College of Arts and Sciences

Astrophysical Neutrinos: Revealing Neutrino Properties at the Highest Energies

Continue reading… Michelson Postdoctoral Prize Lecture – Nicole Bell

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.

Continue reading… Phonon Anharmonicity and Phase Transitions in Bulk and Nanoparticle ZnSe under High Pressure – Bernard Weinstein

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.

Continue reading… The structure of a vortex and critical current through the BCS-BEC crossover – Mohit Randeria

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.

Continue reading… Fractional vortices and composite domain walls in nanomagnets – Oleg Tchernyshyvov

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.

Continue reading… Molecular Imaging with Ultrafast Electron Diffraction – Chong-Yu Ruan

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,

Continue reading… Treading a Fine Line: One-Dimensional Semiconductor Physics in Carbon Nanotubes – Michael Fuhrer

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.

Continue reading… Electron Interactions and Phase Coherence in Metals – Norman Birge

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.

Continue reading… Point defects in ZnGeP2 – Walter Lambrecht

Continue reading… The determination Liquid Crystal Device parameters by means of renormalized transmission spectroscopic ellipsometry – Munehiro Kimura

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.

Continue reading… Experiments with New Soft Solids – Patrick Mather

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.

Continue reading… Frontiers in spectroscopy with the scanning tunneling microscope – Jay Gupta

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.

Continue reading… Electronic Properties of InSb Quantum Wells and Mesoscopic Structures – Michael Santos

Continue reading… Oscillatory interlayer coupling in Co/Pt multilayers with perpendicular anisotropy – Fengyuan Yang

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.

Continue reading… Quantitative modeling of single-molecule RNA force-extension experiments – R. Bundschuh

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.

Continue reading… Between gases and liquids: the paradoxes of near-critical fluidsdynamics – Pierre Carles

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,

Continue reading… Low-dimensional Transport in Nanoscaled Materials – Philip Kim

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.

Continue reading… Mid-infrared Hall effect in ferromagnetic oxides and semiconductors – John Cerne

Continue reading… Cell signalling Biophysics of GTPase-protein interactions: an overview of ideas and ongoing activities – Matthias Buck

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.

Continue reading… ZIP-ping for Dark Matter – Michael Dragowsky

Continue reading… Ground- and Excited-State Attributes of Hexanuclear Rhenium(III) Chalcogenide Clusters – Thomas Gray