College of Arts and Sciences

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.

Continue reading… Near-field optical scanning spectroscopy of photonic nanostructures – Alexander Mintairov

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”

Continue reading… Ab-initio Assisted Process and Device Simulation for Nanoelectronic Devices – Wolfgang Windl

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.

Continue reading… Some examples of theory and computation of properties of transition metal nitrides – Sanjay Khare

Continue reading… Laboratory studies of atmospheric aerosol nucleation – Shan-Hu Lee

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.

Continue reading… Flow diagram and Quantum critical behavior of the two-dimensional metal-insulator transition (2DMIT) – Alex Punnoose

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,

Continue reading… Universal Gelation of Particles with Short-ranged Attraction – Peter J. Lu

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,

Continue reading… Metallic and Magnetic Nanostructured Thin Films

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,

Continue reading… Macrophase ordering in ionomers under external potential – Elshad Allahyarov

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.

Continue reading… Ubiquity of Entanglement – Stanislaw Szarek

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,

Continue reading… Bilayer Quantum Hall Effect – Bahman Roostaei

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.

Continue reading… Nanotubes beyond carbon: theory of gallium nitride and boron nanotubes – Sohrab Ismail-Beigi

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.

Continue reading… Near-field optical scanning spectroscopy of photonic nanostructures – Alexander Mintairov

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.

Continue reading… Phonons in ZnGeN2 and related materials: experiment and theory – Tim Peshek and Tula Paudel

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,

Continue reading… Thermo-acoustic waves near the liquid-vapor critical point : the sound of heat – Pierre Carlès

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],

Continue reading… Identifying defect structures by first principles XANES – Sukit Limpijumnong

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.

Continue reading… Synthesis and Characterization of GaGdN – Cammy Abernathy

Continue reading… Controlling light on the nanoscale: imaging and spectroscopy with ultrahigh spatial and temporal resolution – Markus Raschke

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”,

Continue reading… Polyelectrolytes: A Field-Theoretic Perspective – Yuri Popov

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,

Continue reading… “Spinning” and “twisting” a light beam and other wavefront-shaping tricks performed with suitably patterned liquid crystals – Lorenzo Marrucci

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.

Continue reading… Disorder, Interactions, and Crossovers in Quantum Dots – Ganpathy Murthy

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.

Continue reading… Looking for the Spin Hall Effect in all the Wrong Places – Nitin Samarth

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.

Continue reading… Defect structures in nematic liquid crystal shells – Alberto Fernandez-Nieves

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.

Continue reading… Frustration Phenomena in Liquid Crystals in Contact with Patterned Substrates – Tim Atherton

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.

Continue reading… Elasticity of Polymer Gels (and a Cytoskeleton in the Closet) – Gavin Buxton