College of Arts and Sciences

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.

Continue reading… Odd circuits: stability and jamming in hard granular materials – Nicolas Rivier

Continue reading… Some problems in the non-linear optics of liquid crystals – Tim Sluckin

Continue reading… Michelson Postdoctoral Lecture – Adam Bolton

Continue reading… ACES seminar: Molecularly Engineered Interfaces for Organic Optoelectronics – Zakya Kafafi

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. In this talk, I plan on detailing my life as an experimentalist and how my work has touched on some of these intriguing questions.

Continue reading… Ongoing Mysteries in Astrophysics – Donald Driscoll

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.

Continue reading… Bioinspired molecular optoelectronics – Volodimyr Duzhko

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.

Continue reading… New avenues to computational technology: novel spin transport effects at the nanoscale – Ewelina Hankiewicz

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),

Continue reading… From nano to micro: hierarchical ordering at the nanoscale – Ezekiel Johnston-Halperin

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.

Continue reading… Current, maximum power and optimized efficiency of Brownian heat engine – Mulugeta Bekele

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.

Continue reading… Unraveling Electronic and Spin Structure with Photoemission – Oleg Krupin

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.

Continue reading… Electrical Transport in Individual Nanostructures – Zhixian Zhou

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,

Continue reading… Ultrafast non-equilibrium phenomena of the integer quantum Hall system – Keshav Dani

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.

Continue reading… Heisenberg’s XY model and the Development of Mammalian Visual Cortex – Peter Thomas

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.

Continue reading… Engineering Defect Dynamics in Liquid Crystal Cells – Rolfe Petschek

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.

Continue reading… Transition metal and rare-earth nitrides: a new route to magnetic semiconductors – Walter Lambrecht

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,

Continue reading… Nonlinear Optics in Multilayer Polymer Films – Kenneth Singer

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.

Continue reading… Terahertz spectroscopy of InMnAs – Jason Deibel

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.

Continue reading… Semiconductor Spintronics – Igor Zutic

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,

Continue reading… Fabrication and Characterization of Functional Nanostructures and Applications – Richard Mu

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.

Continue reading… Is the Adiabatic approximation Inconsistent? – Solomon Duki

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,

Continue reading… X-Ray Emission and Resonant Inelastic X-ray Scattering: new probes of electronic structure in complex materials – Kevin Smith

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.

Continue reading… Solution Processable Organic Photovoltaics – Sean Shaheen

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.

Continue reading… Electromechanical coupling effects in semiconductor heterostructures – Lok C. Lew Yan Voon

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.

Continue reading… Simulating non-equilibrium processes over extended time and length scales using parallel kinetic Monte Carlo and parallel accelerated dynamics – Jacques Amar

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.

Continue reading… Expulsion of bend from a smectic liquid crystal: Anology to a type-I superconductor – Ruiting Wang

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,

Continue reading… Suppression of superconductivity in the Hubbard model at intermediate coupling by buckling and breathing phonons – Mark Jarrell