Physics Colloquium

Fall 2011

Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
  Colloquium organizer: Yew San Hor
(Link to main colloquium page)

Green - open date
Yellow - tentative (reserved)
Red - firm commitment

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Date Host Speaker Title of the talk Abstract
Sept. 1 Hor William Ratcliff
NIST
The Multiferroic Renaissance
As early as 1894, Pierre Curie found that there were materials in which the magnetic and dielectric properties were coupled.  These magnetoelectric materials were studied for some time, but the coupling was relatively weak.  Later, people studied multiferroic materials in which both spontaneous ferroelectric and magnetic ordering occurs.   In these materials, the coupling could be much stronger, but the materials were exceeding rare.
   Recently, several new compounds such as TbMnO3 have been discovered and the field has enjoyed a renaissance, culminating in the award of the 2010 James C. McGroddy Prize for New Materials for research on these compounds.  During this talk, I will talk about the history of this field and the current state of the research.  I will focus on one of the most technologically promising materials, BiFeO3 and the critical role neutron scattering has played in understanding the physics of these materials.
Sept. 8 Hor
Jay A. Switzer
S&T
Epitaxial electrodeposition of metal oxides for solid-state memory As the size of nonvolatile memory continues to shrink, there is increasing demand for highly scalable memory devices. An emerging technology is resistive random access memory (RRAM) that is based on resistance switching in materials such as transition metal oxides. Although electronic materials are traditionally processed by physical vapor deposition methods, we use electrodeposition to deposit candidate materials for RRAM. Previously in our group we have shown that electrodeposition can be used to produce epitaxial films of Cu2O, ZnO, Bi2O3, Fe3O4, Tl2O3, PbO2, and CuO. In addition to epitaxial films, we have produced nano-layered materials known as superlattices by pulsing the applied potential during deposition. Besides the low cost of electrodeposition, it also offers advantages such as the ability to deposit conformal layers on nano-crossbar arrays. The materials only deposit onto conducting elements that are electrically poised. The two materials we have electrodeposited for RRAM applications are magnetite, Fe3O4, and vanadium dioxide, VO2.  Both of these materials undergo a metal-to-insulator (MIT) phase transition that we utilize for resistance switching. The MIT for Fe3O4 occurs at 120 K, and the MIT for VO2 occurs at 341 K. Of the two materials, VO2 is the most promising because it shows resistance switching at room temperature.
Sept. 15 Hor
Michael A. McGuire
Oak Ridge National Lab
Iron compounds in energy applications: superconductors, thermoelectrics, and ferromagnets
I will present recent results, primarily experimental, from several iron containing materials. Our interest in these materials is motivated by their relevance to energy applications. Over the past several years, work in our group has focused heavily on iron-based superconductors, and closely related compounds. I will give an overview of this relatively new and active field, and discuss in some detail several of the contributions in this area from Oak Ridge National Laboratory. In the remainder of the presentation, I will address some recent work on the thermoelectric material iron-silicide (FeSi), and the quest for better permanent magnet materials that do not contain rare-earth elements. 
Sept. 22 Hor Alexey Snezhko
ANL
Active Materials: Self Assembly and Control Far-From-Equilibrium
New self-assembled materials composed of simple building blocks that spontaneously organize into desired structures whose constituents can assemble, disassemble, and reassemble autonomously or on command will enable functional materials capable of self-repair, multi-tasking, and even structure adjustment – set of properties currently exhibited only by biological systems. To support this kind of structural complexity and functional diversity these new self-assembled materials should rely on an external energy intake and “live” outside of equilibrium. Ensembles of interacting colloidal particles subject to an external periodic forcing often develop nontrivial collective behavior. We study emergent phenomena in magnetic colloidal ensembles suspended at liquid-liquid or liquid-air interfaces and driven out of equilibrium by alternating magnetic fields. Experiments reveal new types of nontrivially ordered dynamic self-assembled structures (in particular, “asters”, “snakes”, “clams”) emerging in such systems in a certain range of excitation parameters. Transition between different self-assembled phases with parameters of external driving magnetic field is observed.
   Above certain frequency threshold dynamic self-assembled structures spontaneously break the symmetry of self-induced surface flows (symmetry breaking instability) and turn into swimmers. Induced self-propulsion of robust aster-like structures in a presence of small in-plane DC field perturbations has been discovered. A possibility of the directed cargo transport at the interface by self-assembled structures is demonstrated. Some features of the self-localized structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density. Molecular dynamic simulations capturing microscopic mechanisms of the non-equilibrium self-assembly in our system are presented.
Sept. 29 Hor
Eric Majzoub
UMSL
Thermodynamics and Kinetics of Nano-cluster and Nano-confined Complex Hydrides
The equilibrium plateau pressure of a metal hydride at a given temperature is a characteristic thermodynamic quantity, and determines the application and engineering required for a hydrogen storage system. While recent interest has focused on complex metal hydrides such as NaAlH4 and Ca(BH4)2, these compounds are not as easily tunable as the interstitial metallic hydrides through alloying with other metal atoms, due to the strongly ionic character of the cohesive energy. However, the complex hydrides are superior on a wt.% hydrogen basis, and are the preferred materials for vehicular transport. In order to address thermodynamic tunability, we investigate these materials at the nanoscale, where the ratio of surface to bulk atoms impacts the energetics. Recent theoretical work indicates that small clusters of MgH2, for example, can significantly lower the desorption enthalpy with respect to bulk. Small metal or hydride clusters may be incorporated into nanoporous frameworks such as metal organic frameworks (MOFS), block co-polymer (BCP) templates, or nanoporous hard carbons, for example, to prevent agglomeration and perhaps even improve tunability through particle/surface interactions. We present theoretical results for desorption energetics of free nanoclusters of NaAlH4 as a function of temperature and pressure. Prototype geometries for the clusters were generated using a well-validated electrostatic ground state approach to a global optimization of the cluster total energy using a recently developed non-conventional Monte Carlo random walk in energy space. First-principles density functional theory applied to the prototype clusters was used for full free energy calculations of the clusters and decomposition products. Results will be discussed with relation to recent experimental work on incorporation of complex hydrides in nanoporous framework materials, and the importance of the surface chemistry of the frameworks.
Oct. 6 Jentschura Christian Schubert
Universidad Michoacana de San Nicolás de Hidalgo
Application of String Theory Concepts to Phenomenology During the last fifteeen years, first-quantized path integral methods have emerged as a viable, and sometimes superior, alternative to the standard approach in quantum field theory based on second quantization and Feynman diagrams. In this talk, I will first explain the history of this worldline path integral approach, which goes back to Feynman but gained popularity only through certain developments in string theory, and then present a number of sample calculations, with an emphasis onquantum electrodynamics.
Oct. 7
Hale
Shawn M. Kathmann
PNNL
Understanding the Chemical Physics of Nucleation
The chemical physics of nucleation is currently of great interest to the DOE because the new phase’s characteristics, such as catalytic, electronic and optical properties, depend strongly on size, shape, phase, mixing state, and composition. Subtle variations in interaction potentials, excited states, free energetics, quantum nuclear degrees of freedom, etc., can alter nucleation rates by many orders of magnitude and thus motivate the need to understand their relative influences when absolute accuracy remains intractable. Nucleation is complicated because the condensing molecules configurations, bulk and interfacial reactions, energy flow, electrochemistry, neutral and ionic thermodynamics, and electronic structure must be considered and adequately sampled. I will summarize our computational efforts toward understanding the chemical physics of nucleation in aqueous systems including: molecular interactions, reaction coordinates, phase space sampling, computing rate constants, sensitivity analysis, anharmonic effects, potentials of mean force, activation energies, and molecular vs continuum treatments. Finally, a discussion of outstanding challenges for future work will be presented.
Oct. 13 Yamilov Lihong Wang
WashU
Photoacoustic Tomography: Ultrasonically Breaking through the Optical Diffusion Limit Photoacoustic tomography (PAT), combining optical and ultrasonic waves via the photoacoustic effect, provides in vivo multiscale non-ionizing functional and molecular imaging. Light offers rich tissue contrast but does not penetrate biological tissue in straight paths as x-rays do. Consequently, high-resolution pure optical imaging (e.g., confocal microscopy, two-photon microscopy, and optical coherence tomography) is limited to depths within the optical diffusion limit (~1 mm in the skin). Ultrasonic imaging, on the contrary, provides good image resolution but suffers from poor contrast in early-stage tumors as well as strong speckle artifacts.  In PAT, pulsed laser light penetrates the tissue and generates a small but rapid temperature rise, which induces emission of ultrasonic waves due to thermoelastic expansion. The ultrasonic waves, ~1000 times less scattering than optical waves in tissue, are then detected to form high-resolution images at depths up to 7 cm, breaking through the optical diffusion limit. Further depths can be reached by using microwaves or RF waves as the excitation source. PAT, embodied in the forms of scanning photoacoustic microscopy or photoacoustic computed tomography, is the only modality capable of imaging across the length scales of organelles, cells, tissues, and organs with consistent contrast. Such a technology has the potential to enable multiscale systems biology and accelerate translation from microscopic laboratory discoveries to macroscopic clinical practice. PAT may also hold the key to the earliest detection of cancer by in vivo label-free quantification of hypermetabolism, the quintessential hallmark of cancer. The technology is commercialized by several companies.
Oct. 20 Yamilov Clifford Will
WashU
The Confrontation between General Relativity and Experiment We review the experimental evidence for Einstein's general relativity. Tests of the Einstein Equivalence Principle support the postulates of curved spacetime, while solar-system experiments strongly confirm weak-field general relativity. We discuss the results of the recently concluded Gravity Probe B experiment, and of  observations of binary pulsar systems. Future tests of the theory in the radiative and strong-field regimes may be possible using gravitational-wave observatories on Earth and in space, and using observations of stars orbiting the central black hole in our galaxy.
Oct. 27 Hor
Thomas Vojta
and
Alexey Yamilov
Annual Nobel Prize(s) colloquium:
"Discovery of the accelerating expansion the Universe"
and
"Discovery of quasicrystals"
2011 Nobel Prizes in Physics and Chemistry

Nov. 3 Yamilov
Eighteenth Annual Laird D. Schearer Prize Competition

Nov. 10 Yamilov
Xiaodong Yang
S&T
Manipulating Light with Engineered Optical Nanostructures
Manipulation of light with engineered optical nanostructures will play a critical enabling role in nanotechnologies of the future. The strong light confinement in high-Q photonic crystal nanocavities and subwavelength plasmonic nanostructures enhances light-matter interactions, which permits lots of fundamental studies and integrated nano-optics applications. In this talk, I will present controlling light with high-Q silicon photonic crystal nanocavities, including digital cavity resonance tuning with atomic layer deposition, and the thermal-optic tuning of all-optical analogue to electromagnetically induced transparency (EIT) for slow light applications. I will then describe my recent studies on subwavelength plasmonic nanostructures, including optical forces enhancement with hybrid plasmonic waveguides for applications of optomechanics, and optical nanocavities of indefinite metamaterials with size-independent resonant frequencies. These results present opportunities and challenges in understanding optical physics in nanoscale and building future photonic integrated circuits on which optics, mechanics and electronics are synergistically combined.
Nov. 17 Hor Kim Fook Lee
Michigan Technology University
Optical Quantum Technology for Communication, Imaging and Biophotonics
Superposition and entanglement are the foundations for quantum information processing. One of the great challenges of quantum information science is to develop tools to prepare and manipulate the quantum state of a physical system. I will discuss the generation of high purity fiber based entanglement source in telecom-band for practical quantum communication such as entanglement distribution over a distance of 100 km in a WDM network environment. Generation of photon-pair in silicon-on-insulator waveguide is also presented. I will talk about my recent works of developing a new protocol based on weak coherent states for quantum key generation [Phys. Rev. A 83, 030302(R) (2011)], optical phase-space imaging through Kirkwood-Rihaczek distribution [Phys. Rev. A 81, 063826 (2010)], and optical phase-space-time-frequency tomography for biophotonics [Optics Express 19, 9352-9363 (2011).]
Dec. 1 Dubois
Jeff Shinpaugh
East Carolina University


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