larger-scale functional light harvesting devices. Using time-resolved optical spectroscopy, we aim to understand the nature of discrete interfacial electronic states and their role as crucial intermediates promoting efficient interactions between extended systems (e.g., charge transfer). Our research has suggested the importance of such intermediate interfacial states in both hard and soft nanomaterial heterostructures, including semiconductor quantum dots and organic semiconductors. We aim to understand the fundamental impact of “molecular-like” interfacial states on macroscopic material properties, such as charge transport and light harvesting. For example,

Optical Material Science: Electrodynamics of Nanoscale Assembly, and Lifetime and Degradation Science for Photovoltaics – Roger H. French Mon. April 30th, 2012
12:30 pm-1:30 pm

The optical properties and electronic structure of materials are critical to the development of new optical materials,(1) novel processes of nanoscale assembly, and the viability of advanced energy technologies. They are the origin of the electrodynamic van der Waals-London dispersion (vdW-Ld) interactions (2) which play a universal role in wetting, interfacial energies, and nanoscale assembly.(3) The challenge of nanotechnology is for science to span more than nine orders of magnitude in dimension. Advanced energy technologies, with their 25 or 50 year capital lifetimes, challenge us to span 24 orders of magnitude in time so as to control degradation processes, damage accumulation,

Magnetoresistance in Two Dimensions – Arnold J. Dahm Mon. April 23rd, 2012
12:30 pm-1:30 pm

We present measurements of the magnetoresistivity of a weakly interacting 2D electron liquid in an unexplored region near the boundary of the 2D electron gas supported by a liquid helium surface. The magnetoresistivity is calculated by Dykman in the self-consistent Born approximation. For fields greater than a field B0, the magnetoresistivity is proportional to (muB)^3/2, where mu is the mobility. Electron-electron interactions cause a crossover to the Drude behavior as the density is increased. All of our data scale with the density-dependent parameter B0 as B/B0, with the magnitude of the magnetoresistivity scaled as 1/n. For low electron densities and fields less that B0,

Electronic structure of disordered solids – David A. Drabold Mon. April 16th, 2012
12:30 pm-1:30 pm

Understanding the physics of structurally disordered materials is a challenge to experimentalists and theorists alike. In this talk, I discuss the character of electronic states in disordered materials and emphasize the interplay between structure and electronic properties. I begin by discussing the consequences of atomic structural disorder on electron states. As shown long ago by Anderson, disorder in atomic coordinates creates spatially confined or “localized” electron eigenstates near the Fermi level. I explore these states with large and realistic structural models and suitable electronic structure techniques. I begin with the structure of electron states in large and realistic models of a-Si,

The role of molecular beam epitaxy in fundamental physics through an example: assessing the impact of disorder on the v=5/2 fractional quantum Hall effect – Mike Manfra Fri. March 30th, 2012
11:30 am-12:30 pm

Thirty years after its initial discovery, the fractional quantum Hall effect continues to challenge our understanding of electronic correlations in low dimensions. Throughout this history advances in molecular beam epitaxy (MBE) have played an important role. Presently, the fragile v=5/2 fractional quantum Hall state is the subject of intense scrutiny. It is theoretically conjectured that the v=5/2 state is described by the Moore-Read Pfaffian wavefunction, possessing excitations obeying non-Abelian braiding statistics. If experimentally confirmed, excitations with non-Abelian braiding statistics may provide a platform for proposed schemes of topologically-protected quantum computing. While there are many aspects to the physics at v=5/2,

Micro and Nano Technology at the Lurie Nanofabrication Facility – Robert Hower Fri. March 23rd, 2012
12:30 pm-1:30 pm

This seminar will give an overview of micro and nano technologies at the University of Michigan Lurie Nanofabrication Facility (LNF). In addition, we will present examples of research accomplishments and applications of these technologies in diverse fields including but not limited to Electrical Engineering, Physics, Life Sciences, Biomedical Engineering and Chemical Engineering.
Operated by the University of Michigan Solid-State Electronics Laboratory (SSEL), the LNF has extensive experience in microelectronics, micromechanics, optoelectronics, and micro and nano technologies based on silicon, compound semiconductor, and organic materials. It offers a complete laboratory for the fabrication of nanofabricated semiconductor and polymer electronic and optoelectronic devices and circuits,

Anisotropic response in molecular crystals and the development of Modulated Orientation Sensitive Terahertz Spectroscopy (MOSTS) – Andrea Markelz Mon. March 19th, 2012
12:30 pm-1:30 pm

Since the mid 1980’s there have been predictions of protein structural vibrations with ~ 1meV energies, which corresponds to the terahertz frequency range. These large scale motions involve the correlated movement of many atoms and are associated with the conformational motions involved in protein function. There have been many attempts to measure these modes, but the energy range overlaps with that of local librational motions of the surface side chains and the solvent, and these contributions give rise to a strong glass-like response. In this talk I will discuss our development of a technique to isolate the large scale structural contribution from a glassy background called Modulated Orientation Sensitive Terahertz Spectroscopy (MOSTS).

III-Nitride Light-Emitting Diodes for Solid-State Lighting – Hongping Zhao Mon. March 12th, 2012
12:30 pm-1:30 pm

Energy efficiency and renewable energy technologies have significant importance for achieving sustainable energy systems in modern society. Lighting accounts for more than 22% of the total electrical energy usage in US, and technologies based on solid state lighting (SSL) utilizing semiconductor-based material has tremendous promise to replace the existing lighting devices. As compared to traditional incandescent and fluorescent lamps, SSL is more energy-efficient, reliable, and environmentally-friendly. Once widely used, SSL could lead to the decrease of worldwide electricity consumption for lighting by >50% and reduces total electricity consumption by >10%. The U.S. Department of Energy describes SSL as a pivotal emerging technology that promises to fundamentally alter lighting in the future.

HgTe as a Topological Insulator – Laurens Molenkamp Mon. March 5th, 2012
12:30 pm-1:30 pm

HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a topological insulator. The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating.
Our transport data provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2-dimensional topological insulator.

High Tc superconductivity in cuprates: A status report – Mohit Randeria Fri. February 17th, 2012
12:30 pm-1:30 pm

25 years after their discovery, the microscopic problem of high Tc superconductivity in cuprates is still not “solved”. I will focus on summarizing the experiments that show us that the observed phases, with varying carrier concentration, challenge three paradigms of 20th century condensed matter physics. (i) The parent Mott insulator cannot be understood within band theory; (ii) the superconducting state and phase transition force us to go beyond a BCS mean-field description; and (iii) the “normal” metallic state cannot be described within Landau Fermi liquid theory. I will then briefly describe some of the success in theoretically understanding the superconducting state and indicate open questions about the normal state.

Quantum Signatures of Optomechanical Instability and Synchronization in Optomechanical Arrays – Jiang Qian Mon. February 13th, 2012
12:30 pm-1:30 pm

Optomechanical systems couple light stored in an optical resonant cavity to the motion of a mechanical motion of the cavity walls. Single optomechanical cells have been successfully fabricated in a wide variety of systems. Recent experiments have further demonstrated setups, such as photonic crystal structures, that in principle allow to confine several optical and vibrational modes on a single chip.
In the first part of my presentation I will demonstrate the emergence of a robust, long-living and highly non-classical mechanical state in a standard single cell optomechanical setup. I will show that under some parameters, the longtime steady state of the mechanical degrees of freedom has significantly negative Wigner density.

Fe pnictide superconductors – David Singh Mon. February 6th, 2012
12:30 pm-1:30 pm

The 2008 discovery of high temperature superconductivity in doped LaFeAsO by Kamihara and co-workers provided the second class of high Tc materials, the other being the cuprate family discovered in 1986 by Bednorz and Mueller. This discovery was revolutionary in that many of the properties of the iron based superconductors are radically different from those of the cuprates, apparently requiring a new and broader understanding of the physics of high temperature superconductivity. The purpose of this talk is to discuss the chemistry and physics of the new superconductors in relation to cuprates. So far, many puzzles remain. The materials appear to be much more band-like and show much stronger signatures of metallic (Fermi surface related) physics than cuprates,

The Incredible Shrinking Tuning Forks – Nanowire Electromechanical Systems at Radio and Microwave Frequencies – Philp Feng Mon. January 30th, 2012
12:30 pm-1:30 pm

Nanoscience today enables many fascinating low-dimensional structures and new materials with previously inaccessible properties. Nanostructures with mechanical degrees of freedom offer compelling characteristics that make them interesting for both fundamental studies and technological applications. This talk will describe my collaborative research efforts in exploring vibrating nanowires, and in engineering these very thin nanowires into functional and high-performance nanoscale electromechanical systems (NEMS). I will show NEMS resonators operating in the very-high and ultra-high frequency (VHF/UHF, 30MHz – 3GHz) ranges, based on silicon nanowires enabled by a hybrid bottom-up/top-down process. Exploiting the interesting properties of thin silicon nanowires, we have developed an all-electronic,

Graphene Optics and Electronics – Marcus Freitag Mon. December 5th, 2011
12:30 pm-1:30 pm

Graphene is a two-dimensional material with conical bands that touch at the Dirac or Charge-Neutrality point. Its zero bandgap and atomically thin body allow it to switch between n-type and p-type conduction when assembled into a field-effect transistor geometry. The current modulation however is limited due to a finite minimum conductivity at the Charge Neutrality point, which prevents us from using graphene for digital electronic applications.
We therefore investigate graphene as an optical and analog electronic material, where the low on-off ratio is less of a problem. Especially the high frequency (RF) electronic applications are promising since graphene can be gated efficiently and has high carrier mobility.

Charge carrier dynamics in heterostructured semiconductor nanocrystals and nanocrystal solids – Michail Zamkov Mon. November 21st, 2011
12:30 pm-1:30 pm

In the first part, I will present a novel strategy for processing of colloidally stable semiconductor nanoparticles (also known as nanocrystals or quantum dots) into all-inorganic solid films, deployable for photovoltaic applications. The method relies on encapsulation of semiconductor nanocrystal arrays within a matrix of a wide-band gap inorganic material, which preserves optoelectronic properties of individual nanoparticles, yet, renders the nanocrystal film photoconductive. The photovoltaic performance of fabricated nanocrystal solids is demonstrated through the development of prototype solar cells exhibiting stable and efficient operation in ambient conditions.
The second part of the presentation will focus on ultrafast electron processes taking place in heterostructured nanocrystals comprising metal (Au) and semiconductor (CdS) material domains.

A physicist walks into a biology department… – Robin Snyder Mon. November 14th, 2011
12:30 pm-1:30 pm

I present two recent projects in theoretical ecology and point out the connections to math loved by physicists. The first concerns life in a variable environment: when should an organism buffer itself against environmental variation and when should it try to take advantage of environmental variation? The second concerns the viability of mussel populations in a marine reserve network when shifting ocean currents cause fluctuating dispersal between reserves.

The search for Majorana Fermions in semiconductor nanowires – Roman Lutchyn Mon. November 7th, 2011
12:30 pm-1:30 pm

The exploration of topological phases of matter is one of the main challenges in condensed matter physics. Among the exciting recent developments in this direction are the discoveries of the new phases of matter with many intriguing properties such as topological insulators and superconductors. In my talk, I will focus on topological superconductors and discuss how to realize spinless p-wave superconductivity in semiconductor/superconductor heterostructures. I will show that such a non-trivial topological state emerging at the interface supports zero-energy modes that can be occupied by Majorana fermions. These quasi-particles, which are exotic in the sense that they are at the same time their own antiparticles,

Theoretical studies of magnetic and structural thermodynamics using effective Hamiltonians – Kirill Belashchenko Mon. October 31st, 2011
12:30 pm-1:30 pm

Effective configurational and spin Hamiltonians are commonly used to study magnetic and structural thermodynamics. For some purposes, such as the description of phase transitions in substitutional alloys, they can be routinely constructed by high-throughput first-principles calculations. As an illustration of this standard approach, I will describe the calculation of the phase diagrams of Gd-doped EuO and EuS using the cluster expansion technique [1]. Many problems, however, require physical insight for the selection of the relevant degrees of freedom and for an adequate representation of their interactions. The main focus of this talk will be on such problems, including the structural phase transitions at the Cr2O3 (0001) surface and the magnetic thermodynamics of the parent compounds of ferropnictide superconductors [2].

Photorefractive Polymers for an Updatable Holographic Display – Cory Christenson Mon. October 24th, 2011
12:30 pm-1:30 pm

Holography is a technique commonly used to display objects in three-dimensions, as it has the potential to accurately reproduce all features of the light from a real object. Holographic telepresence has been a compelling fantasy for decades, but modern science has failed to deliver such a system, primarily due to the computational power required and the lack of a suitable recording material. I will discuss my graduate work at the University of Arizona on the use of organic photorefractive polymers as a medium for updatable 3D holographic displays. These exhibit a reversible index change in response to light, and the wavelength sensitivity can be modified using different chromophores,

Energetics and Electronic Structure of Point Defects in Oxide Semiconductors: A Density Functional Approach – Fumiyasu Oba Fri. October 21st, 2011
12:30 pm-1:30 pm

Because of the crucial roles of point defects in the physical properties of pristine and doped oxide semiconductors, a fair amount of experimental research has been devoted to their characterization in previous decades. However, the understanding of the defects is limited, particularly at the atomistic and electronic level. A density functional approach is useful for the study of the defects and has provided various insights into their characteristics. In this talk, I will present our recent results on the defects in several oxide semiconductors, ZnO [1], SrTiO3 [2], BaTiO3 [3], and SnOx [4, 5], obtained using semilocal and hybrid density functional calculations.

Variational Studies on the Kagome Lattice – Jesse Kinder Mon. October 17th, 2011
12:30 pm-1:30 pm

The two dimensional kagome lattice is a highly frustrated spin system. When spins are placed on the vertices of the lattice with an antiferromagnetic interaction, there is no unique classical ground state. The large degeneracy of classical configurations with the same energy appear to give rise to an unusual quantum ground state. In this talk, I will discuss theoretical attempts to understand the ground state of the antiferromagnetic Heisenberg model on the kagome lattice. In the first portion of the talk, I will review several theoretical proposals for the ground state put forth over the past two decades. These fall into two basic categories: spin liquids and valence bond crystals.

Measuring the electronic properties of single semiconductor nanowire heterostructures using advanced optical spectroscopies – Leigh M. Smith Mon. October 10th, 2011
12:30 pm-1:30 pm

There has been intense interest in recent years to control the electronic structure in quasi one-dimensional nanowires through the fabrication of novel axial and radial heterostructures. Unlike materials in higher dimensions, nanowires have the unique ability to grow axial or radial heterostructures between almost any two materials regardless of lattice mismatch or strain. Understanding exactly how the electronic properties of the nanowire are changed through this control is extremely important and requires spectroscopies with high spatial, temporal and spectral resolution. I will discuss a number of examples in which the electronic structure in nanowire heterostructures can be modified either through strain,

Moving spins with heat: spin-Seebeck effect in a ferromagnetic semiconductor and Polarization-induced pn-junctions in wide band gap semiconductor nanowires – Roberto Myers Mon. October 3rd, 2011
12:30 pm-1:30 pm

Many proposed spin-based devices require transfer of spin into non-magnetic materials, which is usually accomplished by driving a charge current from a ferromagnet into a non-magnetic material. Heat can also be used to transfer spins into non-magnetic material using the spin-Seebeck effect, as demonstrated by Uchida et al. in permalloy[1]. We also observed this in GaMnAs [2], a ferromagnetic semiconductor. A different orientation of spin is injected into platinum bars on the hot side of the sample as compared to the cold side, and this spatial distribution of spin currents is unaffected by electrical breaks highlighting that the effect is driven by phonons in the substrate.

First-principles electronic structure calculations in energy research – Emmanouil (Manos) Kioupakis Mon. September 26th, 2011
12:30 pm-1:30 pm

As the world strives to reduce its reliance on fossil fuels, materials innovations can help catalyze the switch to renewable energy and the engineering of energy-efficient devices. Powered by modern high-performance computers, s first-principles methods can provide an understanding of fundamental materials processes at the microscopic level and play an important role in the development of novel energy materials and devices. In this talk, I will present insights garnered from first-principles calculations for the study of the performance of optoelectronic devices for energy. I will discuss the loss by non-radiative recombination in nitride LED light bulbs, the internal reabsorption of light and loss in nitride green lasers and transparent conductors,

The metal insulator transition of VO2: Shining new (synchrotron-based) light on an old problem – Louis Piper Mon. September 19th, 2011
12:30 pm-1:30 pm

The origin of the abrupt metal-insulator transition (MIT) in VO2 has been a subject of debate for several decades and remains an important problem for condensed matter physics. The change from high temperature metallic rutile phase to low temperature insulating monoclinic occurs abruptly at 360 K for bulk VO2. The origin of the MIT, whether structural (i.e. Peierls-like instability due to V-V dimerizing and tilting along the cR axis) or electronic (i.e. Mott-Hubbard transition due to strong electron correlation effects) or some combination of the two still remains a matter of debate. Recent advances in the growth of VO2 compounds have provided an opportunity to really examine this system.

Probing crystal defects by their vibrational modes – Sukit Limpijumnong Tue. July 5th, 2011
11:00 am-12:00 pm

First principles calculations can be used to study many material properties from a fundamental point of view. This talk will cover the calculations of natural vibration frequencies (local vibrational modes) of impurities and defects in crystals. These vibration frequencies can be probed experimentally by infrared spectroscopy techniques. After a brief review of the computation techniques and details, two different cases of local vibrational modes will be explained. The first case covers the vibration frequencies of hydrogen atoms that form strong bonds with N or O in GaN and ZnO. In these cases, the vibration mode is very distinct from the crystal phonon modes.

Uniform Peak Conductivity in Single-Walled Carbon Nanotubes – Jesse Kinder Mon. June 27th, 2011
12:30 pm-1:30 pm

A carbon nanotube is a one-dimensional system in which confinement of charge carriers and an unusual band structure lead to a variety of interesting effects. Many electronic and optical properties of a nanotube depend strongly on its geometry — the way in which a two-dimensional lattice of carbon atoms is rolled up to form the nanotube. In contrast, recent Rayleigh scattering measurements by the Park group at Cornell reveal that the peak optical conductivity is approximately equal for all single-walled carbon nanotubes, independent of their geometric structure. In this talk, I will describe our efforts to understand the origin of this uniform peak conductivity.

Sign reversal in dielectric anisotropy and dielectric relaxation in bent core liquid crystals – Jagdish Vij Mon. June 13th, 2011
12:30 pm-1:30 pm

We investigate the nematic phase of a 4-cyanoresorcinol bisbenozate compound by varying its chain length from C4 to C9 using dielectric and electro-optic spectroscopy. The frequencies and dielectric strengths of the various modes are determined. The results of the sign reversal in the dielectric anisotropy are interpreted in terms of the relative dielectric strengths of the various modes, their relaxation frequencies the order parameter of the system. The results are found to be much more interesting and complex than known so far for the calamitic Mesogens-.

Experimental observation and manipulation of topological surface states – Yulin Chen Mon. May 9th, 2011
12:30 pm-1:30 pm

Three-dimensional (3D) topological insulators (TIs) are a new state of quantum matter with a bulk gap generated by the spin orbit interaction and odd number of relativistic Dirac fermions on the surface. The robust surface states of TIs can be the host for many striking quantum phenomena, such as an image magnetic monopole induced by an electric charge and Majorana fermions induced by the proximity effect from a superconductor. Recently, several classes of materials were theoretically predicted to be the simplest 3D TIs whose surface states consist of a single Dirac cone. By investigating the surface state of these materials with angle-resolved photoemission spectroscopy (ARPES),

Strong-arming electron spin dynamics – Jason Petta Mon. May 2nd, 2011
12:30 pm-1:30 pm

A single electron spin in an external magnetic field forms a two-level system that can be used to create a spin qubit. However, achieving fast single spin rotations, as would be required to control a spin qubit, is a major challenge. It is difficult to drive spin rotations on timescales that are faster than the spin dephasing time and to individually address a single spin on the nanometer scale. I will describe a new method for quantum control of single spins that does not involve conventional electron spin resonance (ESR). In analogy with an optical beam splitter, we use an anti-crossing in the energy level spectrum of our quantum dot “artificial atom”

Scanning tunneling microscopy studies of single magnetic ions in GaAs – Jay Gupta Mon. April 25th, 2011
12:30 pm-1:30 pm

The scaling of electronic devices such as field effect transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations can impact device performance and functionality. Toward this end, the scanning tunneling microscope (STM) is emerging as a useful tool for its capabilities of atomic manipulation, imaging and tunneling spectroscopy. I will discuss our STM studies of Mn acceptors within the surface layer of a p-doped GaAs crystal [1]. We start by sublimating Mn adatoms onto the GaAs (110) surface, prepared by cleavage in ultrahigh vacuum. A voltage pulse applied with the STM tip allows us to replace a Ga atom in the surface with the Mn atom,

Electron-electron interaction and transport in bilayer graphene – Jun Zhu Mon. April 18th, 2011
12:30 pm-1:30 pm

Bilayer graphene, or two layers of graphene stacked together in Bernal stacking, is a unique two-dimensional electron system with hyperbolic bands and a band gap tunable by the application of an electric field through the two layers. In this talk, I will describe our experiments in understanding the mechanisms of carrier transport in a gapped bilayer graphene. I will also show accurate measurements of the effective mass of bilayer graphene, from which we determine its band structure. Our results show that electron-electron interaction renormalizes the bands of bilayer and suppresses the effective mass m*. The manifestation is qualitatively different from that in conventional 2D systems.

Nano is more than size: The role of geometry in the electronic structure of carbon nanostructures – Vince Crespi Fri. April 15th, 2011
12:30 pm-1:30 pm

The atomic-scale order of highly deformable yet chemically inert carbon frameworks animates a wide range of novel structural, optical, and electronic phenomena. For example, the division of surrounding space into two disconnected zones by an impenetrable suspended graphenic sheet enables adsorption of otherwise highly co-reactive species, such as alkali and halogen, in opposite subspaces, with an intense cross-sheet charge transfer that produces a new variant of ionic binding with a uncompensated electrostatic dipoles. New physics also results when the same species is adsorbed to both sides, with unusual “which-side” symmetry breaking at certain chemical potentials. New ways to induce gaps in graphene sheets can also be designed exploiting new stacking physics.

Colloidal Quantum Dot Solar Cells – Randy Ellingson Mon. April 11th, 2011
12:30 pm-1:30 pm

Earth’s need for clean energy becomes more evident with each demonstration of the shortcomings of fossil and nuclear energy sources. All carbon-free and nuclear-free energy sources will play important roles in our energy future, but only solar energy can in principle provide all of our energy needs. I will describe current market and technology landscapes for photovoltaics, introduce the use of quantum dots (QDs) as electronic materials, and provide an overview of the developing field of colloidal QD-based thin film solar cells. Although many R&D efforts pursue the fabrication of thin film photovoltaic devices from solution-based particle and nanoparticle (QD) starting materials,

Ab-initio Heat Transfer: Predicting thermal transport in nanostructures and materials from the atoms up – Derek Stewart Mon. April 4th, 2011
12:30 pm-1:30 pm

While electronic transport has been the focus of intensive research for nearly a century, thermal transport has proven difficult to quantify and model. However, a predictive model for thermal conductivity can improve our understanding of thermoelectric materials, thermal resistance barriers, nanoscale heat transport, and even geologic heat transfer. In this talk, I will discuss the development of a new first principles framework to model thermal transport in materials and nanostructures. Using density functional perturbation theory, we are able to calculate both harmonic and anharmonic interatomic force constants. Coupling these terms with a Boltzmann transport approach, we are able to demonstrate excellent agreement between the calculated and measured lattice thermal conductivities of technologically relevant semiconductors (silicon,

Toward Graphene-Based Photovoltaics – Liang-shi Li Mon. March 28th, 2011
12:30 pm-1:30 pm

Solution-processable thin-film solar cells can be competitive with silicon-based ones in terms of electricity output/cost ratio and therefore have great potential in solar energy utilization. Due to the requirement for efficient light harvesting, however, so far the most successful low-cost thin-film solar cells require materials containing either rare or toxic metals. In my talk I will discuss our efforts in making graphene, which is primarily made of carbon, an active component for solar energy conversion. Our work is based on the synthesis of solution-processable colloidal graphene quantum dots with tunable size and bandgap. Our spectroscopic studies have shown that the graphene quantum dots have some unique electro-optical properties,

InN and ZnO: Unexpected Commonalities – Steven Durbin Mon. February 7th, 2011
12:30 pm-1:30 pm

InN is an infrared bandgap semiconductor (although it hasn’t always been that way); ZnO is an ultraviolet bandgap material used in applications from gas sensors to breakfast cereals. Surprisingly, these ostensibly disparate materials are more closely related than we might think: for both, p-type doping is problematic, the surface exhibits significant electron accumulation, and undoped samples are characterized by a large background electron concentration. In this talk I will describe some of our work to date geared towards developing a better understanding of these and closely related materials, which have significant device potential for a large variety of applications.

Accurate and efficient solutions of wave propagation problems in periodic media – Catalin Turc Mon. January 31st, 2011
12:30 pm-1:30 pm

Many devices designed to guide and control waves rely on periodic structures on the wavelength scale: these include diffraction gratings (used to squeeze multiple signals onto a single optical fiber, and in our highest-powered lasers), photonic crystals (the most promising route to energy-efficient ultra-fast optical computation on a chip), meta-materials (allowing the control of waves in ways impossible in naturally-occurring media), and solar cells. I will present a class of efficient and accurate numerical methods based on boundary integral equations for the solution of wave propagation problems in piece-wise homogeneous periodic media. The main ideas behind these algorithms are (1) a novel representation of quasi-periodic Green’s functions that converges fast and (2) acceleration techniques based on equivalent sources and FFTs.

Ferromagnetic semiconductors and the role of disorder – Bruce Wessels Mon. December 6th, 2010
12:30 pm-1:30 pm

Magnetic semiconductors having Curie temperature greater than 300 K are of interest for a wide variety of spintronic device applications. Short-range order has been reported to stabilize ferromagnetism in transition metal-doped III-V compound semiconductors. While both theory and experiment have centered on dilute magnetic semiconductors where the magnetic ions substitute randomly for cation sites, there is increasing evidence that correlated substitution needs to be considered. Evidence of correlated substitution comes from structural analysis (extended x-ray absorption fine structure (EXAFS)), magnetic property and recent magneto-optical property measurements as well as theory. Furthermore, there is growing theoretical and experimental support that ferromagnetic semiconductors with Curie temperatures well above room temperature can be formed through correlated substitution.

Chemical Design of Magnetic Nanomaterials – Ana Cristina Samia Mon. November 29th, 2010
12:30 pm-1:30 pm

Nanosized magnetic materials continue to attract great interest due to their wide range of potential applications from data storage to medical diagnostics and therapy. Each application demands unique magnetic characteristics of the nanoparticles. Hence, the ability to tailor their properties is of considerable importance. A first step towards understanding the correlation between magnetic properties and nanostructure is the development of robust synthetic approaches to prepare magnetic materials with controllable nanoarchitectures. In this presentation the synthesis of metal and metal oxide magnetic nanoparticles using wet chemical approaches is presented. By carefully tuning the synthesis conditions, different sizes, shapes and compositions can be prepared.

Imaging 3D spatiotemporal hemodynamics of single cortical vessels in vivo using two-photon laser scanning microscopy – Peifang Tian Mon. November 22nd, 2010
12:30 pm-1:30 pm

The dynamics response of individual cerebral vessels to sensory-stimuli is crucial to form a mechanistic understanding of functional imaging technologies, such as functional MRI (fMRI), as well as for understanding neurovascular dysfunction, as occurs in stroke and dementia. Using optical imaging technologies such as two-photon laser scanning microscopy and the rat primary sensory cortex as our animal model, we have characterized the stimulus-evoked cerebral hemodynamic response on the level of single arterioles and capillaries throughout a significant three-dimensional volume (~1-2mm3) in vivo. Further, we will relate this characterization to the underlying neuronal electrical activity and the angioarchitecture. In this talk,

Stories of Large Scale Graphene – Yong Chen Mon. November 15th, 2010
12:30 pm-1:30 pm

Graphene has rapidly risen in the past few years to become one of the most actively researched topics in condensed matter physics and nanoscience due to its numerous remarkable properties and potential applications. Perhaps the best known method to make graphene has been the “scotch tape” technique (to exfoliate graphite), used just a few years ago by graphene pioneers Geim and Novoselov (who were awarded the physics Nobel in 2010) to unlock the novel physics of graphene. This simple method, however, produces only very small graphene flakes (typically tens of microns) and therefore is not sufficient for large scale production of graphene to fully realize its potentials.

Modeling defects, microstructure, and shape evolution in orientationally ordered soft materials: nematic elastomers and lipid vesicles – Robin Selinger Mon. November 8th, 2010
12:30 pm-1:30 pm

Liquid crystal elastomers, sometimes called “artificial muscles,” combine the elastic properties of rubber with the molecular order properties of liquid crystals. These fascinating materials stretch, shrink, bend or flap in response to changes in temperature, illumination, or applied fields, due to strong coupling between orientational order and elastic strain. Their mechanical response under applied strain is also peculiar, showing in some geometries a pronounced plateau in the stress-strain curve, accompanied by formation of a striped microstructure with director rotation in alternating directions. We construct a mesoscale model of this system based on a simple free energy functional and implement it using finite element elastodynamics.

Polymeric materials for printable electronic applications: from synthesis to device characterization – Genevieve Sauve Mon. October 25th, 2010
12:30 pm-1:30 pm

Conjugated polymers are considered by many as leading candidates to produce the next generation of electronics. This belief is based upon several factors: (a) they can be solution-processed using established printing technologies to give flexible, lightweight functional thin films, allowing for low-cost and large-scale production. (b) Several already have desirable properties for practical applications. In the first part of this talk, I will discuss strategies to optimize transistors and show results for well-defined poly(3-alkylthiophene)s and block copolymers of poly(3-hexylthiophene). In the second part of the talk, I will talk about my research here at CWRU on polymer-based photovoltaics.

Absorption/Expulsion of Oligomers and Linear Macromolecules in a Polymer Brush – Sergei Egorov Mon. October 18th, 2010
12:30 pm-1:30 pm

The absorption of free linear chains in a polymer brush was studied with respect to chain size and compatibility with the brush by means of Monte Carlo simulations and Density Functional Theory / Self-Consistent Field Theory at both moderate and high grafting densities using a bead-spring model. Different concentrations of the free chains were examined. When free chains are incompatible with the brush, all oligomeric species are almost completely ejected by the polymer brush irrespective of their length. For compatible case, we find that in going from shorter to longer chains, the absorbed amount undergoes a sharp crossover from weak to strong absorption.

Fractionalization in Mesoscopic Rings – Smitha Vishveshwara Mon. October 11th, 2010
12:30 pm-1:30 pm

A spectacular phenomenon that can occur in strongly correlated low dimensional systems is that of fractionalization. In such electronic systems, quasiparticles excitations can carry a fraction of the electron’s charge and can have anyonic quantum statistics which is neither fermionic nor bosonic. Here, mesoscopic ring geometries are introduced as a means of bringing out novel signatures of both charge fractionalization and non-Abelian anyonic statistics. It is shown that power maps of quasiparticle motion around a thin ring can act as measures of fractionalization complementary to recent cutting-edge studies in etched quantum wires. A proposal is presented for probing the non-Abelian statistics of recent tour de force realizations of fractional vortices in superconducting mesoscopic rings.

Exciton-Plasmon Interactions and Fano Resonances in Nanostructures – Alexander Govorov Mon. October 4th, 2010
12:30 pm-1:30 pm

Coulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several interesting effects: energy transfer between nanoparticles (NPs), plasmon enhancement, reduced exciton diffusion in nanowires (NWs), exciton energy shifts, Fano interference effect, and non-linear phenomena [1-3]. Using transport equations for excitons, we model exciton transfer in NWs and explain the origin of the blue shift of exciton emission observed during recent experiments with hybrid NW-NP assemblies [2]. We also look at optical responses of artificial light-harvesting complexes composed of chlorophylls, bacterial reaction centers, and NPs [3]. We show that, using superior optical properties of metal and semiconductor NPs, it is possible to strongly enhance the efficiency of light harvesting in such complexes [3].

Embedded nanopillars for solar cell applications – Jingbiao Cui Mon. September 27th, 2010
12:30 pm-1:30 pm

Nanopillar radial junctions achieved by embedding nanopillars in absorbing thin films have potential for improved performance in solar cells due to increased junction area and improved charge carrier collection. This type of structure is still in its initial stage of development by using expensive and complicated microfabrication processes. An economic approach to the fabrication of nanopillar p-n junction solar cells has been investigated recently using a simple two-step electrodeposition method. The nanopillar heterojunctions are composed of n-type ZnO nanopillar arrays embedded in p-type Cu2O thin films, showing improved performance as compared with the planar thin film structures. While much effort is needed to optimize the device overall performance,

Studies of reflection-band defects in 1D polymeric photonic crystals – Guilin Mao Mon. September 20th, 2010
12:30 pm-1:30 pm

Disorder or variation of the periodic structure of 1D-photonic crystal can lead to defects in the reflection band, characterized by one or more spectrally narrow transmission peaks inside that band. At or near such defects, changes in the effective group velocity of the light result in interesting optical phenomena such as Faraday rotation enhancement and gain enhancement in a distributed feedback (DFB) photonic crystal laser. In this talk, we present experimental results on and a theoretical interpretation of magneto-optic rotation and DFB lasing within the reflection band of CLiPS polymer multilayers.

Triboelectric Charging in Granular Systems – Daniel Lacks Mon. September 13th, 2010
12:30 pm-1:30 pm

Have you ever received a shock when you touched a doorknob after shuffling across a carpeted floor? The culprit, known as triboelectric charging, is also responsible for phenomena as innocuous as a rubbed balloon that makes your hair stand on end, or as dramatic as a lightning strike. While it is familiar to every child, fundamental understanding of triboelectric charging is so poor that even the most basic questions are still being debated, such as whether the transferred charge species are electrons or ions. Scientific progress is difficult because triboelectric charging is a non-equilibrium process (separated surfaces are neutral at equilibrium) that involves changes in electron states and occurs at a level of one electron per 100,000 surface atoms (physical and/or chemical defects at this low level likely control the behavior).

Massively parallel Density functional calculations for thousands of atoms: KKRnano – Alexander Thiess Mon. August 30th, 2010
12:30 pm-1:30 pm

Existing highly precise density functional method for electronic structure calculations are mostly restricted to the treatment of at maximum a few hundred inequivalent atoms. This limitation leaves many open questions in material science e.g. on complex defects and defect-defect interaction unresolved. KKRnano is a new massively parallel DFT-algorithm in the framework of the KKR Green function method which we developed and optimized for large-scaled applications of thousands of atoms. In order to deal with the enormous computational requirements of such calculations we have implemented four levels of parallelization, which allows for an efficient use of hundreds of thousands of processors on the latest generation of supercomputers as Blue Gene.

Michelson Postdoctoral Lecture 2:Optical Atomic Clocks – David Hanneke Tue. May 11th, 2010
12:30 pm-1:30 pm

The most precise measurement techniques involve time, frequency, or a frequency ratio. For example, for centuries, accurate navigation has relied on precise timekeeping — a trend that continues with today’s global positioning system. After briefly reviewing the current microwave frequency standards based on the hyperfine structure of cesium, I will describe work towards atomic clocks working at optical frequencies. Among these are standards based on trapped ions or on neutral atoms trapped in an optical lattice. A frequency comb allows the comparison of different optical frequencies and the linking of optical frequencies to more-easily-counted microwave ones. Though still in the basic research stage,

Michelson Postdoctoral Lecture 1: Entangled Mechanical Oscillators and a Programmable Quantum Computer: Adventures in Coupling Two-Level Systems to Quantum Harmonic Oscillators – David Hanneke Mon. May 10th, 2010
12:30 pm-1:30 pm

The two-level system and the harmonic oscillator are among the simplest analyzed with quantum mechanics, yet they display a rich set of behaviors. Quantum information science is based on manipulating the states of two-level systems, called quantum bits or qubits. Coupling two-level systems to harmonic oscillators allows the generation of interesting motional states. When isolated from the environment, trapped atomic ions can take on both of these behaviors. The two-level system is formed from a pair of internal states, which lasers efficiently prepare, manipulate, and read-out. The ions’ motion in the trap is well described as a harmonic oscillator and can be cooled to the quantum ground state.

Atom Mapping and Correlated Functional Imaging of Nanowires – Lincoln J. Lauhon Mon. April 26th, 2010
12:30 pm-1:30 pm

Nanowires are nanoscale in two dimensions and microscale in a third dimension, providing a wealth of opportunities to exploit novel nanoscale electronic, optical, magnetic, and thermal properties in devices with well-defined microscale electrical contacts. An attendant challenge is the establishment of quantitative structure-property relationships that enable rational engineering of new and/or superior function. In semiconductors, the dopant concentration determines the carrier concentration, so correlated studies of dopant distribution and local conductivity are important when intentional or unintentional inhomogeneities are present. In materials that undergo phase-changes near room temperature, such as vanadium oxide (VO2), the crystal structure influences the conductivity, so local mapping of phase domains is important to understanding and controlling switching behaviors.

Controlling Spin and Magnetism in Quantum Dots – Rafal Oszwaldowski Mon. April 19th, 2010
12:30 pm-1:30 pm

A promising approach for the next generation of applications for information storage and processing comes from the field of spintronics (spin-electronics) that seeks to use spin of carriers, rather than just their charge [1]. Commercial spintronic devices, such as computer hard drives, are based on metallic magnetic multilayers, and utilize the magnetic moment associated with spin to read magnetically stored information. Unfortunately, for advanced functions such as signal processing and digital logic, these structures are of limited use, and it would be more desirable to control spin and magnetism in semiconductors. Carrier-mediated magnetism in such semiconductors as (In,Mn)As, (Ga,Mn)As or (Cd,Mn)Te [2],

Diffusion Tensor Imaging: A Guided Tour – Cheng Guan Koay Thu. April 15th, 2010
12:30 pm-1:30 pm

Diffusion tensor imaging (DTI) is a noninvasive magnetic resonance (MR) technique for investigating tissue microstructure and white matter architectural organization in the brain. In this talk, we will present a basic introduction to DTI and give a guided tour through recent developments in the analysis of diffusion tensors from the least squares estimations of the diffusion tensor to the elliptical cone of uncertainty for characterizing uncertainty of the major eigenvector (or principal axis) of the diffusion tensor.

Through A Glass, Darkly: Obtaining Quantitative Information from Microscope Images of Liquid Crystals – Tim Atherton Tue. April 13th, 2010
12:30 pm-1:30 pm

Liquid Crystalline phases are identified by their beautiful textures when viewed under the polarizing microscope. These two-dimensional textures contain much information about the ordering of the liquid crystal, but it is generally difficult to extract quantitative information from them since the mapping from the order parameter field to the image is not injective. More sophisticated imaging methods have been developed such as Fluorescence Confocal Microscopy and Near-field Scanning Optical Microscopy, which offer three-dimensional images of LC ordering but nonetheless pose further challenges to their interpretation. In this talk I discuss several examples of how these techniques may be used to reveal the fundamental physics of surface ordering,

Hard tetrahedra and Quasi-Crystals – Rolfe G. Petschek Mon. April 5th, 2010
12:30 pm-1:30 pm

I will describe the packing of hard tetrahedra. Contrary to recent speculations, Monte Carlo simulations show that at finite temperatures this Platonic solid packs with quite high volume fractions and has very complicated, probably quasi-crystalline phases and (likely) a modestly complicated phase diagram. An analytic high pressure equation of state for hard particles that summarizes this data is given and compared to the simulation data. Interestingly, this system is shown robustly to have an only modestly first order transition from an isotropic fluid to a quasi-crystal (or quasi-crystal-like) structure. Contrary to speculations made two decades ago and not yet contradicted clearly in the literature,

Periodic networks in heterogeneous materials: theory and multiscale homogenization for soling heat transfer and deformation problems – Viktoria Savatorova Mon. March 29th, 2010
12:30 pm-1:30 pm

All materials consist of some heterogeneity. In many cases heterogeneity can affect the properties of the whole sample, and this fact stimulates the desire to create heterogeneous materials with definite desired properties. Most of man-made materials such as composite materials, porous structures, powders, have periodical structure. In nature geological materials have structures, close to periodical. This is the reason to investigate the behavior of heterogeneous materials with periodical structure. In the majority of cases heterogeneous materials with a number of components, having different properties, cannot be described by direct considering each of the heterogeneities. The way to avoid intractable problems is to replace the heterogeneous medium by an equivalent homogeneous.

Ultrafast physics in photosynthesis: Mapping sub-nanometer energy flow – Naomi Ginsberg Thu. February 25th, 2010
12:30 pm-1:30 pm

In the first picoseconds of photosynthesis, photoexcitations of chlorophyll molecules are passed through a network of chlorophyll-binding proteins to a charge transfer site, initiating the conversion of absorbed energy to chemical fuels. The remarkably high quantum efficiency of this energy transfer relies on near-field coupling between adjacent chlorophyll molecules and their interaction with protein phonon modes. Using two-dimensional electronic spectroscopy, we track the time-evolution of energy flow in a chlorophyll-protein complex, CP29, found in green plants. The results from these nonlinear four-wave mixing experiments elucidate the role of CP29 as a light harvester and energy conduit by drawing causal relationships between the spatial and electronic configurations of its chlorophyll molecules.

Single cell studies using microfluidic devices – Amy Rowat Thu. February 18th, 2010
12:30 pm-1:30 pm

Cells that are genetically identical can exhibit differences in phenotype, however, such variation remains masked in bulk measurements. To capture variability among individual cells, as well as the behavior of subpopulations of cells, requires studies with single cell resolution. Here I will describe a new class of microfluidic devices that enables studies at the single cell level. First, I will describe a microfluidic device that enables measurements of the mechanical properties of individual cells. The ability of cells to deform through narrow spaces is central in physiological contexts ranging from immune response to metastasis. To elucidate the effect of nuclear shape on the deformability of neutrophil cells,

Imaging coherent electron transport in graphene – Jesse Berezovsky Thu. February 11th, 2010
12:30 pm-1:30 pm

The coherent flow of electrons through a graphene device is an intriguing physical problem, which must be understood for future quantum technologies. We have developed a low-temperature scanning probe technique for mapping the effect of a single movable scatterer on coherent transport in graphene. We obtain images of conductance vs. scatterer position that provide a spatial view of the interference of electron waves that create universal conductance fluctuations and weak localization.

Principles and Applications of Extrinsic (Doped) Organic Semiconductors – Calvin Chan Mon. February 1st, 2010
12:30 pm-1:30 pm

Organic semiconductors have garnered much attention for many promising applications, including organic light-emitting diodes, photovoltaic cells, thin-film transistors, thin-film batteries and spintronic devices. Despite this demand, robust and efficient organic electronic devices have been limited by the quality of organic semiconductor materials and a poor understanding of their underlying physics. Extrinsically doped organic semiconductors provide an avenue to overcoming the intrinsic material limitations that impede device performance. Moreover, fundamental doping studies provide insight into the nature of molecular charge transfer between organic moieties. For example, host molecules doped with highly electropositive donor species result in pronounced shifts of the Fermi-level towards the unoccupied molecular states [1].

Gigahertz dynamics of a strongly driven single spin in diamond – G. D. Fuchs Mon. January 25th, 2010
12:30 pm-1:30 pm

Nitrogen vacancy (NV) center spins in diamond have emerged as a promising solid-state system for quantum information and communication. Techniques to manipulate a single spin have been used to study the long room temperature spin coherence times of NV centers as well as their interactions with nearby electron and nuclear spins. There remain major challenges, however, both in understanding the physics of these defects and in the development of technologies based on their quantum properties. We extend coherent control of individual spins to the chip level by fabricating coplanar waveguide structures on diamond substrates to apply high-intensity microwave fields. Within large driving fields,

ZnGeAs2: A Novel Semiconductor for Photovoltaics – Tim Peshek Mon. January 4th, 2010
12:30 pm-1:30 pm

> I will motivate the fabrication of tandem thin film devices based solely on II-IV-V2 compounds as a target for wide-scale PV deployment. Third generation solar cells must overcome the Shockley-Queisser (SQ) limitation of single diode solar cells; the fabrication of multiple junction solar cells is one avenue to circumvent the SQ limit. Currently, the high cost of highly efficient multiple junction cells (e.g. the >40% efficient Ge/GaAs/InGaP triple junction) prohibits these devices from being widely deployed in the market. The challenge to condensed matter physics is to identify semiconductors that have optimal properties for multi-junction cells, including material abundance and low manufacturing costs.

Spin Fluctuations in Magnetic Quantum Dots – Andre Petukhov Mon. December 14th, 2009
12:30 pm-1:30 pm
Quantum Simulation of Strongly Correlated Quantum Dots Out of Equilibrium – Jong Han Mon. November 30th, 2009
12:30 pm-1:30 pm

The study of strong correlation physics out of equilibrium has become one of the most exciting fields in condensed matter theory of today. The physical systems of interest include quantum dots displaying the zero-bias-anomaly (ZBA) due to the Kondo phenomena. Recently, significant progress has been made in the strong-correlation community toward deeper understanding of nonequilibrium many-body state of nanoscale electronic devices under finite source-drain bias. I introduce the recently formulated imaginary-time theory of steady-state nonequilibrium which extends the equilibrium theory into nonequilibrium by introducing complex chemical potentials. Due to its similarity to the equilibrium theory, this formalism becomes very powerful when combined with equilibrium tools such as quantum Monte Carlo method.

From Water Splitting to Hydrogen Storage: The Art of First-Principles Predictions in Materials Design – Shengbai Zhang Mon. November 9th, 2009
12:30 pm-1:30 pm

Green and renewable energy is important to our environment, for sustainable energy supply, and offers new opportunities for economical growth. In the past, materials research has played an essential role in the development of the science bases necessary for green energy technology. In this talk, I will discuss two recent examples using first-principles density functional theory to predict new materials that are relevant to our energy mission. In particular, I will discuss how to optimize the electronic structures of titanium oxide for water splitting using unconventional codoping. I will also discuss using graphene oxide as a light substrate to anchor transition metal element for enhanced binding to non-polar molecules such as the dihydrogen for room-temperature storage in solids.

Quantum Mechanics of Point Defects and Diffusion in α-Al2O3 – Arthur Heuer Mon. November 2nd, 2009
12:30 pm-1:30 pm

Ab-initio DFT calculations have been made of native point defects – aluminum vacancies and interstitials and oxygen vacancies and interstitials – and point defect clusters, in both pure sapphire (α-Al2O3) and sapphire doped with the aliovalent solutes Mg and Ti. These calculations have been carried out by and in collaboration with Hine, Frensch, Finnis and Foulkes of Imperial College, London and have resolved the corundum “conundrum” (corundum is the mineral name forα-Al2O3 . The conundrum in question is the “buffering” evident in self-diffusion data of pure and doped crystals and how to interpret the sizeable activation energy found experimentally for oxygen diffusion.

Computer Simulations of Self-Assembly of Metallo-Supramolecular Networks – Elena Dormidontova Sat. October 31st, 2009
12:30 pm-1:30 pm

Using Monte Carlo simulations we studied formation of reversible metallo-supramolecular networks based on 3:1 ligand-metal complexes between end-functionalized oligomers and metal ions. The fraction of 1:1, 2:1 and 3:1 ligand-metal complexes in reversibly associated structures was analyzed as a function of oligomer concentration, c and metal-to-oligomer ratio. We studied the onset of network formation, which occurs in a limited range of metal-to-oligomer ratios at sufficiently large oligomer concentrations as well as the properties of metallo-supramolecular networks. We found that the mesh size of the network decreases with oligomer concentration and reaches its minimum at the stoichiometric composition, where the high-frequency elastic plateau modulus approaches its maximal value.

Magnetic Properties of Rare Earth Doped GaN – John M. Zavada Mon. October 26th, 2009
12:30 pm-1:30 pm

Rare earth (RE) doped GaN has been widely investigated for applications in displays and optical applications due to the strong visible infared (IRR) emissions from RE3+ ions in such a wide-band-gap material. In recent years this material systems has also beome an importatn candidate as a dilute magnetic semiconductor (DMS). Room temperature ferromagnetism has been observed in GaN doped with different REs inclouding Gd, Eu and Er. However, considerbable debate continues as to the origin of this ferromagnetic behavior. Thsi talk will focus on research concerning the magnetic properties of RE doped GaN films and the claim it is a DMS material.

Nanoscale memristive devices for memory and logic applications – Wei Lu Mon. October 19th, 2009
12:30 pm-1:30 pm

Memristor (a word created from “memory” and “resistor” ) has been claimed as the ” missing circuit element”and research on nanoscale memristor devices has gained substantial interest recently after the development of a simple device model last year. Memristors or memristive systems are two-terminal electrical switches that exhibit both hysteresis (memory) and non-linear resistance characteristics. These properties allow them to act as promising candidates for ultra-high density memory and logic applications. In this talk, I will discuss our studies on a silicon-based memristive system. As a digital memory, the device is fully compatible with CMOS processing and exhibits excellent performance in terms of scaling potential,

A birds-eye view of nonlinear optics: using scale invariance to optimize the molecular response – Mark Kuzyk Wed. October 14th, 2009
12:30 pm-1:30 pm

Nonlinear optical materials show great promise in a broad range of applications from cancer therapies and medical imaging to increasing the speed of the internet. Making such applications possible requires molecules that interact more strongly with light. After more than three decades of research, many new molecules have been synthesized with larger nonlinear response, made larger mainly by increasing the number of electrons and decreasing the energy gap; but, the best molecules have remained a factor of 30 below the fundamental limit. I will discus how the fundamental limits and scale invariance are applied to the design of better molecules.

Thermal Transport and Thermoelectric Energy Conversion in Nanomaterials – Li Shi Mon. October 5th, 2009
12:30 pm-1:30 pm

The high charge carrier mobility and thermal conductivity of carbon nanotubes and graphene have attracted interest in their applications for nanoelectronics and thermal management. On the other hand, the suppressed lattice thermal conductivity of semiconducting nanowires and thin films may give rise to enhanced figure of merit of thermoelectric materials. In an effort to better understand the potentials and challenges of these nanomaterials-enabled designs, we have developed a set of experimental methods to characterize electron and phonon transports in individual nanostructures. Our recent experiments have demonstrated the measurement of the thermal conductance together with the chirality of the same individual single- walled carbon nanotube,

Band structure information from soft x-ray spectroscopy – Andrew Preston Mon. September 28th, 2009
12:30 pm-1:30 pm

The optical and electric properties of a material are entirely dependent on the ordering of its electrons. In crystalline materials quantum effects constrain the electrons to bands that are best described in terms of crystal momentum. It is this electronic “band structure” we need to understand in order to fully harness novel materials. Soft x-ray based spectroscopic techniques provide a way to directly measure electronic band structures. In this talk I will present an overview of synchrotron based x-ray absorption, emission, photoemission, and resonant emission spectroscopy. Focusing on zinc oxide – a transparent conducting oxide with numerous potential applications –

Formation and properties of Cu_2S-CdS and Ag_2S-CdS Nanorod Heterostructures – Denis Demchenko Mon. September 21st, 2009
12:30 pm-1:30 pm

A partial cation exchange has been used to synthesize Cu_2S-CdS and Ag_2S-CdS nanocrystal heterostructures, with two very different morphologies. Cu^+ cation exchange takes place preferentially at the ends of CdS nanorods, Cu_2S segments grow into the nanorod from both ends. Ag^+ exchange is non-selective, Ag_2S islands nucleate and grow over the entire surface of the nanorod. This leads to very different patterns, striped Ag_2S-CdS superlattice with several equidistant Ag_2S segments in a CdS nanorod, and an asymmetric Cu_2S-CdS heterostructure with Cu_2S segments at the ends of the CdS nanorod. We use first-principles calculations to explain the formation patterns in the two nanostructures,

Recent Advances in Organic (Opto)electronic Materials – Oksana Ostroverkhova Wed. September 16th, 2009
12:30 pm-1:30 pm

There is growing interest in using organic (opto)electronic materials for applications in electronics and photonics. In particular, organic semiconductor thin films offer several advantages over traditional silicon technology, including low-cost processing, the potential for large-area flexible devices, high-efficiency light emission, and widely tunable properties through functionalization of the molecules. Over the past decade, remarkable progress in materials design and purification has been made, which led to applications of organic semiconductors in light-emitting diodes, polymer lasers, photovoltaic cells, high-speed photodetectors, organic thin-film transistors, holographic displays, and many others. Most of the applications envisioned for organic semiconductors rely on their (photo)conductive and/or luminescent properties.

Ballistic Quasiparticles in Superfluid 3He: A Non-Newtonian Gas – George Pickett Mon. May 18th, 2009
12:30 pm-1:30 pm

We can cool superfluid 3He to below 100 microkelvin where the number of unpaired 3He atoms is only of the order of 1 in 10^8. Here these quasiparticle excitations move ballistically as they are so tenuous that collisions are highly improbable. This dilute gas has very strange properties, since the Bardeen-Cooper-Schrieffer dispersion curve is quite unlike that for a classical gas. This makes the dynamics very unusual since even at the lowest temperatures and smallest quasiparticle energies the momentum they carry is very large. That means that we can detect this gas by its damping effect on a mechanical resonator even though by room temperature standards it would represent a reasonably good vacuum.

Chirality and Kondo Physics in Graphene – Herb Fertig Mon. April 27th, 2009
12:30 pm-1:30 pm

Graphene, a two-dimensional network of carbon atoms, exhibits unique electronic properties because it supports low energy, massless, Dirac-like quasiparticles. The quantized Hall effect in this system has an unusual set of plateaus, whose locations may be interpreted in terms of a geometric “Berry’s phase” related to the chirality of the Dirac particles. The chiral nature of these states also leads to an unusual edge state structure, particularly near filling factor nu=0. We examine the transport behavior of the nu=0 graphene system in light of its unusual edge state structure. When electron-electrons interactions are included, we find a magnetic domain wall structure at the edge that is electrically conducting and behaves like a Luttinger liquid.

Landau Level Spectroscopy of Graphene – Zhigang Jiang Mon. April 20th, 2009
12:30 pm-1:30 pm

Graphene, a single atomic sheet of graphite, is a monolayer of carbon atoms arranged in a hexagonal lattice. The unique electronic band structure of graphene exhibits an unusual low-energy linear dispersion relation, radically different from the parabolic bands common to all previous two-dimensional systems. Most interestingly, the charge carriers in graphene mimic relativistic, massless Dirac particles, leading to intriguing new phenomena. In this talk, I focus on two projects related to graphene that complement each other: magneto- transport measurements in high magnetic fields, and infrared optical studies of graphene. In the transport experiments, we discovered a room temperature quantum Hall effect in graphene [1] and new quantum Hall phases in the extreme quantum limit [2,3].

First-principles theory of coloration on WO3 upon charge insertion – Peihong Zhang Mon. April 13th, 2009
12:30 pm-1:30 pm

Electrochromic matrials exchibit reversible and persistent change of the optical properties, hence the color, upon applying an electrical pulse that injects both electrons and compensating ions into the materials. Despite much research effort, a first-principles theory for the coloration mechanism in this material has not emerged. Although the connection between the appartent color of materrials and their optical properties is obvious, so far there has been no calculations of the color of materials from first-principles. In this talk, I will discuss first-principles investigations of the coloration of WO3 upon charge insertion, using sodium tungsten bronze (Na_xWO3) as a model system.

Screening Plasmonic Materials using Nanopyramidal Arrays – Teri Odom Mon. April 6th, 2009
12:30 pm-1:30 pm

Surface plasmon polaritons (SPPs) are responsible for optical phenomena including negative refraction, surface enhanced Raman scattering, and nanoscale focusing of light. Although many materials support SPPs, the choice of metal for most applications has been based on traditional plasmonic materials such as Ag and Au because there have been no side-by-side comparisons of different materials on well- defined, nanostructured surfaces. This talk will describe a platform that not only enables rapid screening of a wide range of metals under different excitation conditions and dielectric environments but that also can identify unexpected materials for biosensing. Nanopyramidal gratings were used to generate SPP dispersion diagrams for Al,

Fast Protonic Conductivity in Crystalline Materials: Highly Sulfonated Aromatics – Yuriy Tolmachev Tue. March 31st, 2009
12:30 pm-1:30 pm

Polymer Electrolyte Fuel Cells are expected to replace internal combustion engines as power sources in transportation during our lifetime. The talk will discuss briefly main issues impeding commercialization of PEFC technology as well as the PEFC research at Kent State. Particular attention will be drawn to the need for high-temperature proton conducting membrane. The rest of the talk deals with our recent discovery of fast protonic conductivity in highly sulfonated arenes and its implications for the development of low-humidity high-temperature PEFC membranes.

Dipole in a Magnetic Field, Work, and Quantum Spin – Robert Deissler Mon. February 23rd, 2009
12:30 pm-1:30 pm

Place an atom in a nonuniform static external magnetic field and, because of the interaction between the atom’s magnetic moment and the magnetic field gradient, the atom will accelerate. This, of course, is what occurs in the classic Stern-Gerlach experiment. An important and fundamental question, which has been neglected in the literature, is whether or not the magnetic field is doing work on the atom. It is shown that, while the magnetic field does no work on the electron orbital contribution to the magnetic moment (the source of translational kinetic energy being the atom’s internal energy), whether or not it does work on the electron-spin contribution to the magnetic moment depends on whether the electron has an intrinsic rotational kinetic energy associated with its spin.

Synthesis of Novel Fuel Cell Membranes with Aligned Proton Conducting Pathways – Matt Yates Mon. February 16th, 2009
12:30 pm-1:30 pm

Novel approaches have been developed to engineer the microstructure of proton conducting membranes to enhance proton transport. Polymer composite and ceramic membranes were synthesized in which proton conducting pathways are aligned through the plane of the membrane. For polymer composite membranes, electric fields are applied during membrane synthesis to cause proton conducting domains aggregate into connected chains aligned through the membrane. For ceramic membranes, surfactant mediated crystallization is employed to direct crystal growth, resulting in aligned proton conducting paths. For both types of membranes, the engineered microstructure results in significantly enhanced proton conductivity through the membranes and improved performance of the membranes in fuel cells.

Molecular materials for dynamic holography and lasing applications – Jarek Mysliewiec Wed. February 4th, 2009
12:30 pm-1:30 pm

The subject of the presentation will be focused on molecular materials like liquid crystals, photochromic polymers or modified DNA-dye systems and their possible applications for lasing and dynamic optical information recording. Results on optical information processing were obtained in a typical degenerate two- or four-wave mixing experiments. For amplified spontaneous emission measurements, nano- or picosecond lasers of different wavelengths (355 nm and 532 nm) were used.

Bent-core nematic liquid crystals: Opportunities and mysteries – Jim Gleeson Mon. December 1st, 2008
12:30 pm-1:30 pm
Phonon expansion and dispersion: Condensed matter channels: Material diagnosis – Dov Hazony Mon. November 24th, 2008
12:30 pm-1:30 pm

Propagating basic acoustic pulses may behave as phonons. They can be characterized and utilized to evaluate channels through which they have travelled.

Terahertz Time-Domain Measurement of Ballistic Electron Resonance in a Single-walled Carbon Nanotube – Zhaohui Zhong Mon. November 17th, 2008
12:30 pm-1:30 pm

The terahertz (~ 100 GHz to 10 THz) electrical properties of nanomaterials are of relevance both to the fundamental science of low-dimensional systems and to the operation of next-generation smaller and faster electronics. I will describe the first terahertz time-domain electrical measurements of single-walled carbon nanotube transistors. A ballistic electron resonance is directly observed with a picosecond-scale period corresponding to the roundtrip transit of an electron along the nanotube. The electron velocity is found to be constant and equal to the Fermi velocity, showing that the high-frequency electron response is dominated by single-particle excitations rather than collective plasmon modes. These results demonstrate a powerful new tool for directly probing picosecond electron motion in nanostructures.

Room temperature ferromagnetism in semiconducting oxides – Chandran Sudakar Mon. November 10th, 2008
12:30 pm-1:30 pm

Diluted magnetic semiconductors are formed when magnetic transition metal ions are doped in small concentrations into a semiconductor host lattice. The first reports of ferromagnetism being observed at room temperature in a dilutely doped semiconducting oxide film attracted a great deal of attention, but were also met with considerable skepticism. These materials would have enormous potential for developing new classes of electronic devices, but there were concerns that the observed magnetism arose from impurity contributions, rather than any intrinsic property of the material. Although there has been some progress made in understanding these systems, in particular, the important role played by oxygen vacancies,

Spin injection, transport, and control in Silicon – Ian Appelbaum Mon. November 3rd, 2008
12:30 pm-1:30 pm

The intrinsic angular momentum of an electron (spin) – and its associated magnetic moment – can encode information: spin “up” or “down” can be interpreted as “0” or “1”, and potentially be used as the physical realization of a new paradigm of computing beyond electronics. However, this concept of spin-electronics (“spintronics”) needs to be built using a material where the electron spin orientation is preserved over long times (to enable many gate operations) and long distances (so that many devices can be integrated). Silicon, the materials basis for electronics, has been known for decades to have an extraordinarily long spin lifetime,

Coupling nanomechanical motion to electromagnetic fields through the Casimir effect and surface evanescent waves – HoBun Chan Fri. October 24th, 2008
12:30 pm-1:30 pm

The miniaturization of mechanical devices opens new opportunities for investigating and exploiting novel phenomena that occur for components in close proximity. The Casimir force, for example, originates from the zero-point quantum fluctuations of the electromagnetic fields. I will describe experiments that investigate the Casimir effect in micromechanical devices. In particular, we demonstrate the strong boundary dependence of the Casimir force on silicon surfaces with an array of nanoscale trenches. In another effort, subwavelength structures are fabricated on the surface of metal films to strongly modify their interaction with light. The evanescent fields channel the optical energy to specific locations, resulting in strong and localized field enhancement.

Charge Transport Phenomena in MilliKelvin Germanium and Detectors of the Cryogenic Dark Matter Search – Kyle Sundqvist Mon. October 20th, 2008
12:30 pm-1:30 pm

The Cryogenic Dark Matter Search (CDMS) seeks to detect putative weakly-interacting massive particles (WIMPS), which could explain the dark matter problem in cosmology and particle physics. By simultaneously measuring the number of charge carriers and the energy in non-thermalized phonons created by particle interactions in intrinsic Ge and Si crystals at a temperature of 40 mK, a signature response for each event is produced. This response, combined with phonon pulse-shape information, allows CDMS to actively discriminate candidate WIMP interactions with nuclei apart from electromagnetic radioactive background which interacts with electrons. The challenges associated with these techniques are unique. Carrier drift-fields are maintained at only a few V/cm,

High Resolution Spectroscopy of the Quantum Hall Liquid – Oliver Dial Mon. October 13th, 2008
12:30 pm-1:30 pm

The single particle spectrum of an electronic system is a measure of the ease of inserting a single, whole electron into the system at a particular energy. A peak in this spectrum indicates that there is a long-lived state available for electrons at that particular energy — in essence, that it is possible to form a quasiparticle at that energy. By revealing the energies of the quasiparticles, these spectra teach us about the many-body ground state of the system. However, experimental difficulties have prevented the measurement of this spectrum in two dimensional electron systems until recently. I will present tunneling measurements of the single particle spectrum of a 2D system in a Gallium Arsenide quantum well performed using a novel pulsed technique called timed domain capacitance spectroscopy.

Pedigrees and Partition Functions – Joseph Abraham Mon. October 6th, 2008
12:30 pm-1:30 pm

I will review some key concepts and computations in statistical genetics and discuss some analogies with the calculations on disordered spin systems. No prior knowledge of genetics is assumed.

Electrical, Mechanical, and Electromechanical Studies of Carbon Nanotubes and Graphene – James Hone Wed. September 24th, 2008
12:30 pm-1:30 pm

This talk will report on studies of the properties of carbon nanotubes of known chiral index, as determined by Rayleigh scattering spectroscopy. These properties include the mechanical stiffness, the electromechanical response, and basic electrical transport properties. The behavior of heterojunctions between nanotubes of different chiral indices will also be described. We have measured the mechanical properties of suspended graphene sheets, which show ultrahigh stiffness and the highest tensile strength ever measured. Finally, I will describe recent work on the use of graphene for high-frequency resonators.

Heterostructured quantum dots: growth and characterization – Kurt Eyink Mon. September 15th, 2008
12:30 pm-1:30 pm

Quantum dots (QDs) have been receiving considerable attention lately due to the unique properties, which arise due to the confinement of the electron and holes in a lower band gap material. The InAs on GaAs material system is one of the most studied combinations in which quantum dots form during epitaxy. These QDs form in a Stranski Krastanov manner via a self-assembly process in which the dots nucleate at a critical adatom coverage on a wetting layer of InAs. QDs may be vertically aligned by using the residual strain above a buried dot layer to enhance the nucleation of the second layer of dots.

Collective molecular motor using chiral liquid crystalline thin films – Hiroshi Yokoyama Mon. September 8th, 2008
12:30 pm-1:30 pm
Near-field optical scanning spectroscopy of photonic nanostructures – Alexander Mintairov Mon. April 21st, 2008
12:30 pm-1:30 pm

I will describe the experiments of using high spatial resolution near-field temperature-dependent magneto-photoluminescence to study optical and structural properties of variety semiconductor quantum dots emitting from violet to near-infrared. The probing of the mode fields in micro-disk and photonic crystal cavities using near-field technique will also be discussed.

Ab-initio Assisted Process and Device Simulation for Nanoelectronic Devices – Wolfgang Windl Mon. April 14th, 2008
12:30 pm-1:30 pm

The continuing miniaturization of traditional semiconductor devices deep into the nano-realm and novel concepts such as molecular devices require an unprecedented attention to the detailed geometry and electronic properties on the atomic scale. This talk will examine the role of atomistic modeling – mostly on the basis of quantum mechanical ab-initio methods – for current and future semiconductor process and device simulations. First, we will discuss atomistic enhancements of traditional process modeling to include nano-scale effects and the nanoscale characterization problem for conventional devices, where traditional characterization techniques cannot provide the needed information anymore. We discuss a coupled experimental-theoretical approach based on analytical transmission electron microscopy techniques (Z-contrast spectroscopy and electron energy-loss spectroscopy) that can detect single dopant atoms and even allow to “see”

Some examples of theory and computation of properties of transition metal nitrides – Sanjay Khare Mon. April 7th, 2008
12:30 pm-1:30 pm

A convergence of many factors has caused the emergence of growing synergy between theoretical and experimental research in condensed matter and materials science. Our current research interests that are benefiting from this symbiosis will be briefly discussed. Part of this co-development is due to first principles computational approaches that have ripened to such a degree that they can simulate materials properties with predictive power. With such ab initio methods, macroscopic properties can be theoretically correlated to microscopic causes such as bonding between individual atoms. As a specific example of such theoretical work, using density functional theory (DFT) we will show the structure of the recently discovered noble metal nitride of Pt and N.

Laboratory studies of atmospheric aerosol nucleation – Shan-Hu Lee Mon. March 31st, 2008
12:30 pm-1:30 pm
Flow diagram and Quantum critical behavior of the two-dimensional metal-insulator transition (2DMIT) – Alex Punnoose Mon. March 24th, 2008
12:30 pm-1:30 pm

Early speculation that an electron gas in two dimensions is always an insulator was upset when experiments in relatively high mobility systems showed signs of metallic behavior. Systematic experiments forced us to re-examine the interplay between electron-electron interactions and disorder. I will show that a disordered Fermi-liquid model can comprehensively describe the observed transport properties of the 2DMIT. In particular, it will be shown that the MIT corresponds to a quantum critical point (QCP), whose existence was recently uncovered theoretically. Predictions for the thermodynamic properties in the vicinity of the QCP of the 2DMIT will be discussed. New experimental evidence will be presented in support of this scenario.

Universal Gelation of Particles with Short-ranged Attraction – Peter J. Lu Thu. February 28th, 2008
12:30 pm-1:30 pm

Nanoscale or colloidal particles are exceptionally important in many realms of science and technology. They can dramatically change the properties of materials, imparting solid-like behavior to a wide variety of complex fluids, from yoghurt to cast ceramics. This behavior arises when particles aggregate to form mesoscopic clusters and networks. The essential component leading to aggregation is an interparticle attraction, which can be generated by many physical and chemical mechanisms. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation (DLCA), long-understood as a purely kinetic phenomenon, which can form solid-like gels at arbitrarily low particle volume fraction,

Metallic and Magnetic Nanostructured Thin Films Mon. February 25th, 2008
12:30 pm-1:30 pm

The correlation between structure and magnetism in magnetic materials continues to offer exciting opportunities at the nano-scale. For example the fabrication of novel magnetic materials in ultra-thin film form has led to perpendicular magnetic anisotropy and in some cases also enhanced magneto-optical behavior. In addition, understanding magnetic anisotropy is important to tailor desired properties for a given function. Thus, research on nano-magnetism is driven by its fundamental scientific importance as well as possible applications. For example, the magneto recording industry has projected stored areal-data-densities in the tera-bit/inch-square range for the next few years. As areal density grows at unprecedented rates,

Macrophase ordering in ionomers under external potential – Elshad Allahyarov Mon. February 18th, 2008
12:30 pm-1:30 pm

Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion-like ionomer by the imposition of a strong electric field. We observe that proton transport through this polymer electrolyte membrane is accompanied by morphological changes that include the formation of structures aligned along the direction of the applied field. The polar head groups of the ionomer side chains assemble into clusters, which then form rod-like formations, and these cylindrical structures then assemble into a hexagonally ordered array aligned with the direction of current flow. For dry ionomers, at current densities in excess of 1 A/cm2 these rod-like clusters undergo an inner micro-phase separation,

Ubiquity of Entanglement – Stanislaw Szarek Mon. February 11th, 2008
12:30 pm-1:30 pm

Entanglement is thought to be the critical resource in quantum computing and quantum communication. We explain how this physical concept is related to ideas and problems in mathematics, and particularly in functional analysis, convex analysis and high- dimensional geometry. This allows to show that the phenomenon of entanglement is ubiquitous: for example, for 8 qubits (one may say, a qubyte), the proportion of the states that are entanglement-free is smaller than 10^(-19990), when measured by the standard Euclidean volume. One may similarly compare various classes of quantum maps (or channels), related with dynamical changes of the physical system.

Bilayer Quantum Hall Effect – Bahman Roostaei Mon. February 4th, 2008
12:30 pm-1:30 pm

In a closely spaced double quantum well (DQW), electrons are thought to form an interlayer coherent state when a perpendicular magnetic field is applied such that the total Landau level filling factor one. The low energy topological excitations of the electron gas in these structures includes charged pseudo-spin vortices and anti-vortices. Using the Hartree-Fock approximation, we show that there are new excited states with interwoven spin and pseudo-spin and that their presence in the system can explain new experimental results. The excitations of DQW’s (called merons) also have important effects on transport in these systems. These objects carry charge, vorticity,

Nanotubes beyond carbon: theory of gallium nitride and boron nanotubes – Sohrab Ismail-Beigi Mon. January 28th, 2008
12:30 pm-1:30 pm

Although atomically-thin nanotubes of other elements are now fabricated, carbon nanotubes are probably the best known examples of nano-materials. They provide ideal cases for studying a variety of nanoscopic effects: e.g., geometric and quantum confinement of electrons, enhanced Coulomb interactions in one dimension, and curvature effects. However, as materials for use in electronics applications, carbon nanotubes have some drawbacks. Here, we describe our recent theoretical work on the physical properties of two other classes of nanotubes based on gallium nitride and boron, highlighting what is novel and potentially useful in each case.

Near-field optical scanning spectroscopy of photonic nanostructures – Alexander Mintairov Mon. December 3rd, 2007
12:30 pm-1:30 pm

I will describe the experiments of using high spatial resolution near-field temperature-dependent magneto-photoluminescence to study optical and structural properties of variety semiconductor quantum dots emitting from violet to near-infrared. The probing of the mode fields in micro-disk and photonic crystal cavities using near-field technique will also be discussed.

Phonons in ZnGeN2 and related materials: experiment and theory – Tim Peshek and Tula Paudel Mon. November 19th, 2007
12:30 pm-1:30 pm

This seminar will consist of three practice talks for the MRS Fall meeting. Tim will first talk about an experimental determination of the free energy of formation of GaN from its elements. In his second talk, he will present growth of ZnGeN2 single crystals and their micro-Raman spectra, which will be compared to Tula’s calculations of these spectra. Tula will describe the calculation of phonons and related infrared and Raman spectra in the family of II-IV-N2 nitrides with II=Zn, IV=Si,Ge, and Sn. His talk will focus on infrared reflectance spectra on ZnSiN2 and the trends of the phonon spectra in this series of materials.

Thermo-acoustic waves near the liquid-vapor critical point : the sound of heat – Pierre Carlès Wed. November 14th, 2007
12:30 pm-1:30 pm

A fluid near its liquid-vapor critical point exhibits puzzling heat transfer dynamics, as temperature relaxation becomes faster and faster near the critical point (an observation which contradicts the expected critical slowing down of diffusive processes). The reason behind this seemingly paradoxical behavior is a fourth mode of heat transfer named the Piston Effect, which results from a subtle coupling between local heat diffusion phenomena and long-scale acoustic propagation. So far, only the average effect of this acoustic propagation had been observed in experiments and modeled theoretically. Recently however, the Japanese team of Akira Onuki achieved a direct observation of the acoustic waves produced by a local and rapid input of heat in a near-critical fluid,

Identifying defect structures by first principles XANES – Sukit Limpijumnong Mon. November 12th, 2007
12:30 pm-1:30 pm

First principles calculations allow one to model materials from fundamental quantum mechanics without bias. Because the calculations contain detailed atomic coordinates and electron distributions as well as their wave functions, most measurable properties including the X-ray Absorption Near Edge Structures (XANES) can be simulated. We will present results of our recent research activity in utilizing (pseudopotential) first principles calculations of defects by supercell approach to XANES spectra. By comparing the simulated XANES with experimental measurements detailed geometry of defects can be positively identified. The examples covered in my talk include: (a) the identification of the manganese location in PZT crystal [1],

Synthesis and Characterization of GaGdN – Cammy Abernathy Mon. November 5th, 2007
12:30 pm-1:30 pm

The past decade has seen a rise in interest in the area of dilute magnetic semiconductors (DMSs). In part, this has been directed by a push towards harnessing the spin of electrons for device usage in the field of spintronics. This field has potential for increased speed, power efficiency and storage density in devices. Although transition metal dopants have been extensively studied in GaN-based DMS materials, rare earth dopants are coming under increasing scrutiny. GaGdN, with a reported colossal magnetic moment, Curie temperature above room temperature, and much lower dopant concentration level than transition metal doped GaN has sparked increased interest in rare earth dopants.

Controlling light on the nanoscale: imaging and spectroscopy with ultrahigh spatial and temporal resolution – Markus Raschke Mon. October 29th, 2007
12:30 pm-1:30 pm
Polyelectrolytes: A Field-Theoretic Perspective – Yuri Popov Mon. October 15th, 2007
12:30 pm-1:30 pm

Field-theoretic methods are not new to polymer physics. Their basic idea is to replace the particle-based description of the polymer in terms “monomers” or “beads” with a description in terms of collective variables, or fields (e.g. “density field” or “charge field”). Even the lowest order of this approach – the mean-field approximation – has already produced some remarkable results, most notably for block copolymers. However, for certain polymer systems, such as polyelectrolytes, the mean-field approximation in most cases produces trivial results, and fluctuations must be taken into account. Recent developments in analytical and numerical field-theoretic techniques allowed a new, “beyond the mean field”,

“Spinning” and “twisting” a light beam and other wavefront-shaping tricks performed with suitably patterned liquid crystals – Lorenzo Marrucci Thu. October 11th, 2007
12:30 pm-1:30 pm

The so-called “helical modes” of an electromagnetic wave are characterized by a helical shape of the wavefront. They carry quantized angular momentum of an orbital kind, as opposed to the spin-like angular momentum that can be associated with circularly polarized waves. In a way, one can consider helical modes as “twisted” light beams that are “spinning upon themselves” while propagating. In the talk I will present a novel method for generating helical waves of light by letting a circularly-polarized non-helical wave pass through an azimuthally inhomogeneous birefringent plate made of a suitably patterned liquid crystal, a device dubbed “q-plate”. The q-plate converts the variation of spin angular momentum associated with the switching of light polarization handedness into orbital angular momentum,

Disorder, Interactions, and Crossovers in Quantum Dots – Ganpathy Murthy Mon. October 8th, 2007
12:30 pm-1:30 pm

In ballistic/chaotic quantum dots the single-particle states are controlled by Random Matrix Theory below the Thouless scale. The three pure Random Matrix ensembles correspond to dots without an orbital B field and no spin- orbit coupling (Orthogonal), dots without an orbital field and with spin-orbit coupling (Symplectic), and dots with an orbital field (Unitary). At weak coupling, the low-energy physics is described by the Universal Hamiltonian. We will be concerned with interacting electrons in a dot in a crossover between two RMT ensembles, such as the Orthogonal and the Unitary, and we will show that in the crossover, special RMT correlations develop and the states become strongly correlated in the electronic sense.

Looking for the Spin Hall Effect in all the Wrong Places – Nitin Samarth Mon. October 1st, 2007
12:30 pm-1:30 pm

The spin and anomalous Hall effects are related phenomena that arise from spin-dependent electrical transport in solids in the presence of spin-orbit coupling. Conventional wisdom has motivated many experimental studies of these effects in systems where spin-orbit coupling effects are inherently strong. I will describe two recent experiments on systems with manifestly weak spin-orbit coupling that suggest a need to cast a wider net in explorations of the spin and anomalous Hall effects. Our studies surprisingly reveal: a. An electrically-tunable anomalous Hall effect in paramagnetic 2DEGs formed in a magnetically-dilute, wide bandgap semiconductor quantum well; [1] b. A room temperature spin Hall effect in an n-doped wide bandgap semiconductor.

Defect structures in nematic liquid crystal shells – Alberto Fernandez-Nieves Mon. September 24th, 2007
12:30 pm-1:30 pm

We use double emulstions drops to experimentally realize a system to investigate the defect structure in spherical shells of nematic liquid crystal. The ground state of this system is predicted to exhibit a tetrahedral arrangement of four surface defects in a structure reminiscent of a baseball. Instead, we find a much richer set of coexisting defect structures dictated by the inevitable thikness of even the thinnest shells, and the resultant variation of the shell thickness around the sphere. These structures are characterized by a varying number of disclination lines and pairs of surface point defects, one each on the inner and outer surfaces of the nematic shell.

Frustration Phenomena in Liquid Crystals in Contact with Patterned Substrates – Tim Atherton Mon. September 17th, 2007
12:30 pm-1:30 pm

Recent experimental advances in fabricating micropatterned surfaces offer the display industry the possibility of constructing new types of display with such desirable properties as bistability, enhanced contrast ratio and reduced power consumption. The physical principles that underlie these displays are rather more complex than the conventional Twisted Nematic (TN) display, and it is essential to understand their operation in order to produce optimized devices. In this talk, I explore an industrial problem involving the fabrication of switchable liquid crystal lenses using striped surfaces, and show how the solution provides an insight into the fundamental physics of liquid crystals including elastic anisotropy and the energetics of smectic structures.

Elasticity of Polymer Gels (and a Cytoskeleton in the Closet) – Gavin Buxton Mon. September 10th, 2007
12:30 pm-1:30 pm

Using computer simulations we can investigate the elastic properties of random elastic networks of struts. As the connectivity of the network increases a transition is observed between systems which deform through the bending of the struts to systems which deform through the stretching of the struts. I would like to talk a little about this bending to stretching transition and the implications of this transition to materials science and biology.

Odd circuits: stability and jamming in hard granular materials – Nicolas Rivier Mon. May 21st, 2007
12:30 pm-1:30 pm

A dry granular material is modelled as a graph of spherical grains linked by purely repulsive contacts. Its stability (jamming) is insured by odd circuits that prevent the grains from rolling on each other. A topological dynamical matrix is associated with the graph. The odd circuits gathered on the largest R-loop are responsible for the high density (independent of the size of the material and of the dimension) of low-energy excitations and for the extended corresponding eigenstates in (disordered) granular matter at the jamming transition.

Some problems in the non-linear optics of liquid crystals – Tim Sluckin Mon. April 30th, 2007
12:30 pm-1:30 pm
Michelson Postdoctoral Lecture – Adam Bolton Mon. April 30th, 2007
12:30 pm-1:30 pm
ACES seminar: Molecularly Engineered Interfaces for Organic Optoelectronics – Zakya Kafafi Mon. April 23rd, 2007
12:30 pm-1:30 pm
Ongoing Mysteries in Astrophysics – Donald Driscoll Wed. April 11th, 2007
12:30 pm-1:30 pm

We are at the brink of a Golden Age of Astrophysics with the promise of answers to many long-outstanding questions, including:
What is the nature of Dark Matter?
What source powers Active Galactic Nuclei?
Where do Gamma-Ray Bursts come from?
Where do the highest energy Cosmic Rays come from?
With an unprecedented number of experiments both active and coming online, there is a real hope that many of these questions may be answered in the near future.
I have been lucky enough to be associated with some of the world’s most advanced astrophysical experiments.

Bioinspired molecular optoelectronics – Volodimyr Duzhko Fri. April 6th, 2007
12:30 pm-1:30 pm

The versatility and rich functionality of living cells in Nature inspire researchers from many disciplines. For example, artificial replication of photosynthesis, which is an efficient solar-to-chemical energy conversion process in plants, promises a breakthrough in reducing our dependence on exhaustible and environmentally harmful fossil energy sources. I will discuss several novel concepts in bioinspired molecular electronics and optoelectronics recently enabled by the controlled self-assembly of artificial organic molecules into functional architectures with preprogrammed photo-physical properties. The molecular-scale picture of the underlying fundamental physical processes such as the photo-excited states relaxation, energy and charge carrier transport, and interfacial charge transfer, in these self-assembled heterogeneous architectures will be described.

New avenues to computational technology: novel spin transport effects at the nanoscale – Ewelina Hankiewicz Mon. April 2nd, 2007
12:30 pm-1:30 pm

The exponential increase of computational speed over time through miniaturization, known as Moore’s law, is now a thing of the past. This increase in speed is no longer due to our ability to make smaller devices, but in the control of heat dissipation. This is the so called problem 2020 when the temperature of a miniaturized computer, based on current power consumption trends, would be equal to the temperature of the sun. Clearly, this would be hazardous to the health of the consumer. Manipulation of electron spins through electric fields (so called spintronics) is one of the avenues sought for a new generation of devices.

From nano to micro: hierarchical ordering at the nanoscale – Ezekiel Johnston-Halperin Mon. March 19th, 2007
12:30 pm-1:30 pm

The overall goal of controlling structural and electronic materials properties at nanometer length scales can be thought of as the intersection of two distinct but correlated challenges. The first is the synthesis/fabrication of individual nanoscale structures and the second is the arrangement of those structures into tailored nano- and micro-scale assemblies. Motivated by these twin challenges, the development of the superlattice nanowire pattern transfer (SNAP) technique has enabled the fabrication of highly ordered arrays of hundreds of nanowires (both metallic and semiconducting) at pitches down to 16 nm and aspect ratios up to 106. As a result of the hierarchical ordering of these assemblies (ranging from nanometer to micrometer length scales),

Current, maximum power and optimized efficiency of Brownian heat engine – Mulugeta Bekele Fri. March 2nd, 2007
12:30 pm-1:30 pm

A tiny heat engine is modeled as a Brownian particle in a sawtooth potential (with or without load) moving through a highly viscous medium driven by the thermal kick it gets from alternately placed hot and cold heat reservoirs. We found closed form expression for the current as a function of the parameters characterizing the model. Depending on the values these model parameters take, the engine is also found to function as a refrigerator. Expressions for the efficiency as well as for the refrigerator coefficient of performance (COP) are also reported. Study of how these quantities depend on the model parameters enabled us in identifying the points in the parameter space where the engine performs with maximum power and with optimized efficiency.

Unraveling Electronic and Spin Structure with Photoemission – Oleg Krupin Mon. February 26th, 2007
12:30 pm-1:30 pm

Angle-resolved photoemission is widely recognized as a versatile tool for studies of the electronic structure and Fermi surface topology of new structures and materials with electronic and magnetic properties potentially interesting for modern electronics and future spintronics applications. In my talk I will present two examples showing that it also can be used to study effects directly associated with the spin structure. At the example of the rare-earth metal surfaces I will consider an appearance of the Rashba effect at real surfaces/interfaces and associated electron and spin structures. In the past this effect has attracted considerable attention due to proposing it for manipulating the electron spin by an electric field.

Electrical Transport in Individual Nanostructures – Zhixian Zhou Tue. February 20th, 2007
12:30 pm-1:30 pm

Understanding the fundamental physical properties of individual nano scale materials is an essential and fundamental part of the research in nanoscience, since these nanostructures are not only potential building blocks of nanotechnology but also provide unique opportunities for studying a wealth of quantum mechanical phenomena. We have investigated various one-dimensional and quasi one-dimensional nano scale materials employing state-of-the-art nanofabrication techniques and electrical transport measurements for a wide temperature range. As two examples, I will present and discuss our experimental results on in situ fabricated individual single wall carbon nanotube devices and individual superconducting nanowires of NbSe2.

Ultrafast non-equilibrium phenomena of the integer quantum Hall system – Keshav Dani Mon. February 19th, 2007
12:30 pm-1:30 pm

The non-equilibrium properties of a system are typically understood by assuming instantaneous scattering between particles. However, for very early (femtosecond) timescales, one sees that the interactions are not instantaneous. They are instead the quantum interference of particle wavefunctions which last for a finite duration in time. Within this time, interactions are reversible; fundamental approximations, like Fermi’s Golden Rule, fail; and the system forms coherences that govern the non-linear optical response. I will present some recent studies of the ultrafast non-linear optical response of the integer quantum Hall system. We access the as-yet-incomplete interaction between photoexcited carriers and collective excitations of the quantum Hall system,

Heisenberg’s XY model and the Development of Mammalian Visual Cortex – Peter Thomas Mon. February 12th, 2007
12:30 pm-1:30 pm

The architecture of the primary visual cortex, the first cortical area devoted to processing visual information, exhibits fascinating spatial organization. Individual nerve cells in this area are strongly tuned to respond to specific orientations (edges, contours, line segments) in the visual field. The map of preferred orientation as a function of position in the cortical sheet shows a mixture of order and disorder on different spatial scales. The patterns observed experimentally show regions of smooth progression in preferred orientation interspersed with line and point singularities. A natural framework for studying the formation of these patterns is a variant of Heisenberg’s planar magnetic or XY model in which the Hamiltonian includes cooperative local interactions and longer range competitive interactions.

Engineering Defect Dynamics in Liquid Crystal Cells – Rolfe Petschek Mon. February 5th, 2007
12:30 pm-1:30 pm

Defects and slow dynamics of defects in a type of liquid crystal cell that allows the manufacture of wide-viewing angle flat panel liquid crystal displays is a very significant problem or “deal breaker”. I will describe the physics of this cell and its planar degenerate (“XY”-like) order parameter. How and if the dynamics of defects in this order parameter can be changed with reasonable (manufactureable) changes in the liquid crystal cell is of significant practical as well as scientific interest. Gaps in the electrodes, easily realized by lithography, result effective fields on the XY order parameter and allow manufacture of such cells.

Transition metal and rare-earth nitrides: a new route to magnetic semiconductors – Walter Lambrecht Mon. January 29th, 2007
12:30 pm-1:30 pm

Transition metal and rare-earth nitrides have both potential as magnetic semiconductors. I will present two case studies: Mn-doped ScN, which, unfortunately, might be a spinglass instead of a ferromagnetic semiconductor, and Gd-doped GaN, which was claimed to have COLOSSAL magnetic moments. Along the way, I will discuss recent computational advances to study magnetic exchange interactions and strongly correlated systems, such as non-collinear magnetic linear response calculations and LSDA+U.

Nonlinear Optics in Multilayer Polymer Films – Kenneth Singer Mon. November 27th, 2006
12:30 pm-1:30 pm

Case Western was recently awarded a National Science Foundation Science and Technology Center. This 5 year ~$20M once renewable grant is housed in the Department of Macromolecular Science and Engineering and is named the Center for Layered Polymer Systems (CLiPS). The center will focus its efforts on research and education relating to a polymer coextrusion process capable of fabricating multilayer polymer films. The films can possess up to thousands of layers with each layer as thin as a few nanometers. One of the three scientific/engineering platforms is aimed at exploring optical properties and applications of these films. In this seminar,

Terahertz spectroscopy of InMnAs – Jason Deibel Mon. November 13th, 2006
12:30 pm-1:30 pm

Diluted magnetic semiconductors (DMS’s) based on III-V semiconductors such as GaAs and InAs have drawn considerable interest over the past two decades as possible materials for use in spintronic devices. These are devices in which both the charge and spin of the electron are exploited. Future DMS-based devices could include hard drives, spin-based transistors, and magnetic random access memory (MRAM). In order for such applications to be realized though, a detailed understanding of the carrier dynamics and spin magnetism of DMS’s must be developed. For magnetic semiconductors such as GaMnAs and InMnAs, it is crucial to study the influence of the Mn dopant on the electronic and optical characteristics of the parent semiconductor.

Semiconductor Spintronics – Igor Zutic Mon. November 6th, 2006
12:30 pm-1:30 pm

Spin-polarized transport and the related field of spintronics [1] rely on lifting of spin degeneracy in various physical properties. A different behavior for “spin up” and “spin down” in metallic magnetic structures has been shown to lead to large magnetoresistive effects which were successfully applied to computer hard drives and nonvolatile magnetic random access memory. On the other hand, recent materials progress in semiconductors has opened a possibility to consider novel effects in spin-polarized transport which could also be useful for spin-based logic. We focus on our proposal for bipolar spintronics in which carriers of both polarities (electrons and holes) contribute to spin-charge coupling.

Fabrication and Characterization of Functional Nanostructures and Applications – Richard Mu Mon. October 30th, 2006
12:30 pm-1:30 pm

My talk may consist of two folds. First, I would like to take this unique opportunity to give a brief introduction of Fisk University, and research and educational activities in general. Then, I would like to highlight our on-going research activities of the nanoscience research team in the areas of /1/) */_Fabrications_/* of complex/functionalized semiconductor, metal, glass, and glass-ceramic materials in zero-, one-, and two-dimensions with pulsed lasers and electron-beam deposition (PLD/PED), and with physical vapor transport techniques; /2/) */_Materials Characterizations_/* to evaluate and understand physical, chemical, and optical properties of low dimensional nanostructures; and /3/) */_Sensory Developments_/* that include high efficiency nanostructured solar cells,

Is the Adiabatic approximation Inconsistent? – Solomon Duki Mon. October 23rd, 2006
12:30 pm-1:30 pm

The adiabatic theorem is the basis of an approximation scheme that was discovered at the dawn of quantum mechanics and that has been in widespread and continuous use ever since. Applications range from two-level systems (such as nuclei undergoing magnetic resonance or atoms interacting resonantly with a laser field) to quantum field theory (where a low-energy effective theory is derived by integrating out fast, high-energy degrees of freedom). Two decades ago, Berry uncovered the beautiful geometric structure underlying the adiabatic approximation, leading to a resurgence of interest in the subject and to new applications. More recently, it has been proposed that Berry phase effects lead to quantum phase transitions that lie outside the usual Landau-Ginzburg-Wilson paradigm.

X-Ray Emission and Resonant Inelastic X-ray Scattering: new probes of electronic structure in complex materials – Kevin Smith Mon. October 16th, 2006
12:30 pm-1:30 pm

Detailed electronic structure measurements are required in order to fully understand many physical phenomena in solids. While photoemission spectroscopy is often the electronic structure probe of choice, there are many sample and environmental constraints that must be satisfied before meaningful data can be obtained with this spectroscopy. I will discuss the development of a new technique, high resolution synchrotron radiation-excited soft x-ray emission spectroscopy, which can probe solid state electronic structure in circumstances where photoemission spectroscopy is inapplicable. When the incident synchrotron radiation is tuned close to a core level absorption edge, the x-ray emission process is more appropriately described as a resonant x-ray scattering event,

Solution Processable Organic Photovoltaics – Sean Shaheen Mon. October 9th, 2006
12:30 pm-1:30 pm

Organic photovoltaics (OPV) have demonstrated power conversion efficiencies under AM1.5 illumination of 5%, a value high enough to attract attention from industry and national laboratory researchers. I will discuss issues in the photophysics, charge transport, molecular morphology and band structure that limit current devices and discuss new materials and device approaches that may yield higher efficiencies.

Electromechanical coupling effects in semiconductor heterostructures – Lok C. Lew Yan Voon Mon. October 2nd, 2006
12:30 pm-1:30 pm

Electromechanical coupling effects are known to significantly impact the physical properties of wurtzite (nitrides, ZnO, …) semiconductor nanostructure devices. However, there has not been to date a systematic study of the fully-coupled multiphysics problem and there are discrepancies within and between experimental and theoretical studies. We present a systematic study of various contributions to the problem for quantum-well structures. Some of the results obtained so far include a resolution of discrepancies among theoretical calculations and the original study of dynamic and nonlinear piezoelectric effects.

Simulating non-equilibrium processes over extended time and length scales using parallel kinetic Monte Carlo and parallel accelerated dynamics – Jacques Amar Tue. September 26th, 2006
12:30 pm-1:30 pm

A long-standing obstacle to the understanding of non-equilibrium processes in condensed-phase systems is that many important processes occur on time-scales that are not easily accessible with conventional methods such as molecular dynamics. In addition, simulations over extended length-scales are often necessary. Accordingly, the development of methods which can extend non-equilibrium simulations over longer time- and length-scales are of interest. We first discuss recent progress in developing and applying efficient algorithms for parallel kinetic Monte Carlo simulations of non-equilibrium processes. Applications to simulations of epitaxial growth and island coarsening are discussed. We then discuss recent progress in extending the recently developed temperature-accelerated dynamics method to larger systems via parallel accelerated dynamics.

Expulsion of bend from a smectic liquid crystal: Anology to a type-I superconductor – Ruiting Wang Mon. September 18th, 2006
12:30 pm-1:30 pm

Using an atomic force microscope to nanopattern a substrate for liquid crystal alignment, a bend distortion is imposed on a liquid crystal. In regions of large bend the smectic-A phase melts into the nematic phase, and the width of the melted region is measured as a function of temperature. The results are consistent with type-I superconducting (nematic to smectic-A) behavior, wherein a large magnetic field (bend or twist distortion) induces an order to disorder transition. A model that accounts for non mean-field behavior is presented.

Suppression of superconductivity in the Hubbard model at intermediate coupling by buckling and breathing phonons – Mark Jarrell Mon. September 11th, 2006
12:30 pm-1:30 pm

Recent quantum Monte Carlo Dynamical Cluster calculations show that the Hubbard model displays superconductivity at temperatures relevant to the cuprate high temperature superconductors [1] suggesting that spin fluctuations may be responsible for superconductivity in these materials [2]. Nevertheless, recent experiments (ARPES, isotope effect,…) show evidence of very strong electron-phonon interactions. We study the role of buckling, breathing and local phonon modes in the doped Hubbard model. We find that the synergistic interplay of antiferromagnetic correlations and the electron-phonon interaction strongly enhances polaron formation, antiferromagnetism, and the strength of the spin-mediated pairing interaction[3]. Despite the latter effect, the buckling and breathing phonons suppress the superconductivity in the region of parameter space relevant for cuprate superconductors,

Michelson Postdoctoral Prize Lecture – Nicole Bell Mon. May 1st, 2006
12:30 pm-1:30 pm

Astrophysical Neutrinos: Revealing Neutrino Properties at the Highest Energies

Phonon Anharmonicity and Phase Transitions in Bulk and Nanoparticle ZnSe under High Pressure – Bernard Weinstein Mon. April 24th, 2006
12:30 pm-1:30 pm

Resonant multi-phonon interactions strongly modify the life-times of the TO(Gamma) and LO(Gamma) normal modes in many bulk semiconductors.[1] The optically active confined and surface/interface modes in nanoparticles[2] are subject to enhanced anharmonic coupling because of the loss of q-conservation, the mixing of LO and TO polarities, and the presence of surfactant. Raman scattering experiments, carried out under variable pressure conditions, are a revealing probe of these phenomena due to the ability to tune the vibrational modes in and out of resonant decay channels, and to cycle through structural phase transitions [3] that often modify volume and/or surface disorder. Recent work at Buffalo has shown that unusually strong changes occur in the Raman spectra of bulk and nanorod ZnSe under applied hydrostatic pressure.

The structure of a vortex and critical current through the BCS-BEC crossover – Mohit Randeria Mon. April 10th, 2006
12:30 pm-1:30 pm

Recently, there has been dramatic progress in experimental studies of the BCS-BEC crossover in trapped atomic Fermi gases. In this talk I will begin with a brief overview of the field and comparison of experiments and theory. I will then describe recent theoretical work on the evolution of structure of the vortex through the BCS-BEC crossover and on calculations of the critical current which show that the unitary Fermi gas (with infinite scattering length) is the most robust superfluid state in the entire crossover.

Fractional vortices and composite domain walls in nanomagnets – Oleg Tchernyshyvov Mon. April 3rd, 2006
12:30 pm-1:30 pm

We provide a simple explanation of complex magnetic patterns observed in ferromagnetic nanostructures. To this end we identify elementary topological defects in the field of magnetization: ordinary vortices in the bulk and vortices with half-integer winding numbers confined to the edge. Domain walls found in experiments and numerical simulations in strips and rings are composite objects containing two or more of the elementary defects.

Molecular Imaging with Ultrafast Electron Diffraction – Chong-Yu Ruan Mon. March 27th, 2006
12:30 pm-1:30 pm

Ultrafast molecular imaging represents an emerging frontier.In particular, recent developments in the ultrafast electron diffraction (UED) have demonstrated the ability to image the rearrangements of chemical bonds in complex systems with resolutions of ~0.01A and ~1 ps, respectively. These new limits provide the means for the determination of transient structures of molecules, surfaces and nanostructures, including reactive intermediates and nonequilibrium structures of complex energy landscapes. By freezing structures on the ultrafast timescale, we are able to develop concepts that correlate structure with dynamics. Examples include structure-driven radiationless process, and nonequilibrium structures exhibiting negative temperature, bifurcation, or selective energy localization in bonds.

Treading a Fine Line: One-Dimensional Semiconductor Physics in Carbon Nanotubes – Michael Fuhrer Mon. March 20th, 2006
12:30 pm-1:30 pm

The growth of individual, long (> 1 mm), high-quality single- or few-walled carbon nanotubes (CNTs) on substrates by chemical vapor deposition has allowed the careful study of the intrinsic electronic properties of this material. Recently we have made electrical measurements on semiconducting CNTs up to 800 microns in length in a field-effect transistor (FET) geometry, and determined that the charge carrier mobility is greater than 100,000 cm2/Vs at room temperature, exceeding that of the best known semiconductors. Analysis of the FET behavior at higher drain bias indicates that semiconducting CNTs do not experience curren t saturation due to optical phonon emission (as observed in metallic CNTs) but rather show saturation of the carrier velocity at ~2 x 107 cm/s,

Electron Interactions and Phase Coherence in Metals – Norman Birge Mon. March 6th, 2006
12:30 pm-1:30 pm

At low temperatures, conduction electrons in disordered metals maintain quantum phase coherence over times often exceeding one nanosecond — several orders of magnitude longer than the time between elastic collisions. Phase coherence is broken by inelastic collisions, which also relax the energy distribution of the electrons toward thermal equilibrium. Theory predicts that the phase coherence time should increase as the temperature is lowered, whereas many experiments show a saturation of the phase coherence time at temperatures below 1 K. The issue of whether those observations reflect a fundamental, intrinsic decoherence mechanism, or an extrinsic, sample-dependent source of decoherence has been controversial.

Point defects in ZnGeP2 – Walter Lambrecht Mon. February 27th, 2006
12:30 pm-1:30 pm

ZnGeP2 is a semiconductor material used in nonlinear optical frequency conversion. To advance these applications it is necessary to gain a better understanding of the native point defects in this material. I will present results of our studies of the basic electronic structure of the main defects and their interactions and compare them with available experimental data mostly from electron paramagnetic resonance studies. One problem we have not yet fully resolved is the nature of the Zn-vacancy, which according to experimental data shows a distorted structure while local density functional calculations indicates an undistorted structure is preferred in energy. Possible ways to resolve this discrepancy will be discussed.

The determination Liquid Crystal Device parameters by means of renormalized transmission spectroscopic ellipsometry – Munehiro Kimura Mon. February 20th, 2006
12:30 pm-1:30 pm
Experiments with New Soft Solids – Patrick Mather Mon. February 13th, 2006
12:30 pm-1:30 pm

The design and synthesis of soft polymeric materials with tailored properties is an area of emphasis in our group. This talk will focus on two unique materials, covering synthesis through properties, revealing structure property relationships as we now understand them. (i) The first part of the talk will examine new liquid crystalline elastomers that allow either one-way or two-way shape memory behavior, depending on underlying phase behavior dictated by chemical composition. Uses of these materials as reversible embossing media and artificial muscles will be discussed. (ii) Next, I will discuss hybrid hydrogels that combine the inorganic-organic POSS moiety (polyhedral oligomeric silsesquioxane) with hydrophilic groups in a multiblock architecture.

Frontiers in spectroscopy with the scanning tunneling microscope – Jay Gupta Mon. February 6th, 2006
12:30 pm-1:30 pm

The scanning tunneling microscope is a versatile tool to study nanoscale structures with atomic resolution through a combination of manipulation and spectroscopic capabilities. By a process of inelastic scattering, tunneling electrons can probe vibrational, configuration and spin-flip excitations with single-atom sensitivity at low temperatures (T less than 5K). I will discuss examples where inelastic electron tunneling spectroscopy (IETS) has been applied to build isotope-selected molecule cascades, study two-state dynamics of molecular hydrogen, and probe magnetic spin-flip energies of single manganese atoms. Molecule cascades are precise arrangements of carbon monoxide molecules in which the motion of one molecule induces a cascade of motion similar to a row of dominos.

Electronic Properties of InSb Quantum Wells and Mesoscopic Structures – Michael Santos Mon. January 30th, 2006
12:30 pm-1:30 pm

In narrow-gap semiconductors, electrons have properties that are much different than in free space. For example, the effective mass in InSb is nearly two orders of magnitude smaller than the mass in free space. This property can be exploited in applications, such as magnetic read heads or ballistic transport devices, where a high mobility or a long mean free path is required. The strength of the interaction between an electrons spin and a magnetic field is also enhanced in InSb. The consequences of a small effective mass and large spin-orbit coupling are seen in far-infrared spectroscopy and charge transport measurements performed on structures with nanometer-scale dimensions in one or more directions.

Oscillatory interlayer coupling in Co/Pt multilayers with perpendicular anisotropy – Fengyuan Yang Mon. December 5th, 2005
12:30 pm-1:30 pm
Quantitative modeling of single-molecule RNA force-extension experiments – R. Bundschuh Mon. November 28th, 2005
12:30 pm-1:30 pm

Single-molecule force-extension experiments are an emerging tool for the study of biomolecules. For a molecule like RNA that has to fold into a specific structure in order to perform its biological function a crucial question is if such experiments can reveal this structure. I will show how the polymer physics of the backbone, the statistical physics of RNA folding, and the very detailed knowledge of the relevant free energy parameters can be combined into a computational model of such force-extension experiments. This model quantitatively reproduces experimental results for short molecules. However, for long molecules it shows that simple force-extension measurements are not able to reveal the structure of an RNA molecule due to a self-averaging effect.

Between gases and liquids: the paradoxes of near-critical fluidsdynamics – Pierre Carles Mon. November 21st, 2005
12:30 pm-1:30 pm

The thermophysical properties of fluids near their liquid-vapor critical point are governed by universal critical phenomena, formalized theoretically after the works of Kaddanoff, Widom and Wilson in the early seventies and afterwards. But the influence of these peculiar properties on the dynamics of fluids close to the critical point is a relatively new subject, which finds its early developments around 1990. In this talk, we will present how the paradoxical nature of such fluids, intermediate between gases and liquids, leads to complex and before-unexpected thermal and mechanical responses, among which a fourth mode of heat transfer still unknown some 15 years ago.

Low-dimensional Transport in Nanoscaled Materials – Philip Kim Mon. November 7th, 2005
12:30 pm-1:30 pm

The use of modern state-of-the-art device fabrication techniques and the development of new methods of nanosclae material synthesis/manipulation enable us to investigate at the mesoscopic scales. In these length scales the nanoscaled materials have exhibited a variety of unique physical phenomena due to the enhanced quantum confinement of electrons in reduced dimensions. In this presentation, we will discuss our recent investigation of mesoscopic transport phenomena in carbon nanotubes, nanowires, and graphene, where quantum mechanically enhanced low dimensional effects are predominant. The subjects include, (1) growth/manipulation of ultralong nanotubes and electrical characterization of them, (2) electric field effect in mesoscopic thermoelectric transport in nanotubes,

Mid-infrared Hall effect in ferromagnetic oxides and semiconductors – John Cerne Mon. October 31st, 2005
12:30 pm-1:30 pm

Strongly correlated materials ranging from diluted magnetic semiconductors (DMS) to transition-metal oxides, such as ruthenate perovskite (RP) compounds and high temperature superconductor cuprates, are revolutionizing fundamental concepts in condensed matter physics and show great potential for applications to spin-based electronics and multifunctional devices. These materials exhibit unusual properties, such as carrier-mediated magnetism, metamagnetism, quantum criticality and non-Fermi liquid behavior that continue to challenge the condensed matter community. Despite the wide range of properties exhibited by these materials, they all exhibit anomalous behavior in dc Hall effect measurements. In this talk I will discuss why one should and how one can explore the Hall effect as a function of frequency using magneto-polarimetry.

Cell signalling Biophysics of GTPase-protein interactions: an overview of ideas and ongoing activities – Matthias Buck Mon. September 26th, 2005
12:30 pm-1:30 pm
ZIP-ping for Dark Matter – Michael Dragowsky Mon. September 19th, 2005
12:30 pm-1:30 pm

Astrophysical evidence has long implied the existence of non-luminous matter on the scale of galaxies. In the last few years experimental cosmology has emerged as a precision science, providing further evidence for non-luminous matter on extragalactic distance scales. The evidence points to matter that is non-baryonic and non-relativistic in nature. Detection of this so-called dark matter in terrestrial detectors via scattering from nuclei is feasible provided sufficiently low energy threshold and the means to suppress electron recoil backgrounds is achieved. The Cryogenic Dark Matter Search collaboration has developed novel detectors with these capabilities. Athermal phonon sensors are patterned by photolithography onto thick semiconductor absorbing substrates along with ionization electrodes.

Ground- and Excited-State Attributes of Hexanuclear Rhenium(III) Chalcogenide Clusters – Thomas Gray Mon. September 12th, 2005
12:30 pm-1:30 pm
Michelson Postdoctoral Prize Lecture – Yaroslav Tserkovnyak Mon. May 9th, 2005
12:30 pm-1:30 pm

Lecture 1: SPONTANEOUSLY-SYMMETRY-BROKEN ARCHIMEDES SCREWS: In the first technical lecture, I will use the tool box developed in treating time-dependent magnetoelectronic problems to consider a more general class of nonequilibrium phenomena in heterostructures with arbitrary spontaneous symmetry breaking. Motivated by the richness of physics in magnetic nanostructures, we are led to explore analogous phenomena in other symmetry-broken systems. As a first step in this direction, I will discuss a systematic way to construct “Archimedes screws” that are operated by semiclassical control of local orders. The “pumping” is achieved by means of a time-dependent order parameter which characterizes a broken symmetry and is controlled by external fields.

Monte Carlo simulations of inhomogeneous order in nematic liquid crystal cells: optical applications – Antoni C. Mitus Tue. May 3rd, 2005
12:30 pm-1:30 pm

We will present the results of Monte Carlo simulations of nematic liquid crystals described by Lebwohl-Lasher-Rapini model. Detailed information on local order makes possible a calculation of diffractive index in case of inhomogeneous NLC order due to inhomogeneous electric field on the surface, resulting from laser illumination. The chess-box type of order will be discussed. A new phase diagram in variables external field-anchoring force will be discussed. The method for calculation of diffraction efficiency in an external modulated electric field will be discussed. Some applications to twisted NLC cell will be presented. Further possibilities of the approach will be discussed.

Forces on Small Scales – Liwei Chen Mon. April 25th, 2005
12:30 pm-1:30 pm

Atomic Force Microscopy (AFM) has become an indispensable tool in nanoscience and nanotechnology. In this talk, I will not only show routine application of topographical imaging with nanometer resolution, but also demonstrate further studies that benefit from quantitative measurements of small forces. Examples include complete force profiles between SAMs, interfacial electric dipole, and energy level alignment in organic thin-film-transistors.

Scanned Probe Magnetic Resonance: The Magnetic Resonance Force Microscope – Chris Hammel Mon. April 18th, 2005
12:30 pm-1:30 pm
Focused laser beams and liquid crystals: Three-dimensional imaging and control of topological defects and measurements of colloidal interactions – Oleg Lavrentovich Mon. April 11th, 2005
12:30 pm-1:30 pm

Orientational order is a universal feature of numerous soft-matter systems, most notably liquid crystals. These systems are extremely flexible, producing a rich variety of complex 3D patterns of order parameter. Non-destructive techniques to study and control these patterns are in a great demand. This talk discusses how a tightly focused laser beam can serve as a tool to image complex patterns of the director and to manipulate them. (1) In the fluorescence confocal polarizing microscopy (FCPM), the focused laser beam allows one to image 3D patterns of orientational order. We employ the property of anisotropic media to align fluorescent dye molecules.

Spin separation in cyclotron motion – Leonid Rokhinson Mon. April 4th, 2005
12:30 pm-1:30 pm

The ability to manipulate spin of charge carries in a controllable fashion is central to the rapidly developing field of spintronics, as well as for the development of spin-based devices for quantum information processing. Electrical injection of spin-polarized currents is proven to be a formidable challenge. We realized a solid-state analog of the famous Stern-Gerlach experiment in atomic physics, with spin-orbit interactions playing the role of the gradient of magnetic field. We achieved spatial separation of spins and bipolar spin filtering using cyclotron motion in a weak magnetic field.

Investigations of Light Harvesting and Enhanced Nonlinear Optical Properties in Organic Dendrimers and Branched Macromolecules – Theodore Goodson III Mon. March 28th, 2005
12:30 pm-1:30 pm

Organic conjugated macromolecules have received great attention due to their use in optical and electronic applications. Certain molecular aggregate systems have shown enhanced nonlinear optical properties by virtue of excitonic coupling in the multi-chromophore system. Organic dendrimers and other branched multi-chromophore systems (where the chromophores are covalently attached) have also shown characteristic properties of strong intramolecular interactions which have been utilized in light harvesting processes, light emitting diodes, as well as for enhanced nonlinear optical effects. The mechanism of the strong intramolecular interactions in branched chromophores depends on the nature of the branching center, the geometrical orientation of covalently attached chromophores,

Correlations Stablize Blue Phases – Lech Longa Mon. February 28th, 2005
12:30 pm-1:30 pm
Magnetoresistance in Parallel Fields – Julia Meyer Mon. February 21st, 2005
12:30 pm-1:30 pm

In addition to its large scale in-plane properties, transport in (quasi) two-dimensional electron systems is sensitive to microscopic details in the transverse direction. An efficient tool to study the interplay between both is a parallel magnetic field, which probes the structure of wave functions perpendicular to the plane. Due to the Berry-Robnik symmetry effect, one finds that the magnetoresistance contains information on the geometry of the confining potential as well as characteristics of the disorder.

Sensitive Detection of Radiation Trapping in a Cold Dilute Gas – Samir Bali Mon. February 14th, 2005
12:30 pm-1:30 pm

Radiation trapping in an illuminated gas of atoms refers to the reabsorption of spontaneously emitted photons. This reabsorption prevents the formation of colder denser atomic samples for quantum degenerate studies in ultracold trapped gases. Furthermore, the decoherence induced by this reabsorption significantly affects a vast variety of important experiments that rely on the preparation of macroscopic coherent atomic samples, for example, experiments that propose a practical implementation of a quantum computer. Clearly, it is of interest to devise experimental techniques that detect extremely small amounts of radiation trapping in dilute gases. We will show that the photon statistics of the light scattered from a trapped sample of cold dilute gas is a highly sensitive non-invasive probe of radiation trapping in the sample.

Challenge of Public and Workforce Education in Nanotechnology: Science vs Science Fiction – David Smith Mon. February 7th, 2005
12:30 pm-1:30 pm

Dave will discuss nanotechnology education as a challenge in both the areas of public education in science (for the K-99 audience ) and as another battle of hype against reality; science fiction against science. How he currently teaches freshman-level nanotechnology will be discussed as will the experimental platform he uses, called the Nanopedia, the Web Encyclopedia of Nanotechnology, for which he has a modest NSF grant, shared with Alexis Abramson of the engineering school. He will particularly discuss some of the science fiction aspects of Michael Chrichton’s novel “Prey,” about nanotechnology gone awry, asking for audience comments on whats right and what’s not.

First-principles investigations of p-type doping in ZnO – Sukit Limpijumnong Mon. December 6th, 2004
12:30 pm-1:30 pm

ZnO is one of the top candidates for blue light optoelectronics because of its wide bandgap properties. However, fabricating high quality p-type ZnO has proven to be difficult. While none of the group-I doping yields p-type behavior and Nitrogen doping shows only limited success, doping with larger group-V elements, which should cause high strain and has low solubility on the oxygen site, show some preliminary surprising success. Based on first principles calculations, we have developed a model involving complex doping that could explain the observed p-type behavior in P-, As-, and Sb-doped ZnO and fits well with experimental growth/annealing conditions.

Optical control in semiconductor dots for quantum operations – Duncan Steel Mon. November 29th, 2004
12:30 pm-1:30 pm

Semiconductor quantum dots have optical properties similar to simple atomic systems, unlike higher dimensional semiconductor structures that are dominated by manybody physics associated with the continuum states. They also provide a potentially ideal electronic structure appropriate for quantum computing. The data shows that these structures can be coherently controlled on a time scale short compared to the quantum decoherence time and that entangled states of qubits (represented by exciton optical Bloch vectors) can be created. The system is remarkably robust against pure dephasing and we have been able to demonstrate a simple conditional quantum logic device involving multiple Rabi flops of the exciton and biexciton.

Entropy Applications to Engineering and Health Science – Miron Kaufman Mon. November 8th, 2004
12:30 pm-1:30 pm

I will describe two applications of entropy. The first one is relevant to mixing in polymer processing. The other one is relevant to developing a diagnostic tool for low back pain. I analyze the advection of light particles carried by a high viscosity fluid set in motion in a long channel of rectangular cross section and covered by an infinite plate moving at constant velocity. This is a model of a screw extruder, a basic tool for polymer processing. I present an analytic solution of the creeping flow (zero Reynolds number). I analyze the quality and rate of mixing of the particles by calculating Renyi entropies.

Resonant charge carrier tunneling in nanocrystalline Si/amorphous SiO2 superlattices – Volodimyr Duzhko Mon. November 1st, 2004
12:30 pm-1:30 pm

Electronic transport and charge carrier trapping in the nanocrystalline Si/amorphous SiO2 superlattices were investigated by impedance spectroscopy, dc photoconductivity, and transient photocurrent measurements. The method for evaluation of the density of interface traps from the impedance spectroscopy measurements was developed to controll the quality of the superlattices. Transport of charge carrier in the superlattices at low temperatures was found to be the resonant tunneling of holes between quantum confined states of the valence band of Si nanocrystals. The measured electronic structure of the valence band of Si nanocrystal is in good agreement with the TB calculations. Application of the nanocryslalline Si/amorphous SiO2 superlattices to non-volatile memory cells will be discussed.

Exploring the terahertz region with a narrowband tunable source – Peter Powers Mon. October 25th, 2004
12:30 pm-1:30 pm

The generation of widely tunable coherent terahertz (THz) frequencies is of great interest for a variety of applications in basic and applied sciences. Broadband THz sources, particularly those based on femtosecond lasers, have already shown much promise in addressing these applications. In this seminar, I will present a narrow bandwidth approach to THz generation by means of difference frequency generation (DFG) between two lasers. Provided that one or both of the laser sources are tunable, the DFG technique offers a pathway to tunable narrowband THz frequencies. Our results based on mixing the output of two seeded optical parametric generators will show narrow-bandwidth and tunable operation from 1.6 to 4.5 THz.

Old Method, New Results: Ultra-High Mobility in a Simple Organic Crystalline Semiconductor – Brett Ellman Mon. October 11th, 2004
12:30 pm-1:30 pm

Devices based on organic semiconductors are a new, growing sector of the electronics market. For use as, e.g., field-effect transistors, a primary determinant of the utility of a material is the mobility, the proportionality coefficient between charge velocity and electric field. We report detailed pulsed-laser time-of-flight measurements on a simple crystalline system, 1,4-diiodobenzene, with a room temperature mobility at least an order of magnitude larger than competing inorganic materials (e.g., amorphous silicon). I will discuss purification, crystal growth, and a detailed analysis we have developed to extract other properties besides mobilities from the time-of-flight data. Finally, I will mention ab-initio calculations of electronic structure that indicate that the iodine constituents possibly play a major role in transport.

Coherent time-resolved vibrational spectroscopy of surfaces and interfaces – Alex V. Benderskii Mon. October 4th, 2004
12:30 pm-1:30 pm

Time- and frequency-domain 3-wave mixing spectroscopies (infrared + visible Sum Frequency Generation, SFG) are presented as the lowest-order nonlinear techniques that are both surface-selective and capable of measuring vibrational coherences. Application to ordered Langmuir-Blodgett monolayers shows vibrational quantum beats in time domain, which are connected to the frequency-domain spectrum by a simple Bloch-type model. The beat pattern is shown to be sensitive to the molecular order in the film. A mixed time-frequency domain version of this technique is applied to study ultrafast dynamics of the hydrogen bond network of interfacial water. Along with the vibrational dephasing, we observe ultrafast spectral diffusion of the OD stretch of D2O at the CaF2 surface,

The mystery of the thermotropic biaxial nematic phase – Satyen Kumar Mon. September 27th, 2004
12:30 pm-1:30 pm

The biaxial nematic phase was predicted more than three decades ago and discovered in lyotropic liquid crystalline systems by Yu and Saupe in 1980. However, several attempts to invent and synthesize new thermotropic materials likely to form this phase did not succeed. Several reports of its discovery in the thermotropic materials have proven to be without merit. While investigating the structure of smectic phases in bent-core (or, banana) mesogens, we came across unusual diffraction pattern in their nematic phase. The experimental investigations that followed and modeling, have established their biaxial nature. Recently synthesized materials exhibit the existence of, both, the uniaxial and the biaxial nematic phase.

A Gas of Excitons: Moving and Trapping Electronic Quasi-Atoms – David Snoke Mon. September 20th, 2004
12:30 pm-1:30 pm

An exciton is bound state of a free, negatively charged electron and a postively charged hole in a semiconductor. Excitons act in many ways like hydrogen atoms which can move through a semiconductor and interact with each other much like a gas of atoms. While often excitons have very short lifetime (a few picoseconds), we have developed methods to extend their lifetime by several orders of magnitude, up to 10 microseconds, so that they can travel hundreds of microns across a semiconductor structure. We can also apply a force to the excitons to cause them to move in the direction we want,

Spin injection from ferromagnetic Fe contacts into GaAs/AlGaAs spin LEDs – Athos Petrou Mon. September 13th, 2004
12:30 pm-1:30 pm

Electron spin injection efficiencies up to 40% have been obtained in Fe/AlGaAs(n) Schottky barriers. The spin polarized electrons are collected by a GaAs well and recombine with unpolarized holes. The optical polarization of the emitted excitonic electroluminescence yields a direct measurement of the electron spin polarization in the well. We will concentrate on the following aspects of the spin injection process in this system: a. Phonon assisted recombination b. The effect of confined electrons in the GaAs quantum well c. The transition from a bulk-like three-dimensional to a two-dimensional regime in wide well spin LEDs

Defect dynamics in nematic liquid crystals – Maurizio Nobili Thu. August 12th, 2004
12:30 pm-1:30 pm
Optical rectification and electro-optic sampling in the THz regime using electro-optic polymers – Michael Hayden Mon. May 10th, 2004
12:30 pm-1:30 pm

I will describe the adaptation of electro-optic (EO) polymer technology to terahertz (THz) generation and detection. The generation of wide bandwidth THz radiation (mid-IR to far-IR) with a smooth frequency response using low power laser sources is very desirable for scientific and technological applications such as vibrational analysis of biomolecules, medical imaging, non-contact electrical measurements, and homeland security. EO polymers have the potential to make a significant impact in this field due to their large nonlinearities, ease of processing, and low dispersion from the optical to the THz region. I will describe the fabrication of 100-300 m thick polymer films with electrooptic coefficients greater than 50 pm/V and report their use as emitters and sensors of THz radiation.

Evidence for Neutrino Oscillation and Massive Neutrinos: The Resolution of the Solar Neutrino Problem at SNO and KamLAND – Karsten Heeger, Michelson Postdoctoral Prize Lecture Mon. April 26th, 2004
12:30 pm-1:30 pm

Unambiguous evidence for novel neutrino properties has recently been obtained from observations of solar and reactor neutrinos. Combined with previous solar neutrino experiments the results from SNO and KamLAND are evidence for neutrino oscillation. The Sudbury Neutrino Observatory (SNO) studies neutrinos from the 8B decay in the Sun to search for neutrino flavor change. SNO’s unique measurement of all neutrino flavors has provided model- independent evidence for the flavor transformation of solar neutrinos. Its results imply that neutrinos have mass. This observation explains the long-standing Solar Neutrino Problem, the deficit of the observed electron solar neutrino flux compared to solar model predictions.

Surface Structure Determination of a Diacetylene of Monomer and Polymer LB Monolayers by AFM as Compared to Electron Diffraction – J. B. Lando Mon. April 19th, 2004
12:30 pm-1:30 pm

Langmuir Blodget (LB) monolayer films of the lithium salt of 10,12-nonacosadiynoic acid monomer and polymer LB monolayers were studied by AFM and electron diffraction. The fast Fourier transform of the AFM image was compared to electron diffraction results. The comparison gave insights into both sets of results. The analysis and conclusions will be discussed.

Quantum Criticality near Zero Temperature Phase Transitions – Meigan Aronson Mon. April 12th, 2004
12:30 pm-1:30 pm

It is possible to drive magnetic ordering temperatures in certain metallic magnets to zero temperature by means of applied field, pressure, or compositional variation. We have been using neutron scattering measurements to study the development of dynamic and spatial correlations near one such T=0 antiferromagnetic transition in the heavy fermion system CeRu2Ge2, doped with Fe. We establish that the dynamical susceptibility is controlled equally by energy, absolute temperature, and wave vector magnitude, measured relative to the propagation wave vector of the parent finite temperature antiferromagnet. Unusually, critical slowing down affects a very broad range of wave vectors, suggesting that the quantum critical point is caused by the collapse of the moment magnitude,

Magnetic interactions and properties of 3d-5d/4d nano-particles: exchange interactions mediated by non-magnetic metallic atoms – Oleg N. Mryasov Mon. April 5th, 2004
12:30 pm-1:30 pm

The search for technological solutions for ultra high density magnetic storage devices requires to achieve thermal stability and higher signal to noise ratio for dramatically decreasing media grain size and geometrical dimensions of the field sensing elements. In this work we consider a few specific examples where it is necessary to consider atomic scale effects and correspondingly to achieve a quantitative level of understanding magnetic interactions as possible future design factors for ultra-high density magnetic storage devices. One of the possible and most researched options to advance in the magnetic recording media is to use large magneto-crystalline anisotropy (MCA) materials,

Magnetic Resonance of Hyperpolarized Noble Gases – Jason C. Lea Woods Mon. March 1st, 2004
12:30 pm-1:30 pm

Laser polarization of 3He and 129Xe generates nuclear spin polarizations 100,000 times greater than Boltzmann equilibrium at 2 Tesla and 300 K. The advent of these hyperpolarized gases has led to a wealth of research and applications in atomic and materials physics, chemistry, and medicine. After a brief introduction to magnetic resonance and hyperpolarization, the physics of optical pumping and spin exchange in the production of hyperpolarized gases will be introduced and discussed. Several novel applications, including nuclear polarization transfer and diffusion imaging in normal and diseased human lungs, will be detailed. New research on the discovery of ferromagnetism in glass spin-exchange cells will also be treated,

The Development of Optical Coherence Tomography at CASE and the Optical Related Bio-Images – Zhilin Hu Mon. February 23rd, 2004
12:30 pm-1:30 pm

The presentation is to draw a picture of Optical Coherence Tomography (OCT), which has tremendous applications in biomedical research and clinic disease diagnosis, with a great potential commercial market. The fundamental principle of the OCT is to use the theory of the coherent optics or the electromagnetic waves. The focus of the OCT study is to get high resolution, high contrast and high imaging speed. I would like to discuss how to improve the performance of OCT, and also to demonstrate a few kinds of OCT images and real time movies collected by the OCT devices developed in the group at Case Western Reserve University.

Theory of electron tunneling in ultra-small aluminum grains – Gustavo A. Narvaez Mon. February 16th, 2004
12:30 pm-1:30 pm

The ongoing revolution in information technology drives advances in the ability to synthesize, manipulate, and probe nanometer-size materials. Among the new developments, single-quantum-level tunneling spectroscopy emerges as a powerful tool to study the electronic structure of individual metal nanograins. I will present a microscopic theory of electron tunneling in ultra-small aluminum grains, which helps understanding tunneling spectroscopy experiments. The observed high density of resonances and the asymmetry of the tunneling spectra agains voltage-bias reversal can be understood within the new theory. I will also show that the local electrostatic environment of metal grains significantly change the spectroscopy in the nanoscopic regime.

Structure and Dynamics of Interfaces in the MBE Growth on (110) and (100) Crystal Surfaces – Artem Lewandowsky Mon. February 2nd, 2004
12:30 pm-1:30 pm

The general phenomenology of multilayer epitaxial growth and erosion on square (001) and rectangular (110) symmetry crystal surfaces is discussed. Recently observed transitions between two kinds of ripple states on (110) surfaces are studied within a unified model. Predictions are made about several novel interface states, intervening via consecutive transitions between two rippled states on (110) surfaces. Surface morphology, coarsening dynamics, and far-from-equilibrium transitions on (001) surface are studied. Intermediary interface states with many-sided pyramids on (001) surface are predicted and characterized.

Electron Spin Manipulation in Semiconductor Nanostructures – Alexander Efros Mon. January 26th, 2004
12:30 pm-1:30 pm

Manipulating electron spin is one of the central problems in the growing field of semiconductor spintronics. This is of critical importance for quantum computing and information processing. Here I discuss the “optical initialization” and “optical read out” of the spin of an electron localized in a quantum dot [1]. We suggest using the combined effects of -optical pulses and transverse magnetic field for the optical pumping of the electron spin in QDs and the initialization of single spin Q-bit. The calculation shows that ~100% spin polarization can be reached as a result of several repetitions of this procedure. For “read out” of the single electron spin we suggest using a resonance fluorescence of trions excited resonantly by circularly polarized light,

Visualizing Functional Connections in the Brain with Magnetic Resonance Imaging – Shella Keilholz Mon. December 15th, 2003
12:30 pm-1:30 pm

Magnetic resonance imaging, widely utilized for obtaining excellent anatomical images of soft tissue, has increasingly been applied to the study of brain function. By manipulating image acquisition, information about parameters such as perfusion, diffusion, neural activity, blood volume, and blood oxygenation can be obtained. Combining this functional contrast with fast, high-resolution imaging techniques creates a flexible tool for probing neural networks and their interactions.

The use of antibodies coupled to quantum-dot filled microspheres – Maureen McEnery Mon. December 1st, 2003
12:30 pm-1:30 pm
On the Origin of Soft-Vibrational Modes in Glass-Forming Liquids – Ulrich Zurcher Mon. November 17th, 2003
12:30 pm-1:30 pm

When liquids are cooled rapidly, particles can no longer move freely and the liquid becomes a glass. Above the glass temperature Tg, relaxation in supercooled liquids obeys the Vogel-Fulcher law, τ ∼ exp[-E/(T-T0)] with T0 < Tg. The physical origin of this behavior is still largely unknown. We review some theoretical models for glassy relaxation. We then examine elastic properties of supercooled liquids and explore the role of stress in glassy relaxation.

Level Statistics of Complex Systems: A Random Matrix Approach – Pragya Shukla Mon. November 10th, 2003
12:30 pm-1:30 pm

In general, the physical systems are quite complex in nature. Our approximate knowledge of the complicated interactions in these systems manifests itself by a randomization of various generators of the dynamics. The operators associated with wave dynamics e.g Hamiltonian, electromagnetic waves in a microwave cavity, or signals in a brain etc. can therefore be modeled by random matrices.

The choice of a suitable random matrix model of a complex system is very sensitive to the nature of its complexity. The statistical spectral analysis of various complex systems requires, therefore, a thorough probing of a wide range of random matrix ensembles which is not an easy task.

Oxide-semiconductor materials for quantum computation – Jeremy Levy Mon. November 3rd, 2003
12:30 pm-1:30 pm

I will describe the goals and activities of our Center for Oxide-Semiconductor Materials for Quantum Computation (COSMQC, http://cosmqc.net). In our proposed architecture, quantum information is stored in electron spins, which form the basis for qubits. Spin-polarized electrons are created in Ge/Si quantum dots using optical orientation. Fast optical gating of Zeeman (one-qubit) and Heisenberg (two-qubit) interactions proceeds via optical rectification in an epitaxial ferroelectric. This approach takes advantages of native excitations and interactions in the solid state: (i) electron spins, charges, and photons for initialization and readout, (ii) quantum-mechanical (Heisenberg, Zeeman, spin-orbit) and classical (magnetic, electric, optical intensity) couplings for quantum gating,

Counting electrons on helium with a single electron transistor – Mike Lea Mon. October 27th, 2003
12:30 pm-1:30 pm

Electrons on liquid helium have been proposed as qubits [1], using excited Rydberg states [2] as the |0> and |1> quantum states. This requires the trapping of single electrons in quantum wells, the excitation of Rydberg states using millimetric microwaves and the detection of the quantum states of the electrons. Progress towards these objectives will be described. In particular, we have detected individual trapped electrons on a pool of helium, 0.8 micron deep and 5 micron diameter, using a superconducting single-electron transistor (SET). Electrons on the helium surface induce a positive charge in the SET island and a phase shift in the Coulomb blockade oscillations (CBO) in the SET source-drain current.

Is there a finite temperature phase transitions in bilayer quantum Hall system ? – Jinwu Ye Mon. October 20th, 2003
12:30 pm-1:30 pm

A Mutual Composite Fermion (MCF) picture is proposed to explain the interlayer coherent incompressible phase in bilayer Quantum Hall systems at total filling factor νT12 =1. There are gapped quasi-particles (QP) and quasi-holes (QH) excitations with total fractional charges ±ν1 and ±ν2. The total fractional charges are evenly distributed between the two layers. The QP and QH are asymptotically free even at T=0 . In contrast to previous claims, there is no finite temperature phase transition. Our results are contrasted with previous results from different pictures.

Structure and Properties of Narrow Gap Nitride Films – Rachel Goldman Mon. October 13th, 2003
12:30 pm-1:30 pm

Narrow gap nitride semiconductors have shown significant promise for a wide range of applications, including long-wavelength light-emitters, high performance electronic devices, and high efficiency solar cells. In the case of GaAsN, a consequence of the large N-As size difference is a predicted limited miscibility on the anion sublattice, which often leads to the formation of GaN-rich nanostructures [1-2]. Dilute GaAsN alloys are typically achieved using low growth temperatures, which enable N incorporation without GaN surface conversion. In addition, conflicting results have been reported regarding the mechanism of N incorporation, and recent optical studies have suggested that the shear deformation potential and/or the binary elastic constants may have an unusual composition dependence [3].

Exploiting self-assembly to create polar organic thin films with piezoelectric, pyroelectric, and second order nonlinear optical response – Daniel Dyer Wed. September 17th, 2003
12:30 pm-1:30 pm
Magnetic interactions in metallic anti-ferromagnetic manganese nitride compounds – Walter Lambrecht Mon. September 8th, 2003
12:30 pm-1:30 pm

Electronic structure calculations are used to study the magnetic properties of MnN and Mn3N2. The magnetic moments originate primarily from the Mn t2g orbitals and are in good agreement with neutron diffraction measurements. The ground state is found to be antiferromagnetically ordered along [001] planes in agreement with experiment. By mapping the energy differences between various spin configurations to a Heisenberg model we find that the dominant interactions in MnN are a second nearest neighbor ferromagnetic interaction due to double exchange via nitrogen and a nearest neighbor antiferromagnetic direct exchange interaction about 4 times smaller than the second nearest neighbor interaction.

Dynamic Anchoring of Liquid Crystals: Path to New Applications – Lachezar Komitov Mon. June 2nd, 2003
12:30 pm-1:30 pm

Liquid crystals are widely used as electro-optic active media in display devices. The alignment of the liquid crystal molecules is crucial for the operation of liquid crystal displays. In field-free conditions, the liquid crystal alignment is essentially governed by the surface/liquid crystal interactions. Therefore, these interactions have been the focus of intensive studies for many years. Their importance is clearly demonstrated by surface stabilized ferroelectric liquid crystals (SSFLC) where the ferroelectric state emerges as a consequence of the surface/liquid crystal interactions. It has been shown previously that the changes taking place at the liquid crystal/solid substrate surface affect the behaviour of the liquid crystal volume and,

Electron-Accepting Molecules and Polymers: Theoretical Insights – Douglas Dudis Mon. May 5th, 2003
12:30 pm-1:30 pm

Conjugated molecules and polymers are of intense interest because of their novel electronic, linear and nonlinear optical, electrochemical and biological properties. Technologies benefiting from these materials include photovoltaics, batteries, capacitors, molecular electronics, electrochromics and light emitters among others. While a few reasonably stable p-dopable conductive molecules are commercially available, technologically viable n-dopable materials remain elusive. Suitable n-dopable polymers would, for example, enable the fabrication of all polymer p-n junction devices. This talk will summarize recent experimental and theoretical efforts to understand, design, and synthesize environmentally stable n-dopable (i.e. electron-accepting) materials. The work is an integration of efforts in quantum materials simulations,

Quantum Information Processing using Atomic and Optical Systems – Brian DeMarco Mon. April 28th, 2003
12:30 pm-1:30 pm

Quantum information processing is a rapidly emerging field, with development in atomic, photonic, and condensed matter systems underway. Classical computers (the standard computers of today) are inherently limited by memory storage and computational ability. For example, to store the complete quantum state of 300 interacting spin-1/2 particles in classical memory would require more bytes than the estimated number of protons in the universe. The pursuit of quantum computer technology is motivated by overcoming these limitations. The advantage of quantum computers over classical computers arises from the use of the qubit as a fundamental unit of information. Unlike a bit, which can be in one of two states (“0”

Conductance of molecular nanojunctions – Nikolai Zhitenev Mon. April 21st, 2003
12:30 pm-1:30 pm

Using molecules as possible elements for electronic devices has an enormous appeal. In the size hierarchy of nature, molecules stand just above atoms making them ideal ultimate choice for ever-shrinking electronic devices. Synthetic chemistry offers vast diversity of molecular objects as well as certain degree of fine-tuning of electronic structure similar to the doping of semiconductors. However, wiring the molecules in a macroscopic circuit remains the challenging problem. Many different approaches to contact a small number of molecules have been developed the recent years. Some of the methods exploit the flexibility of tunable contacts to molecules afforded by scanning probes or break junctions.

Swimming towards the dark: a photophobic light-driven elastomeric swimmer – Peter Palffy Mon. April 14th, 2003
12:30 pm-1:30 pm

Liquid crystal elastomers are characterized by strong coupling between orientational order and mechanical strain; optical excitations that result in a change of the order parameter can therefore bring about large mechanical deformations. We have studied the optomechanical response of nematic liquid single crystal elastomers doped with an azo dye.

Our recent experiments have shown a fascinating interaction between light and elastomer samples floating on the surface of a fluid: When exposed to laser light, the samples change both their conformation and position – they swim away from the illuminating light. The observed dynamics is unusal in that it is predicated on the transfer of information as well as energy from the light source to the swimmer.

What causes magnetization relaxation in ferromagnetic transition metals? – Snorri Ingvarsson Wed. March 26th, 2003
12:30 pm-1:30 pm

Much like resistance is to an electric circuit, magnetization relaxation is what restores the magnetization of a ferromagnet to equilibrium when the external stimulus (magnetic field) becomes quiescent. The mathematical equation governing magnetization dynamics in ferromagnets is the Landau-Lifshitz equation, where magnetization relaxation is usually taken into account as a phenomenological damping constant (Gilbert damping). In designing magnetoelectronic devices such as magnetic random access memory, it is obviously very important to understand what lies behind the Gilbert damping constant, and to understand how it can be modified. But what is this Gilbert damping really? In our experimental studies of magnetic thin films we have found that there is a strong correlation between their electrical resistance and Gilbert damping,

Modulation of Membrane Electrogenic Transport. The Role of Charge-Dipole Coupling – Eitan Gross Tue. March 25th, 2003
12:30 pm-1:30 pm

The behavior of a given ion channel can be different in different regions of a cell. For example, Na+ channels in the axons of nerve cells inactivate much faster then in the cell body (soma). These observations suggest that different regions of the cell could respond differently to the same stimulus without differences in channel surface density between the regions. While the mechanism underlying such spatial variations in channel function are unclear, it is important to realize that the regulatory elements within them (i.e. gating charges) are fundamentally responding to the potential gradient at their specific location in the plasmalemma –

Electronic polarization in organic molecules and molecular solids: classical interactions between quantum systems – Eugene Tsiper Mon. March 10th, 2003
12:30 pm-1:30 pm

Electronic structure of molecular solids is strikingly different from the conventional inorganic semiconductors, such as Si. Coulomb interactions between molecules in van der Waals contact, narrow bandwidths and localized nature of charges make electronic polarization a major effect, with energy scale greater than transfer integrals or temperature. We present an approach which treats individual molecules rigorously as quantum-mechanical systems subject to classical non-uniform fields of all other molecules. Atom-atom polarizability tensor is introduced to describe self-consistent intra-molecular charge redistribution. Dielectric tensors of two representative organic molecular crystals are computed to within experimental accuracy. We find quantitative agreements in charge carrier energetics with photoelectron and STM data.

An Atomic Abacus: Trapped ion quantum computing experiments at NIST – Brian DeMarco Mon. February 24th, 2003
12:30 pm-1:30 pm

Trapped atomic ions are an ideal system for exploring quantum information scienc e because deterministic state preparation and efficient state detection are poss ible and coherent manipulation of atomic systems is relatively advanced. In our experiment, a few singly charged Be ions are confined by static and radio-frequ ency electric fields in a micro-machined linear Paul trap. The internal and mot ional states of the ions are coherently manipulated using applied laser light. Our current work focuses on demonstrating the necessary ingredients to produce a scalable quantum computing scheme and on simplifying and improving quantum logi c gates. I will speak about a new set of experiments that was made possible by recent imp rovements in trap technology.

Torsion and stiffening of multi-walled carbon nanotubes – Stergios Papadakis Tue. February 18th, 2003
12:30 pm-1:30 pm

I will describe the fabrication and characterization of nanometer-scale electromechanical oscillator devices which use multi-walled carbon nanotubes as the spring elements. Through atomic-force-microscope force-distance measurements we are able to apply torsional strains to the nanotubes and measure their torsional spring constants and effective shear moduli. The data show that the nanotubes are stiffened by repeated flexing. I will also briefly describe progress on various other nanometer-scale systems we are studying.

Magnetic Properties of YCo5 and SmCo5 – Paul Larson Mon. February 10th, 2003
12:30 pm-1:30 pm

SmCo5 is one of the most important hard magnetic materials, having large magnetic moments and large out of plane magnetic anisotropy energy (MAE). We have calculated and analyzed the magnetic properties of the related compounds YCo5 and SmCo5 using first-principles LAPW calculations including LDA+U. We have found that the large MAE arises from both the Co d bands and the localized Sm f-shell, the latter having a large single-site (crystal field) anisotropy. While the magnetic moments found by replacing Co by Cu or Fe in the unit cell follow expectations, the MAE cannot be as easily explained.

Order at soft-hard interfaces – Pulak Dutta Mon. January 27th, 2003
12:30 pm-1:30 pm

The ordering of soft materials at hard surfaces is by now a familiar process, but the nucleation of hard materials at soft templates is also common in nature (‘biomineralization’). This talk will describe in situ X-ray scattering studies of both types of interfacial phenomena: the ordering of molecular liquids near hard surfaces, and the epitaxial growth of inorganic crystals under floating (Langmuir) monolayers.

Graduating Senior Project Presentations Mon. December 2nd, 2002
12:30 pm-1:30 pm
  • Benjamin Bayat, Volume Logic Diagnostics on Microprocessor and ASICs Chips at IBM
  • Andrew D. Huss, Integration of Sensor Technology for Mini-WHEGS Robot
  • Cameron K. McBride, The Propagation of Light and Gravity through Matter-filled Spacetime with Stabilized Compactified Extra Dimensions
  • Sharon R. Stefanovic, Information and Genetic Evolution
  • Jonathan Wheeler, Investigations of the Optical Properties of CdS Nanoparticles for Potential Applications in Spintronics
Gapped Sequences Alignment, Statistical Significance, and Biomolecular Interactions – Yi-Kuo Yu Mon. November 25th, 2002
12:30 pm-1:30 pm

By comparing the bio-sequences deposited in databases, sequence alignment tools pull out sequences of potential functional similarity to the query. To quantify the significance of the found sequences, one usually ask their associated p-values — how probable it is that a completely irrelevant sequence might be pulled out “by accident”. This important and difficult problem was only solved for a specific type of alignment method — gapless alignment, which is incapable of detecting weak homology. For methods which allow for gaps, the p-values must be obtained through time-consuming shuffling methods. By employing fundamental concepts from statistical physics, we have made important progress in statistics of the gapped alignment.

Diluted magnetic semiconductors based on the layered A2VB3VI compounds – Jeff S. Dyck Mon. November 18th, 2002
12:30 pm-1:30 pm

Currently, there is a great deal of research activity on the incorporation of magnetic ions into semiconductors to produce ferromagnetism. These diluted magnetic semiconductors (DMSs) are of interest both to theorists, because of their unusual mechanisms of magnetic behavior, and to experimentalists, because the manipulation of spin in addition to charge promises devices based on spin polarized transport. The most extensively studied DMS systems to date are based on II-VI and III-V semiconductors doped with manganese. Recently, we discovered a new class of diluted magnetic semiconductors that differ from the traditional compounds in a number of intriguing ways. The narrow band gap tetradymite-type semiconductors with the form A2VB3VI (A = Sb,

Mixed Valence Regime of the Periodic Anderson Model: the proper paradigm for the magnetic properties of f-electron materials? – James E. Gubernatis Mon. November 11th, 2002
12:30 pm-1:30 pm

The f-electron materials (lanthanides and actinides) exhibit a rich array of strongly correlated electron phenomena. Most notable are their anti-ferromagnetism and frequent heavy fermion behaviors. They are generally mixed valence materials. Traditionally, the periodic Anderson model is used to describe the behavior of these materials, and mean-field and variational analyses of this model have lead to the picture that the magnetic properties of the materials are a result of a competition between the RKKY and the Kondo interactions. I will report the results of recent quantum Monte Carlo studies of this model in the mixed-valence regime that find itinerant ferromagnetism to be the dominant magnetic state and that this state does not arise because of the RKKY interaction.

Novel Materials Properties from Reversibly Associated Polymers – Elena Dormidontova Mon. November 4th, 2002
12:30 pm-1:30 pm

In contrast to chemically bonded polymers, reversibly associated polymers (e.g. via hydrogen bonding) have the capability to reversibly change their chain architecture by responding to changes of external conditions. Blends of associating polymers of different chemical structure are capable of self-organization on both micro- and macroscopic level. The phase behavior of associated polymers can be manipulated by changing external factors such as temperature, flow, applied forces, addition of salt and so on. These properties are of potential technological importance for microelectronics, processing and biomedical applications.

We will consider the theoretically predicted phase behavior for blends of associated polymers capable of forming comb-like chains by hydrogen bonding with oligomers.

Functional Polymer Design: Creating Polymer Materials with Tailored Properties – Christoph Weder Mon. October 28th, 2002
12:30 pm-1:30 pm

Motivated by the persistent desire for (novel) materials, which exhibit currently unavailable functionalities, research focused on the creation of polymers with tailored properties has evolved to a central field at the interface of chemistry, materials science, and physics. Polymers, as a group of materials, often offer an attractive combination between ease of processing and final properties. Especially the ability to design their chemical structure virtually at will but also the possibility to control the often very rich phase behavior and supramolecular architectures of polymer systems allows one to minutely tailor the properties of this remarkable class of materials. At hand of selected examples,

Dislocations in silicon and diamond – Malcolm I. Heggie Tue. October 22nd, 2002
12:30 pm-1:30 pm

The study of dislocation core structures from first principles has flourished in the decade since the first calculation which confirmed reconstruction of the single period core of the 90o partial in silicon [1]. Supercell calculations reproduce this result [2] and also find a double period structure which could be degenerate or even lower in energy depending on the elastic environment [3-5]. In addition, more refined approaches have given us simulated EELS spectra and the interactions between point defects and dislocations [1,6,7]. Kinetic Monte Carlo simulations using first principles (or tight binding) energies and activation barriers have developed our understanding of kink dynamics beyond the understanding of the Hirth-Lothe model [8,9].

Atomic Clocks – Kurt Gibble Mon. October 14th, 2002
12:30 pm-1:30 pm
Mechanism of the high-pressure phase transitions from tetrahedrally bonded semiconductors to rocksalt – Maosheng Miao Mon. October 7th, 2002
12:30 pm-1:30 pm

A uniform description is presented for the transition paths of various tetrahedrally bonded semiconductor structures, including wurtzite, zind blende and various SiC polytypes, to the high-pressure rocksalt phase. The enthalpy barriers for these strain induced transitions were calculated from first-principles. A relation between the sound velocity pressure coefficients and the strains that drive the phase transition is pointed out. This relation replaces the Weinstein model which related these transitions to a softening of the transverse acoustic phonons at the zone boundary.

The Materials Machine – Daniel Gall Mon. September 30th, 2002
12:30 pm-1:30 pm

An ongoing challenge in materials physics is to arrange atoms in a controlled manner in order to design materials with the desired physical properties. This can be achieved by developing (1) synthesis methods which control the nanometer scale arrangement of the constituent atoms and (2) tools that predict materials properties solely from their elemental composition and nanostructure. I will illustrate these two aspects of materials physics by presenting results from thin film growth of transition metal nitrides.

An atomistic understanding of growth is developed using a multiple length-scales and dimensionality approach which combines experimental and computational methods to investigate microstructural evolution of entire layers (106-1012 atoms),

Electronic Structure Calculations with Dynamical Mean-Field Theory – Sergej Savrasov Mon. September 23rd, 2002
12:30 pm-1:30 pm

An effort to build a new electronic structure method based on many-body dynamical mean-field theory as an alternative to density functional theory will be reviewed. Several applications of this method to study total energies and photoemission spectra in various phases of plutonium, phonons in Mott Insulators, as well as optical spectra in doped titanites will be discussed.

Transport of Spin in Multilayer Films – Mark Stiles Mon. September 16th, 2002
12:30 pm-1:30 pm

The resistance of certain magnetic multilayers decreases by a factor of two with the application of a magnetic field. The discovery of this so-called giant magnetoresistance effect has led to an explosion in research in magnetic multilayers. In part, this research is motivated by the use of this effect in magnetic field sensors and read heads for magnetic disks. It has lead to the discovery of many other interesting phenomena related to spin transport in these systems. These include magnetic exchange coupling that oscillates in sign as layer thicknesses are varied and the ability to reverse the magnetization of a layer by the spins carried by a current passing through the layer.

Fluorescence confocal polarizing microscopy – a new method for 3D imaging of a director field in liquid crystals – Sergij Shiyanovskii Mon. September 9th, 2002
12:30 pm-1:30 pm

Most of traditional optical methods, e.g. polarizing optical microscopy, provide 2D images as a result of the integration over a vertical direction. We have developed a new fluorescence confocal polarizing microscopy (FCPM) technique, which allows one to visualize 3D patterns of the director field in liquid crystals (LCs). FCPM provides clear and easy recognizable images with sub-micron resolution for different textures and defects in LCs, such as confocal domains in smectic A, disclinations and oily streaks in cholesterics, etc. Computer simulations serve as a powerful complementary tool to the FCPM method to decipher unknown complex director configurations. Our numerical code finds equilibrium structures taking into account finite surface anchoring strength,


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