College of Arts and Sciences

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,

Continue reading… Michelson Postdoctoral Lecture 2:Optical Atomic Clocks – David Hanneke

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.

Continue reading… Michelson Postdoctoral Lecture 1: Entangled Mechanical Oscillators and a Programmable Quantum Computer: Adventures in Coupling Two-Level Systems to Quantum Harmonic Oscillators – David Hanneke

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.

Continue reading… Atom Mapping and Correlated Functional Imaging of Nanowires – Lincoln J. Lauhon

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],

Continue reading… Controlling Spin and Magnetism in Quantum Dots – Rafal Oszwaldowski

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.

Continue reading… Diffusion Tensor Imaging: A Guided Tour – Cheng Guan Koay

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,

Continue reading… Through A Glass, Darkly: Obtaining Quantitative Information from Microscope Images of Liquid Crystals – Tim Atherton

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,

Continue reading… Hard tetrahedra and Quasi-Crystals – Rolfe G. Petschek

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.

Continue reading… Periodic networks in heterogeneous materials: theory and multiscale homogenization for soling heat transfer and deformation problems – Viktoria Savatorova

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.

Continue reading… Ultrafast physics in photosynthesis: Mapping sub-nanometer energy flow – Naomi Ginsberg

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,

Continue reading… Single cell studies using microfluidic devices – Amy Rowat

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.

Continue reading… Imaging coherent electron transport in graphene – Jesse Berezovsky

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].

Continue reading… Principles and Applications of Extrinsic (Doped) Organic Semiconductors – Calvin Chan

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,

Continue reading… Gigahertz dynamics of a strongly driven single spin in diamond – G. D. Fuchs

> 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.

Continue reading… ZnGeAs2: A Novel Semiconductor for Photovoltaics – Tim Peshek

Continue reading… Spin Fluctuations in Magnetic Quantum Dots – Andre Petukhov

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.

Continue reading… Quantum Simulation of Strongly Correlated Quantum Dots Out of Equilibrium – Jong Han

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.

Continue reading… From Water Splitting to Hydrogen Storage: The Art of First-Principles Predictions in Materials Design – Shengbai Zhang

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.

Continue reading… Quantum Mechanics of Point Defects and Diffusion in α-Al2O3 – Arthur Heuer

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.

Continue reading… Computer Simulations of Self-Assembly of Metallo-Supramolecular Networks – Elena Dormidontova

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.

Continue reading… Magnetic Properties of Rare Earth Doped GaN – John M. Zavada

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,

Continue reading… Nanoscale memristive devices for memory and logic applications – Wei Lu

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.

Continue reading… A birds-eye view of nonlinear optics: using scale invariance to optimize the molecular response – Mark Kuzyk

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,

Continue reading… Thermal Transport and Thermoelectric Energy Conversion in Nanomaterials – Li Shi

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 –

Continue reading… Band structure information from soft x-ray spectroscopy – Andrew Preston

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,

Continue reading… Formation and properties of Cu_2S-CdS and Ag_2S-CdS Nanorod Heterostructures – Denis Demchenko

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.

Continue reading… Recent Advances in Organic (Opto)electronic Materials – Oksana Ostroverkhova