College of Arts and Sciences

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,

Continue reading… Dynamic Anchoring of Liquid Crystals: Path to New Applications – Lachezar Komitov

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,

Continue reading… Electron-Accepting Molecules and Polymers: Theoretical Insights – Douglas Dudis

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”

Continue reading… Quantum Information Processing using Atomic and Optical Systems – Brian DeMarco

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.

Continue reading… Conductance of molecular nanojunctions – Nikolai Zhitenev

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.

Continue reading… Swimming towards the dark: a photophobic light-driven elastomeric swimmer – Peter Palffy

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,

Continue reading… What causes magnetization relaxation in ferromagnetic transition metals? – Snorri Ingvarsson

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 –

Continue reading… Modulation of Membrane Electrogenic Transport. The Role of Charge-Dipole Coupling – Eitan Gross

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.

Continue reading… Electronic polarization in organic molecules and molecular solids: classical interactions between quantum systems – Eugene Tsiper

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.

Continue reading… An Atomic Abacus: Trapped ion quantum computing experiments at NIST – Brian DeMarco

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.

Continue reading… Torsion and stiffening of multi-walled carbon nanotubes – Stergios Papadakis

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.

Continue reading… Magnetic Properties of YCo5 and SmCo5 – Paul Larson

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.

Continue reading… Order at soft-hard interfaces – Pulak Dutta

Benjamin Bayat, Volume Logic Diagnostics on Microprocessor and ASICs Chips at IBMAndrew D. Huss, Integration of Sensor Technology for Mini-WHEGS RobotCameron K. McBride, The Propagation of Light and Gravity through Matter-filled Spacetime with Stabilized Compactified Extra DimensionsSharon R. Stefanovic, Information and Genetic EvolutionJonathan Wheeler, Investigations of the Optical Properties of CdS Nanoparticles for Potential Applications in Spintronics

Continue reading… Graduating Senior Project Presentations

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.

Continue reading… Gapped Sequences Alignment, Statistical Significance, and Biomolecular Interactions – Yi-Kuo Yu

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,

Continue reading… Diluted magnetic semiconductors based on the layered A2VB3VI compounds – Jeff S. Dyck

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.

Continue reading… Mixed Valence Regime of the Periodic Anderson Model: the proper paradigm for the magnetic properties of f-electron materials? – James E. Gubernatis

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.

Continue reading… Novel Materials Properties from Reversibly Associated Polymers – Elena Dormidontova

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,

Continue reading… Functional Polymer Design: Creating Polymer Materials with Tailored Properties – Christoph Weder

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].

Continue reading… Dislocations in silicon and diamond – Malcolm I. Heggie

Continue reading… Atomic Clocks – Kurt Gibble

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.

Continue reading… Mechanism of the high-pressure phase transitions from tetrahedrally bonded semiconductors to rocksalt – Maosheng Miao

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),

Continue reading… The Materials Machine – Daniel Gall

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.

Continue reading… Electronic Structure Calculations with Dynamical Mean-Field Theory – Sergej Savrasov

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.

Continue reading… Transport of Spin in Multilayer Films – Mark Stiles

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,

Continue reading… Fluorescence confocal polarizing microscopy – a new method for 3D imaging of a director field in liquid crystals – Sergij Shiyanovskii