| Physics at the University of Virginia | ||||||
| Academics | People | Research | Announcements | Facilities | Administration | Classes |
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Wednesday, February 24, 1999 Note Special Day |
Dr. Edward Gilman [Host: Joseph Poon] | |
| 4:00 PM, Room 204 | Norfolk State and Jefferson Laboratory | |
| Physics Building | “Materials Research at Norfolk State University” |
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Tuesday, November 16, 1999 Note Special Day |
Anthony Campillo | |
| 4:00 PM, Room 204 | University of Virginia - Physics | |
| Physics Building | “NSOM Studies of SiN Membranes and Photonic Bandgap Structures” |
| Thursday, November 18, 1999 | Jackie Johnson [Host: Despina Louca] | |
| 4:00 PM, Room 204 | Argonne National Lab | |
| Physics Building | “Through a Glass Darkly” |
| Thursday, December 2, 1999 | Timothy Newman [Host: E. Kolomeisky] | |
| 4:00 PM, Room 204 | University of Virginia - Physics | |
| Physics Building | “Directed Polymers and Oppressive Population Control” |
| Abstract: In this blackboard talk, I will discuss new results in the field of directed polymers (these are topological objects found in many condensed matter systems, e.g. flux lines in superconductors, domain walls in ferromagnets, and atomic steps on vicinal surfaces). After a detailed introduction, I will describe the physics of "self-localization" of directed polymers, a state induced by elastic interactions between the polymer and its environment. In the latter part of the talk, I will show how the model of self-localization may be mapped to population dynamics, in which, along with local birth and death rules, one imposes a global constraint which fixes the total population. The dynamics of populations under such oppressive control turns out to be very rich, with phases such as "spreading", "collapse", and a novel "pseudo-traveling wave". |
| Thursday, January 27, 2000 | Anna Wyczalkowska [Host: Despina Louca] | |
| 4:00 PM, Room 204 | University of Maryland | |
| Physics Building | “Crossover Critical Phenomena in Fluids” |
| Thursday, February 24, 2000 | Dr. Edward Gilman [Host: Joseph Poon] | |
| 4:00 PM, Room 204 | Jefferson Laboratory and Norfolk State University | |
| Physics Building | “Materials Research at Jefferson Lab” |
| The Center for Materials Research at Norfolk State University supports a number of efforts in materials science and technology. In addition to the surface science and micro-characterization at the Applied Research Center at JLab, there are programs in free electron laser materials processing, laser spectroscopy, NMR, ESR, polymers and organic thin films. An overview of some of these research efforts will be given. In more detail, work on the Colossal Magnetoresistance (CMR) manganites will be presented. These are materials that exhibit both ferromagnetic and metallic behavior below a critical temperature, but are paramagnetic insulators above. The mechanisms of conduction and ferromagnetism in thin films will be presented and related to the structure of these materials at the atomic level. Proposed uses for these materials include read-write heads and as bolometric detectors |
| Thursday, March 2, 2000 | George Minic [Host: Ziad Maassarani] | |
| 4:00 PM, Room 204 | University of Southern California - (USC) | |
| Physics Building | “Holography and the Cosmological Constant Problem” |
| Holography implies that the most probable value for the cosmological constant is zero. |
| Thursday, March 23, 2000 | Professor Gordon Donaldson [Host: Bascom Deaver] | |
| 4:00 PM, Room 204 | University of Strathclyde | |
| Physics Building | “Progress with SQUID Gradiometers and their Application to Biomagnetism and Non-destructive Testing” |
| Thursday, March 30, 2000 | Steve Gensemer [Host: Robert Jones] | |
| 4:00 PM, Room 204 | University of Connecticut | |
| Physics Building | “Characterization and Control of Ultracold Collisions” |
| We have developed several new techniques for observing and altering ultracold collisions in laser-cooled Rb. We have found that the trapping laser in a typical magneto-optical trap (MOT) can increase inelastic collision rates by more than an order of magnitude. We have also observed ultracold collision dynamics in the temporal domain for the first time, by using a series of laser pulses that interact with a colliding pair of atoms at different internuclear distances. Together with other experiments that use repulsive or attractive molecular potentials to reduce or increase inelastic or elastic collision rates, we are developing the tools to understand and utilize the extremely long-range molecular potentials involved (R > 300 bohr radii), which cannot be studied using conventional spectroscopy. |
| Thursday, April 20, 2000 | Prof. Shenting Cui [Host: R. E. Johnson] | |
| 4:00 PM, Room 204 | Dept. of Chemical Engineering at Univ. of Tenn. and Chemical Technology Div. at Oak Ridge Lab. | |
| Physics Building | “Soft Matter in a Tight Spot: The Structural Transition and the Molecular Origin” |
| Phenomena occurring on nanometer scale are attracting increasing scientific interest due to the expected advances in nanotechnology. This talk will describe recent studies on the fluid behavior occurring at a few nanometers when one of the dimensions, the spacing between two solid surfaces, is reduced to a few molecular diameters. Such conditions are encountered in the operation of the computer magnetic disks, preparation of nano-structured materials, colloidal dispersions, and automobile lubrication, etc. When a fluid is confined within spacing comparable to the molecular dimension, interfacial interactions become predominant and this can dramatically alter the structural, thermodynamic and rheological properties of the fluid film. Experiments have found that solid-like behaviors develop for fluids confined between molecularly smooth mica surfaces. This is manifested by the ability of the film to sustain a finite stress, a phenomenon typically associated with solids, and orders of magnitude increase in viscosity. The microscopic mechanism for the phenomenon is still not well understood theoretically. We describe a comprehensive molecular simulation study that demonstrates the solid-like behavior for alkane films narrowly confined in molecularly smooth strong-adsorbing surfaces at ambient temperature and pressure. The results of the study correlate a broad range of experimental observations and point to a mechanism different from that has been previously suggested. |
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Tuesday, May 30, 2000 Note Special Day |
Kamal Benslama [Host: Craig Dukes] | |
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3:00 PM, Room 313 Note Special Time |
University of Regina | |
| Physics Building | “The Measurement of Lambda Polarization at Nomad Experiment” |
| Thursday, July 20, 2000 | Steven Garrett [Host: Bellave S. Shivaram] | |
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10:00 PM, Room 125 Note Special Time |
United Technologies Corporation Professor of Acoustics - Penn State University | |
| Material Science | “Thermoacoustic Refrigeration” |
| The interaction of heat and sound has been a subject of interest to scientists and engineers since 1816 when Laplace corrected Newton's attempt to derive the speed of sound in air from first principles. Glassblowers observed the generation of sound in the presence of temperature gradients even earlier. It was less than twenty years ago that the reverse process - the use of high-amplitude sound to produce refrigeration - was first demonstrated. Due to the discovery of the "hole-in-the-ozone" and the ratification of the Montreal Protocols, research in "thermoacoustics" has accelerated during the past decade. In 1992, an electrically-powered thermoacoustic refrigerator was placed in orbit on the Space Shuttle and a larger thermoacoustic chiller for shipboard electronics was demonstrated for a week on board a US Navy destroyer in 1995. More recently, a heat-driven thermoacoustic device was used to liquefy in excess of 100 gal/day of natural gas by burning part of the gas stream. This presentation will include a simple description of the thermoacoustic heat pumping process and will describe some of the novel devices that have exploited this process. |
| Thursday, September 21, 2000 | Timothy Newman/P0STP0NED [Host: Eugene Kolomeisky] | |
| 4:00 PM, Room 204 | University of Virginia, Physics and Biology | |
| Physics Building | “The hidden world of the diffusion equation” |
| The study of persistent structures in dynamical processes is one of the most interesting recent developments in statistical mechanics. In this talk I shall give a brief review of persistence and its applications. This will be followed by the presentation of some very recent results concerning persistence in the diffusion equation, the foremost of which concerns the discovery of two (astonishingly high) critical dimensions. |
| Thursday, September 28, 2000 | Amit Chattapadhyay [Host: Eugene Kolomeisky] | |
| 4:00 PM, Room 204 | UVA-Chemistry | |
| Physics Building | “Turbulent Shear Flow: Analytic Result for a Universal Amplitude” |
| In the turbulent boundary layer above a flat plate, the velocity profile is known to be proportional to the logarithm of the plate separation distance. We will arrive at this experimental observation from theoretical arguments and will give an estimate of the proportionality constant, which happens to be a universal number. The final calculations will be compared with experimental data." |
| Thursday, October 12, 2000 | Professor Ken Kelton [Host: Joseph Poon] | |
| 4:00 PM, Room 204 | Department of Physics - Washington University | |
| Physics Building | “Nucleation When Diffusion Becomes Important: A Tale of Two Fluxes” |
| The classical theory of nucleation does not properly describe nucleation processes when long-range diffusion becomes competitive with the interfacial processes. A new model is presented for time-dependent homogeneous nucleation in condensed phases, which takes account of the coupled fluxes of interfacial attachment and long-range diffusion. Numerical solutions from this new model show that the time-dependent nucleation rates scale with the dominant mobility and that the steady-state rates and induction times frequently differ significantly from values predicted by the classical theory. Surprisingly, the composition of the region of the parent phase near sub-critical crystal clusters is shifted toward that of the new phase. Consequences are discussed for solid state precipitation and the crystallization of Al-rare earth metallic glasses. |
| Thursday, October 19, 2000 | Robert Konik [Host: E. Kolomeisky] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Quantum Dots: A Tunable Kondo Effect” |
| Thursday, November 16, 2000 | John Larese [Host: George Hess] | |
| 4:00 PM, Room 204 | Brookhaven National Laboratory | |
| Physics Building | “Smoke and Mirrors: The MgO saga” |
| Thursday, November 30, 2000 | Yu Xia [Host: Joseph Poon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Thermoelectric Properties in Sb doped TiNiSn Half-Heusler Alloys” |
| Thursday, February 1, 2001 | Dr. Jongsoo Yoon [Host: B. Shivaram] | |
| 4:00 PM, Room 204 | University of California at Berkeley | |
| Physics Building | “Quantum Wigner crystal and apparent metal-insulator transition of dilute 2D holes in GaAs at B=0” |
| The scaling theory of localization for non-interacting electrons predicts that all states in 2D are localized by any amount of disorder in the zero temperature limit. In contrast, a "metallic" behavior characterized by a decreasing resistivity with decreasing temperaure, and thus an apparent metal-insulator transition as the charge-carrier density is reduced, is observed on many different low disorder dilute 2D systems such as Si MOSFET, GaAs, SiGe, and AlAs at low temperatures. It is found that the charge-carrier density where a metal-insulator transition occurs monotonically decreases with decreasing disorder. In an exceptionally high mobility 2D hole system in a GaAs heterostructure, a metal-insulator transition is observed at a density where the clean quantum Wigner crystal is expected, suggesting that the metal-insulator transition is related to the melting of the Wigner crystal. Details of transport characteristics in this GaAs system at both zero applied magnetic field (B=0) and with magnetic fields applied parallel to the 2D plane will be presented. |
| Thursday, February 8, 2001 | Peter Abbamonte [Host: D Louca] | |
| 4:00 PM, Room 204 | University of Groningen | |
| Physics Building | “Resonant Inelastic X-ray Scattering from Insulating Cuprates” |
| In this talk I will give a general introduction to the technique of inelastic x-ray scattering (IXS) and its use for measuring elementary excitations in condensed matter. I will describe how resonance techniques can be used to achieve (1) enhancement of the inelastic cross section and (2) sensitivity to wave function symmetry. These points will be illustrated with polarization-dependent spectra from the High Tc parent insulators La2CuO4 and Sr2CuO2Cl2, and by their interpretation in terms of symmetry-selection rules. I will conclude with some general statements about the utility of IXS - and where to go from here. |
| Thursday, February 22, 2001 | Dr. Winfried Teizer [Host: J. Poon] | |
| 4:00 PM, Room 204 | University of California at San Diego | |
| Physics Building | “The Density of States in the Quantum Critical Regime of the Metal-Insulator Transition” |
| In the quantum critical regime, Coulomb interactions modify the density of states at the Fermi level and ultimately open a soft Coulomb gap since the decreasing electron mobility inhibits efficient screening. I will present tunneling spectroscopy data on amorphous GdxSi1-x, a material that can be continuously and reversibly tuned through the metal-insulator transition. On the metallic side, we find a signature of strong Coulomb interactions in the density of states, as theoretically expected. As the metal-insulator transition is approached from the metallic side, the tunneling spectrum shows a precursor of a soft Coulomb gap prior to reaching the insulating regime. I will extract the quantitative relationship of the density of states at the Fermi energy and the transport conductivity in the quantum critical regime. |
| Thursday, March 1, 2001 | Gerhard Schoenthal [Host: Eugene Kolomeisky] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “GaAs Varactor Based Frequency Tripler Technology” |
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Monday, March 26, 2001 Note Special Day |
David Cobden [Host: Despina Louca] | |
| 4:00 PM, Room 204 | Univ. of Warwick | |
| Physics Building | “Carbon nanotube quantum wires and quantum dots” |
| Recent experiments on transport through individual single-wall carbon nanotubes with electrical contacts have shown that a broad range of physical phenonema can occur in such molecular-electronic devices [1,2]. I will review our present understanding of metallic nanotubes as interacting quantum wires and dots, with more emphasis on the quantum dot regime. As a result of the one-dimensional (1D) band structure, the spectral characteristics of nanotube dots are in informative contrast with those of dots formed in other systems having 2D or 3D bands. Also, these 1D quantum dots are preserved, effectively floating in vacuum, when the substrate beneath the tubes is etched away. In the resulting tightrope geometry we can begin to study the interplay of the molecule's electronic properties with a wide range of environmental factors. Finally, having achieved surprisingly good electrical contacts between the metal leads and tubes, we find that nanotube dots prove to be an excellent system for studying new aspects of Kondo physics. [1] Cees Dekker, "Carbon Nanotubes as Molecular Quantum Wires". Physics Today 52, 22-28 (1999). [2] Paul McEuen and others, Special reports in Physics World vol 13 issue 6 (June 2000). |
| Thursday, March 29, 2001 | Professor Jingyu Lin [Host: E. Kolomeisky and J. Poon] | |
| 4:00 PM, Room 204 | Kansas State University | |
| Physics Building | “Growth and Optical Properties of III-Nitride Wide Bandgap Semiconductors” |
| III-nitride wide band gap semiconductors (GaN, AlGaN and InGaN) have been a subject of intense recent investigation and have emerged as an important materials system for applications in areas of optoelectronic devices which are active in the blue and ultraviolet (UV) wavelength regions and electronic devices capable of operation at high temperatures, high power levels, and in harsh environments. In this talk, the epitaxial growth and fundamental optical and transport studies of III-nitrides, including GaN epilayers, InGaN and AlGaN alloys, and multiple quantum wells comprising alternating layers of InxGa1-xN/GaN and GaN/AlxGa1-xN will be discussed. In particular, how the picosecond time-resolved photoluminescence studies in providing input for the development of suitable materials quality and device design for specific applications as well as for the understanding of fundamental optical transitions and lasing mechanisms will be presented. |
| Thursday, April 5, 2001 | Prof. Carlos Bustamante [Host: Ian Harrison] | |
| 4:00 PM, Room 304 | University California Berkeley | |
| Chemistry Building | “Recent Advances in Single Molecular Manipulation in Biophysics” |
| Thursday, April 5, 2001 | Derek Stewart [Host: J. Ruvalds] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Giant Magnetoresistance in Spin Valve Multilayers” |
| Thursday, April 12, 2001 | Rama Balasubramanian [Host: D. Louca] | |
| 4:00 PM, Room 204 | James Madison University | |
| Physics Building | “X-ray Diffraction and Spectroscopic Investigations of Nanophased Iron” |
| Nano-phased iron oxides often found in corrosion coatings have properties that are scientifically interesting and industrially important. Naturally weathered iron oxides have particle size in the nanometer regime. Characterizing these oxides therefore require highly specialized spectroscopic tool to accurately quantify the relative % composition of the oxides usually present in a complex mixture. I will be presenting the use of Mossbauer, Micro-Raman and x-ray diffraction tools to accurately characterize each of the oxides in the mixture. Spectroscopic investigations on the influence of chromium in forming nano-phased goethite suggests that with increasing chromium concentration superparamagnetic behavior is enhanced. Micro-structural properties of mechanically alloyed Fe-Al alloys will be presented. |
| Thursday, April 19, 2001 | F. A. Gianturco [Host: V. Celli] | |
| 4:00 PM, Room 204 | University of Rome | |
| Physics Building | “Molecular impurities in He clusters: microsolvation shells and superfluidity” |
| The study of unstable chemical species and of fast chemical reactions has profited greatly, in the last few years, from the development improvements of a novel experimental approach, which has been called the helium cluster beam isolation spectroscopy. In this method a beam of clusters, containing from about 103 to about 105 He atoms, picks up in flight one or more atoms or molecules (neutral or ionized) which can then be studied downstream with laser spectroscopy. The corresponding microscopic understanding of the possible structures of the weakly-interacting 4He atoms around or "outside" the molecular impurities requires the treatment of quantum effects on nuclear motions and the selection of the main, dominant structures within each cluster as a function of its size. In the present work we present new results for the quantum structures and the vibrational and rotational shifts for an OCS molecule in 4He clusters using the Diffusion Monte Carlo (DMC) method. A comparison with the recent results [S. Grebenev et al., J. Chem. Phys. 112, 4485 (2000)] from the experimental group in Göttingen will also be reported. |
| Thursday, April 26, 2001 | Justin Rattner | |
|
7:30 PM, Room Chemistry Auditorium Note Special Time |
Intel Fellow and Director of Intel's Microprocessor Research Lab | |
| Chemistry Building | “Electronics in the Internet Age” |
| Thursday, October 18, 2001 | Arthur Brill [Host: D. Louca] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Electron-nuclear double resonance-mediated dynamic nuclear polarization” |
| Because electronic magnetic moments are orders of magnitude greater than nuclear magnetic moments, for a given magnetic field strength and temperature, electronic paramagnetism far exceeds nuclear paramagnetism. The sensitivities of certain measurements in many branches of physics and chemistry depend upon the extent of nuclear spin polarization. Dynamic nuclear polarization (DNP) processes enable experimenters to transfer electron spin polarization to nuclear spin systems. In this seminar a newly-proposed method, with the potential for achieving as great as 50% polarization of protons and deuterons in much less time and with much lower microwave power than the DNP methods in current use, will be described. |
| Thursday, November 8, 2001 | Robert F. Berg [Host: G. Hess] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “Polymer-like viscosity near the critical point of xenon” |
| This talk will describe two measurements of viscosity very close to xenon's critical point. The first experiment measured the viscosity increase caused by near-critical conditions. It revealed that, close to the critical point, xenon is partly elastic: It can stretch as well as flow. The second experiment, planned for 2002, will look for the shear-rate-induced viscosity decrease predicted by theory. Such "viscoelasticity" and "shear-thinning" are ordinarily seen only in much more complicated fluids such as polymer solutions. Both experiments are designed for the Space Shuttle. We required the Space Shuttle's microgravity because Earth's gravity compresses any fluid near its critical point. A layer of fluid even as thin as a dime collapses under its own weight, increasing the density at the bottom of the sample and distorting the data. Conducting the experiments on the Space Shuttle reduces these density differences by a factor of 100. |
| Thursday, January 17, 2002 | Smitha Vishveshwara [Host: E. Kolomeisky] | |
| 4:00 PM, Room 204 | UCSB | |
| Physics Building | “Carbon Nanotubes: a playground for Luttinger liquid physics” |
| The electronic properties of single-walled metallic carbon nanotubes can be effectively captured by a system of interacting electrons in one dimension. As shown by both theory and experiment, and as discussed here, the nanotube thus exhibits signatures of Luttinger liquid behaviour. Furthermore, as a consequence of Luttinger liquid physics, we find that nanotubes are expected to display bizarre phenomena such as charge fractionalization; anamolous suppression of Andreev current when contacted to superconductors; hosting of entangled quantum states from a superconducting source. |
| Thursday, January 24, 2002 | Phuc Tran [Host: V. Celli] | |
| 4:00 PM, Room 204 | China Lake/Phillip Morris | |
| Physics Building | “Photonic Bandgap Crystal, Flattop Narrow-Band Filter, Modulators, and Switches” |
| Since the concept of Photonic Bandgap Crystal (PBC) was introduced in 1989 as a means to make low lasing threshold lasers, many many more applications of PBC have been suggested, particularly in the telecommunication area. In this talk, we will discuss the R-matrix technique to calculate the transmission through a PBC and the dispersion of surface waves on a PBC. On the application side, we will discuss the design of a flattop, narrow-band, tunable filter for use in Dense Wavelength Division Multiplexing (DWDM) systems. By incorporating non-linearity, we can make an all-optical modulator or switch. A nonlinear Finite Domain Time Dependent (FDTD) technique to simulate such a device will also be presented. |
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Monday, January 28, 2002 Note Special Day |
Barbara Drossel [Host: Alan McKane] | |
| 4:00 PM, Room 204 | Darmstadt(Germany) | |
| Physics Building | “Advection of Particles by Burger's Turbulence” |
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Wednesday, January 30, 2002 Note Special Day |
Eugene Pivovarov [Host: P. Fendley] | |
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3:00 PM, Room 204 Note Special Time |
Caltech | |
| Physics Building | “Non-conventional metals: odd-frequency density waves and d-density wave” |
| We discuss the metallic states which are ordered and therefore are not convetional Fermi liquids. In odd-frequency density waves the order parameter vanishes at zero frequency and there is a conventional Fermi surface. However, the frequency dependence of the gap leads to an unusual temperature dependence for various thermodynamic and transport properties. The d-density wave (DDW) is a candidate for the explanation of the pseudogap phase in superconducting cuprates. The gap vanishes at the nodal points, which causes the pure DDW state to be metallic. However, DDW can coexist with either antiferromagnetic or superconducting phases. We consider a microscopic model which favors DDW and do the mean-field-theoretical derivation of the phase diagram (this work is in progress). |
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Wednesday, February 6, 2002 Note Special Day |
Carlos Bolech-Gret [Host: P. Fendley] | |
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3:00 PM, Room 204 Note Special Time |
Rutgers | |
| Physics Building | “On the Bethe Ansatz Solution of the Two-Channel Anderson Impurity Model” |
| Thursday, February 7, 2002 | Professor I. Takeuchi [Host: Joseph Poon and Ian Harrison] | |
| 4:00 PM, Room 204 | University of Maryland | |
| Physics Building | “Combinatorial approach to materials discovery ” |
| Throughout the history of mankind, scientists and engineers have relied on the slow and random trial-and-error process for materials discovery. The combinatorial approach to materials is an emerging new method of materials research aimed at drastically increasing the rate and efficiency at which new materials are discovered and improved. In order to rapidly survey a large compositional landscape, up to thousands of compositionally varying samples are synthesized, processed, and screened in a single experiment. We have developed thin film combinatorial techniques for exploring new materials phases in a number of key technology areas including electronic, magnetic, and smart materials. Our synthesis tools include combinatorial pulsed laser deposition systems and a UHV combinatorial co-sputtering system. Our rapid characterization tools include scanning SQUID microscopes for mapping properties of magnetic materials libraries and scanning near field microwave microscopes for screening dielectric and conducting materials. Recent developments in investigation of ferromagnetic shape memory alloys and ferroelectric materials will be discussed. |
| Thursday, April 4, 2002 | Jiufeng Tu [Host: D. Louca] | |
| 4:00 PM, Room 204 | Brookhaven National laboratory | |
| Physics Building | “Probing Superconductors with Infrared - Electron-boson coupling in high temperature superconductors” |
| High-temperature superconductivity in an oxide containing quasi-two-dimensional copper-oxygen planes was observed by Bednorz and Muller in 1986. Recently, several high-temperature superconductors without copper or oxygen have been discovered including MgB2 with a Tc of 39K and electric field doped C60 with a Tc as high as 117K. Infrared spectroscopy has emerged as one of the most powerful experimental tools for the study of correlated electron systems and for high-Tc superconductors in particular. This talk will be focused on the infrared studies of two representative high-Tc superconductors: MgB2 (Tc = 39.6 K) and optimally doped Bi2Sr2CaCu2O8+¦Ä (Tc = 91.5 K). Effects of electron-boson coupling are observed in optical conductivities for both systems and their significance with respect to superconductivity will be discussed. In general, having a small free carrier plasma frequency (< 3 eV) seems to be an universal characteristic shared by almost all high-temperature superconductors with a Tc > 30 K, which means that the issue of reduced screening should be treated carefully in all of these systems. |
| Thursday, April 11, 2002 | Xiaoya Qi [Host: D. Louca] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Quantum fluctuations of charge and phase transitions of alarge Coulomb-blockaded quantum dot” |
| We analyze ground-state properties of a large gated quantum dot coupled via a quantum point contact to a reservoir of one-dimensional interacting spinless electrons. We find that the classical step-like dependence of the dot population on the gate voltage is preserved under certain conditions. We point out that the problem is related to the classical one-dimensional Ising model with inverse-square interactions. This Ising universality class further subdivides into (i) the Kondo/Ising class and (ii) the tricritical class. For systems of the Kondo/Ising class, and repulsive electrons, the gate voltage dependence of the dot population is continuous for sufficiently open dots, while taking the form of a modified staircase for dots sufficiently isolated from the reservoir. At the phase transition between the two regimes the magnitude of the dot population jump is universal. For systems in the tricritical class we find in addition an intermediate regime where the dot population jumps from near integer value to a region of stable population centered about a half-integer value. In particular, this tricritical behaviour (together with the two dependencies already seen in the Kondo/Ising class) is realized for non-interacting electrons. |
| Thursday, September 5, 2002 | Paul Fendley [Host: J. Ruvalds] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Dimers on a Triangular Lattice” |
| Thursday, September 12, 2002 | Prof. Cass Sackett [Host: J. Ruvalds] | |
| 4:00 PM, Room 204 | UVA-Physics | |
| Physics Building | “Many Body effects in atomic Bose Condensates” |
| Thursday, October 3, 2002 | Prof. Robert Hull [Host: J. Ruvalds] | |
| 4:00 PM, Room 204 | UVA Materials Sciece and Engineering, MRSEC Director | |
| Physics Building | “The Materials Research Science and Engineering Center at UVA: From Materials to Quantum Cellular Automata” |
| Thursday, October 10, 2002 | Prof. D. Dessau [Host: J. Ruvalds] | |
| 4:00 PM, Room 204 | Univ. of Colorado | |
| Physics Building | “Photomission Spectroscopy of High Temperature Superconductors” |
| Thursday, October 17, 2002 | Dr. S. Miret-Artes [Host: V. Celli] | |
| 4:00 PM, Room 204 | CSIC-Madrid | |
| Physics Building | “Quantum Trajectories and Particle Diffraction"” |
| Thursday, October 24, 2002 | Prof. A. Lichtenberg [Host: Bascom Deaver] | |
| 4:00 PM, Room 204 | UVA-Superconducting Materials, Device and Circuit Research for THz Receivers | |
| Physics Building | “Superconductor Divices” |
| Thursday, October 31, 2002 | Prof. Matthew Neurock [Host: J. Poon] | |
| 4:00 PM, Room 204 | UVA - Chemical Engineering | |
| Physics Building | “Atomically Engineered Active Sites and Environments for Supported Metal Catalysts” |
| Heterogeneous catalysis is responsible for over $400 billion in annual income each year which results from the sale of pharmaceutical and chemical intermediates made via catalytic transformations. Knowledge of the atomic structure at and near the active catalytic site and how it influences reactivity could revolutionize our ability to design more active and selective catalysts. Significant advances in both theory and simulation have occurred over the past decade thus making theoretical chemistry an invaluable partner to experiment in this endeavor. In this talk, we use both ab initio Molecular Dynamics and ab initio-based dynamic Monte Carlo methods to simulate the dynamics and catalytic kinetics. We show that it is now possible to track the nature of the active surface site along with the local “molecular” environment about the active surface ensemble. We specifically probe the influence of surface coverage, bimetallic alloys, and the molecular networks that form at a liquid/metal interface on catalytic activity. This talk will focus on the application of these tools to the selective hydrogenation of olefins, the synthesis of oxygenates, and methanol oxidation for the direct methanol fuel cell. |
| Thursday, November 7, 2002 | Prof. Ying Lu [Host: Y. Yoon] | |
| 4:00 PM, Room 204 | Penn State | |
| Physics Building | “Destructive regime and superconductor-normal metal transition in ultrathin cylinders” |
| We have carried out experiments on ultrathin superconducting cylinders, which revealed the existence of a destructive regime, the loss of superconductivity around half-integer flux quanta even in the zero temperature limit, predicted by de Gennes for doubly connected samples with a diameter smaller than zero-temperature superconducting coherence length [Y. Liu, et al., Science 294, 2332 (2001)]. More recently we have done measurements on the superconductor-normal metal (S-N) transition tuned by magnetic flux in these cylinders, and found evidence for the formation of normal bands near the S-N transition. |
| Thursday, November 14, 2002 | Eugene Kolomeisky [Host: John Ruvalds] | |
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3:30 PM, Room 204 Note Special Time |
University of Virginia | |
| Physics Building | “Ground-state properties of one-dimensional matter and quantum dissociation of a Luttinger liquid ” |
| Thursday, January 23, 2003 | Oleg Tchernyshyov [Host: Paul Fendley] | |
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3:30 PM, Room 204 Note Special Time |
Johns Hopkins | |
| Physics Building | “Valence bond liquids and solids in geometrically frustrated magnets” |
| Frustrated magnets are models of strongly interacting systems. Frustration reveals itself through a vast degeneracy of the classical ground state of a magnet and is responsible for a large number of soft modes that destroy magnetic order. Possible alternative phases that could emerge in place of the Neel state are the valence-bond solid, which breaks some lattice symmetry, and the valence-bond liquid, which has no broken symmetries yet differs from a paramagnet by the existence of a hidden topological order. We have recently found examples of valence-bond liquids and solids in a study of almost classical frustrated antiferromagnets on the pyrochlore lattice and its planar analogs. |
| Thursday, January 30, 2003 | Dr. Mazin [Host: Joseph Poon] | |
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3:30 PM, Room 204 Note Special Time |
Naval Research Lab. | |
| Physics Building | “Novel Copperless Superconductors” |
| I will review some of the recent discoveries in superconductivity outside of the high Tc cuprates domain. I will speak in particular about two-band superconductivity in MgB2, competition of superconductivity and magnetism in MgCNi3, possible p-wave superconductivity vs. Fulde-Ferrel state in ZrZn2 and superconductivity in hexagonal Fe under pressure. |
| Thursday, February 13, 2003 | John Cardy [Host: Paul Fendley] | |
|
2:30 PM, Room 313 Note Special Time |
IAS and Oxford University | |
| Physics Building | “The Effect of Quenched Impurities on First-Order Transitions” |
| In two dimensions, it is known that the presence of any finite concentration of quenched impurities drives a first-order phase transition into a continuous one (or destroys it completely). In three dimensions, this is believed also to occur, above some critical concentration. However, the nature of the continuous transition which may result is very poorly understood. This talk will review the physics behind these results, discuss some of the numerical efforts at understanding the problem, and finally describe one model for which a more complete picture is available. |
| Thursday, February 20, 2003 | Seunghun Hong [Host: Jongsoo Yoon] | |
|
3:30 PM, Room 204 Note Special Time |
FSU | |
| Physics Building | “Hybrid Nanostructures and Devices” |
| Physics, Institute of Molecular Biophysics, and Center for Materials Research and Technology, Florida State University, Tallahassee, FL 32306 Recent dramatic progress of nanotechnology and biological science allows us to combine organic molecules (e.g. biomolecules, self-assembled monolayer etc.) with solid state nanostructures (e.g. metal nanoparticles, carbon nanotubes, microfabricated circuits etc.) to build a generation of new hybrid nanoscale devices. These include nanoscale biological sensors and protein motor-based nanomechanical systems. One promising nano-manufacturing method for these hybrid devices is the surface-templated assembly process. In this strategy, direct deposition methods such as dip-pen nanolithography are utilized to functionalize the desired solid substrate area with organic molecules, and nanostructures (e.g. carbon nanotubes, nanoparticles, proteins etc.) in the solution are specifically assembled onto the functionalized area via molecular recognition mechanism. In this presentation, we will discuss about 1) new properties of hybrid nanostructures and 2) important scientific issues related with the surface-templated assembly process. |
| Thursday, February 27, 2003 | Ryan Kalas | |
|
3:30 PM, Room 204 Note Special Time |
UVA | |
| Physics Building | “Ground-state properties of artificial bosonic atoms, the Bose interaction blockade, and the single-atomic pipette” |
| We analyze the ground-state properties of an artificial atom made out of repulsive bosons attracted to a center for the case when all the interactions are short-ranged. The properties of this artificial bosonic atom, which can be created by optically trapping ultracold particles of alkali vapors, can be varied by adjusting both the strength of "nuclear" attraction and the interparticale repulsion. The dependence of the ground-state energy of the atom on the number of particles has a minimum whose position is experimentally tuneable. This implies that the number of bound bosons has a staircase dependence on external parameters which may be used to create a single-atomic pipette -- a set-up allowing the transport of atoms into and out of a reservoir one at a time. |
| Thursday, March 13, 2003 | Adam Phillips [Host: Jongsoo Yoon] | |
|
3:30 PM, Room 204 Note Special Time |
UVA | |
| Physics Building | “Thermal Measurements for Thin Films” |
| Thursday, March 20, 2003 | Andrew J. Lovinger [Host: Bellave S. Shivaram] | |
|
3:30 PM, Room 204 Note Special Time |
Bell Laboratories, Lucent Technologies | |
| Physics Building | “ELECTROACTIVE POLYMERIC AND ORGANIC MATERIALS FOR THIN-FILM-TRANSISTOR APPLICATIONS” |
| The last few years have seen enormous growth in the area commonly referred to as "plastic electronics". The active elements of all such circuits are field-effect transistors. I will summarize our work on organic and polymeric transistors, which recently resulted in the first prototype of a flexible plastic display. The emphasis of the talk will be on the materials-science aspects, specifically on structure-morphology-processing-property correlations. We have synthesized and characterized a large number of organic and polymeric semiconductors, both p- and n-type, that are processable by vapor- or solution techniques (including printing). I will discuss the requirements that must be satisfied at the various levels of structure (chemical, molecular, unit-cell, crystalline, and bulk) for optimal charge transport. One centrally important requirement in this regard is the orientation of the semiconductor molecules on the substrate. For one organic compound we have also been able to visualize the conductive pathways themselves at a submolecular level of resolution. Lastly, a few applications of these materials will also be briefly discussed. |
| Thursday, April 3, 2003 | Seunghun Lee [Host: Despina Louca] | |
|
3:30 PM, Room 204 Note Special Time |
National Institute of Standards and Technology | |
| Physics Building | “Emergent excitations and novel phase transitions in geometrically frustrated magnets” |
| Strongly correlated systems where the degrees of freedom cannot order despite their strong interactions have constituted an important issue in modern condensed matter physics. Such systems usually have many competing states as ground states that can lead to qualitatively new states of matter. An example is geometric frustration, a magnetic phenomenon in which the topology of the lattice induces a macroscopic ground state degeneracy and prohibits the spin system from ordering. The important issues in this field are: (1) what the nature of the spin liquid phase is and (2) how the system responds to the ground state degeneracy. In this talk, I will address these issues by discussing a spinel antiferromagnet ZnCr2O4. In ZnCr2O4, the magnetic Cr3+ ions form a lattice of corner-sharing tetrahedra with uniform nearest neighbor antiferromagnetic couplings and makes the system as the most frustrated magnet so far. Recently we found by inelastic neutron scattering that a composite spin degree of freedom emerges in the cubic spinel. In the gapless spin liquid phase, spins self-organize into weakly interacting antiferromagnetic hexagonal loops rather than fluctuating individually. The emergence of the composite spin degree of freedom suggests an organizing principle for frustrated systems such that if macroscopic condensation is not possible, interacting degrees of freedom combine to form rigid clusters. We have also shown that the system can lift the degeneracy via a spin-Peierls-like phase transition from the cubic spin liquid to a tetragonal Neel state. Finally, if time allows, I will also discuss how the spin liquid state and the phase transition change in the presence of bond/site disorders or further neighbor interactions. |
| Thursday, April 10, 2003 | Qi Li [Host: J. Yoon] | |
|
3:30 PM, Room 204 Note Special Time |
Penn State Univ. | |
| Physics Building | “Anomalous magnetoresistance effect in ultrathin manganite films” |
| Recently, doped manganites have been studied extensively due to various unusual phenomena observed in the system, such as the colossal magnetoresistance effect which is associated with a metal-insulator transition. Although a fundamental understanding of the system is not yet available, it is well known experimentally that spin, charge, and lattice degree of freedom are strongly coupled. We have studied epitaxial ultrathin manganite films in which bi-axial lattice distortion is imposed due to the lattice strain. Two anomalous effects are found which are not present in single crystals. One is unusually large low field magnetoresistance effect occurring only in compressively strained thin films and the other is giant anisotropic magnetoresistance. The details of the results will be presented and these results are not yet explained using existing models. |
| Thursday, April 17, 2003 | Pillip Kim [Host: Jongsoo Yoon] | |
|
3:30 PM, Room 204 Note Special Time |
Columbia University | |
| Physics Building | “Measurement of Thermal Properties at Mesoscopic Scales” |
| The thermal properties of nanoscale materials are of fundamental interest and also play a critical role in controlling the performance and stability of nanodevices made of these materials. However, the measurements of thermal properties of nanomaterials at a mesoscopic scale have been technically challenging problems. We have fabricated submicron scale devices hybrided with nanoscale materials using state-of-art microfabrication thechniques. The thermal conductivity and thermoelectric power of carbon nanotubes and other nanowires have been measured at mesoscopic levels and exhibit distinticvely different behaviors from bulk material measurement. |
| Thursday, April 24, 2003 | Tom Vandervelde [Host: Jongsoo Yoon] | |
|
3:30 PM, Room 204 Note Special Time |
UVA | |
| Physics Building | “Regulated Self-assembly of Silicon-Germanium Quantum Dots” |
| Thursday, May 1, 2003 | Dr. G. Myneni [Host: Belave Shivaram] | |
|
3:30 PM, Room 204 Note Special Time |
Thomas Jefferson National Accelerator Lab | |
| Physics Building | “Hydrogen in vacuum systems and SRF cavities” |
|
Friday, July 18, 2003 Note Special Day |
Dr. Hwa Shik Youn [Host: George Hess] | |
|
3:30 PM, Room 313 Note Special Time |
Beamline Division Head, Pohang Accelerator Laboratory, Pohang, Korea | |
| Physics Building | “X-ray Microscopy using synchrotron radiation -- a tool of nanotechnology” |
| Thursday, August 28, 2003 | Nick Rizzo [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | Motorola Labs | |
| Physics Building | “The physics of magnetoresistive random access memory (MRAM) based on magnetic tunnel junctions” |
| Resistive Random Access Memory (MRAM) is a new nonvolatile solid state memory that has the potential to be fast, low power, high density, and have unlimited read/write cycles. These combined characteristics make MRAM superior to other memory technologies. For a fully functioning MRAM, the understanding and control of a rich variety of physical phenomena is required. The bit cell is a magnetic tunnel junction, which consists of two thin magnetic layers separated by an ultrathin layer (1 nm) of oxidized Al. The tunneling process itself is an example of a macroscopic quantum effect. The magnetoresistance changes as much as 50% for parallel to antiparallel layer magnetizations. The magnetoresistance is a result of s-d exchange which polarizes conduction electrons to be parallel with the layer magnetizations. The uniformity of the tunnel barrier is critical for well-defined resistance values and to minimize tunneling hotspots. We show a resistance uniformity of less than 1% within a die, which implies an average thickness uniformity of a fraction of an angstrom. The bit shape is typically elliptical, with a magnetic free layer of thickness 4-6 nm. The bit shape defines a shape anisotropy along with a switching field. Variations in lithography and material properties cause a distribution in switching fields - a distribution that must be minimized for error free programming. The state of the bit (0 or 1) is programmed using current pulses that are nanoseconds in duration. The pulses are sent down conductive lines that are surrounded by a thin permeable magnetic film to enhance the generated field. A major challenge to error free programming is to minimize bit switching that is thermally activated. We have characterized thermal activation in our bits and showed that it obeys classic Arrhenius-Neel activation theory for single energy barriers. One way that we have developed to minimize the effects of thermal activation is to use multilayer films to increase the energy barrier to magnetization reversal. In the first part of this talk I will discuss the development of the MRAM read and write process at Motorola, covering the topics described above. I will then review the performance of Motorola's 1 Mb MRAM test vehicle. Finally, I will conclude by listing the major unsolved challenges that MRAM needs solved to become a dominant memory technology. |
| Thursday, September 11, 2003 | Dr. Xin-Nian Wang [Host: Blaine Norum] | |
| 4:00 PM, Room 204 | Lawrence Berkeley Lab | |
| Physics Building | “Nuclear Modification of Jet Fragmentation” |
| Thursday, September 18, 2003 | Xiao Yang [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Weak Coupling Instabilities of Diffusive Fermi Liquid” |
| Thursday, September 25, 2003 | Dr. Bjorvin Hjovarson [Host: B. Shivaram] | |
| 4:00 PM, Room 204 | Uppsala, Sweden | |
| Physics Building | “Playing with dimensionality - magnetism and transport in hydrides” |
| The use of hydrogen to modify the electronic structure in magnetic thin films and heterostructures has opened new routes to tailor magnetic interactions in materials. Hydrogen can, for example, be used to control the strength and character of magnetic interactions. For example, the switching from antiferromagnetic to ferromagnetic order, and vice versa, has been demonstrated for exchange coupled magnetic superlattices. As the sign of the interlayer exchange coupling (J) can be switched by the insertion of hydrogen, J´ arbitrarily close to zero must be accessible. When the exchange interaction between adjacent magnetic layers is completely suppressed, the heterostructure can then be taken to consist of a collection of quasi two-dimensional magnetic sheets, when the ferromagnetic layers are very thin. Consequently, the introduction of hydrogen can be viewed as a route to tune the dimensionality of these structures. |
| Thursday, October 2, 2003 | Dr. Xiao Yang [Host: P. Fendley] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Weak Coupling Instabilities of Diffusive Fermi Liquids” |
| Thursday, October 16, 2003 | Dr. Xiaowang Zhou [Host: J. Ruvalds] | |
| 4:00 PM, Room 204 | UVA- Engineering | |
| Physics Building | “Atomic Scale Structure of Giant Magnetoresistance and Spin Tunnel Junction Multilayers” |
| Thursday, October 23, 2003 | Dr. Ramesh Mani [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Harvard University | |
| Physics Building | “Novel Radiation-induced zero-resistance states in high mobility two-dimensional electron systems” |
| We report the experimental detection of novel zero-resistance states [1], which are induced by electromagnetic wave excitation in ultra high mobility GaAs/AlGaAs heterostructure devices including a two-dimensional electron system. Radiation-induced vanishing-resistance states, which do not exhibit concomitant Hall resistance quantization, are demonstrated in the large filling factor, low magnetic field limit, at liquid helium temperatures. It is shown that the observed resistance minima follow the series B = [4/(4j+1)] Bf with j=1,2,…, where Bf = 2fm*/e, m* is an effective mass, e is electron charge, and f is the radiation frequency. These resistance-minima exhibit an activated resistance as a function of the temperature that leads into zero-resistance states at the lowest temperatures. The dependence of the effect is reported as a function of experimental parameters such as the electromagnetic wave frequency, incident power, temperature, and the current. [1] R. G. Mani, J. H. Smet, K. von Klitzing, V. Narayanamurti, W. B. Johnson, and V. Umansky, Nature 420, 646 (2002). |
| Thursday, October 30, 2003 | Dr. Leah Shaw [Host: E. Kolomeisky] | |
| 4:00 PM, Room 204 | Cornell University | |
| Physics Building | “Statistical mechanics methods for genome-wide modeling of translation” |
| In living cells, DNA serves as the template from which mRNA is synthesized. mRNA is then "read," or translated, by ribosomes to produce proteins. Previous studies have shown a nonlinear relationship between mRNA and protein levels, due to the complexity of the translation process. A model is under development to help explain the quantitative relationship between mRNA and protein levels for all genes in Escherichia coli. Statistical physics methods enable a detailed understanding of a single mRNA with a uniform sequence. Realistic, nonuniform sequences are a far more complex case, but mean field equations provide a good approximation for protein production rates. Details of the model will be discussed, and preliminary results comparing the model to experimental data will be presented. |
| Thursday, December 18, 2003 | J. W. Gadzuk [Host: Joseph Poon and Ian Harrison] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “Scanning Tunneling Spectroscopy of Nanostructures: Mirages in Quantum Corrals” |
| Scanning tunneling microscopy/spectroscopy on so-called Kondo systems consisting of magnetic atoms adsorbed on non-magnetic surfaces has shown that suitable two-dimensional nanostructures can influence the surface electron transport that is a consequential part of the observable STM process. Almost everyone has seen Eigler's stunning STM pictures in which individual atoms were assembled to form a chosen two-dimensional configuration, call it a nanostructure, on the metal substrate.1 Some of these shapes, when closed, are referred to as quantum corrals.2 A particularly intriguing example is an elliptical corral (major axis <15 nm) composed of up to 70 individually-placed atoms or molecules on a surface-state-supporting Cu(111) surface.3 It has been observed with the STM that both the pictorial image and the Fano-related spectroscopic signature of a single Kondo atom4 such as Co placed at one of the foci showed a mirage when STM measurements were made at the opposing unoccupied focus. The generic physics of the resonance electron transfer and transport occuring in a wide variety of surface dynamics processes including those responsible for the quantum mirages will be outlined. The consequent Fano-like spectra depend upon both the position of the STM tip and also on the size and shape, hence 2-D quantum states of the nanostructure confinement. For the 10's of nm corrals of experimental interest, the level spacings are comparable with the Kondo resonance width. This results in non-trivial spectra showing size-dependent oscillatory structure in both the energy-dependent amplitude and in the lineshape asymmetry. Calculated mirage spectra illustrate the useful inter-dependence upon the contrasting nm-scale confinement size and shape and the atomic-scale resonance-defining properties which depend upon the species.5 This is a nice example of a timely problem in high-visibility contemporary science and technology which has usefully and synergistically been addressed by complimentary experimental observation, analytic theory, and computationally-more-intensive modeling. |
| Thursday, January 15, 2004 | Dr. Keith Williams [Host: Joseph Poon] | |
| 4:00 PM, Room 204 | Delft Universities | |
| Physics Building | “Opportunities for new Physics in O-D/1-D Hybrids” |
| Over the last decade, nanocrystals, fullerenes and nanotubes have provided many new opportunities for studies of low-dimensional physical phenomena once accessible only in lithographic heterostructures. These phenomena include spectacular optical properties, ballistic transport, Coulomb blockade, Kondo resonance, and numerous other effects related to electron, exciton, and phonon confinement. This talk will begin by reviewing observations of these phenomena in the nanomaterials; I will then discuss proposals for hybrid 0-D/1-D systems which illustrate what novel physics lies ahead in this promising field. |
|
Monday, January 19, 2004 Note Special Day |
Oleksandr Prokhnenko [Host: Despina Louca] | |
| 4:00 PM, Room 204 | Institute of Physics, Academy of Sciences of the Czech Republic | |
| Physics Building | “Magnetic Structures and Magnetovolume Anomalies in R2Fe17 intermetallic compounds” |
| Effects of high pressure and Mn substitution on the creation and evolution of different magnetic structures in R2Fe17 compounds with non-magnetic rare earths R = Y, Ce, Lu were studied by both microscopic (neutron diffraction) and macroscopic (magnetization) techniques. Main results show instability of the ferromagnetic ground state in all compounds whereas range of stability of incommensurate antiferromagnetic phase expands down to the lowest temperatures. This effect is described in terms of distance dependent interlayer exchange interaction giving the value of the lattice c-parameter as a critical parameter for the appearance of the ferromagnetic – antiferromagnetic transition in studied compounds. |
|
Monday, January 26, 2004 Note Special Day |
Adam Durst [Host: Paul Fendley] | |
|
3:30 PM, Room 204 Note Special Time |
Yale University | |
| Physics Building | “Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas” |
| Recent measurements of a 2D electron gas subjected to microwave radiation reveal a magnetoresistance with an oscillatory dependence on the ratio of radiation frequency to cyclotron frequency. Oscillations grow with radiation intensity, with the minima saturating at zero resistance. We have performed a diagrammatic calculation which yields radiation-induced resistivity oscillations with the correct period and phase. Results are understood via a simple picture of photoexcited disorder-scattered electrons contributing to the dc conductivity. Sufficient intensity drives the calculated minima to negative resistivity, a situation shown by Andreev, Aleiner, and Millis to be unstable to the development of an inhomogeneous current distribution with zero resistivity. Hence, our result, taken together with theirs, provides an explanation for the experiments. |
| Thursday, January 29, 2004 | Jiyeong Gu [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Argonne National Laboratory | |
| Physics Building | “Playing with Superconductivity and Magnetism in Nanoscale” |
| Recent technological advances made it possible to create hybrid nano-structures with high quality ferromagnet/superconductor (F/S) interfaces. The F/S systems have been in focus of intensive experimental and theoretical studies because of both exciting fundamental problems and potential device applications. The physical properties of both F and S films will be strongly modified near the interface due to mutual proximity effect, for example, superconducting correlations penetrate into F and the spin polarization can extend into S. Main questions to be addressed in this talk will be the type and length scale of the induced superconducting correlations and spin relaxation length at the interface. Also, first experimental observation of superconducting switching effect depending on the mutual magnetization directions of F-layers in F/S/F trilayer system will be discussed. |
|
Wednesday, February 4, 2004 Note Special Day |
Igor Altfeder [Host: Joe Poon] | |
|
3:30 PM, Room 204 Note Special Time |
Harvard University | |
| Physics Building | “Anisotropic Quantum Correlations in Nanostructures” |
| Recent advances in scanning tunneling microscopy (STM) have shown that a variety of exotic related phenomena, involving spins and anisotropic interactions, can be realized in atomically flat (MBE grown) thin metal films due to a nontrivial interplay of structural and electronic self-organization. The two of these phenomena will be discussed in my talk: anisotropic Kondo-like "confinement" in metallic quantum wells, and unidirectional charge ordering (CDW stripes) on metal surfaces. In all studied cases, we observe the formation of wire-like objects, where the long-range electronic coherence is spontaneously confined to a single degree of freedom. |
| Thursday, February 5, 2004 | Vadim Oganesyan [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Princeton University | |
| Physics Building | “Nernst effect: from simple to correlated metals” |
| Since the discovery of large Nernst effect in the pseudogap of hole-doped cuprates this transport coefficient is becoming a new tool with which to probe correlated electronic behavior. I will first survey the recent experimental data on materials as varied as superconductors, charge and spin density waves, heavy Fermi liquids and ferromagnets. In some cases I will suggest possible common origins of such behavior and support them with calculations. |
|
Tuesday, February 10, 2004 Note Special Day |
Kirill Shtengel [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Microsoft | |
| Physics Building | “Non-Abelian Anyons and Topological Order in Solids” |
| A concept of topological order originally introduced by Wen in the context of Fractional Quantum Hall Effect has drawn much attention from people working in the fields of HTSC, frustrated magnetism and quantum computation. Topological order is manifested by a non-trivial degeneracy of the ground state on 2D surfaces of non-zero genus (such as a torus) and non-trivial mutual statistics of excitations. E.g., a Z_2 topological order in a magnetic system should lead to spin-charge separation -- one of the interesting (yet unlikely) possible mechanisms for HTSC. From the point of view of quantum computation, one of the biggest challenges is making it fault-tolerant. We hope to use topological properties to encode quantum information in a way that is highly resistant to decoherence. So far one (and only) type of systems where topological order is known to exist are systems with Fractional Quantum Hall Effect. After reviewing the current state of search for topological phases in condensed matter, I will discuss models with non-Abelian topological order. In particular, I will present a version of extended Hubbard model whose low-energy physics can lead to a phase with such non-Abelian order. General arguments for stability of these exotic phases as well as some new ideas about physical implementation of such systems will be presented. If found experimentally, these systems will provide a basis for building a truly fault-tolerant quantum computer. |
|
Wednesday, February 11, 2004 Note Special Day |
Dr. Yang Yu [Host: Joseph Poon] | |
| 4:00 PM, Room 204 | The Research Laboratory of Electronics at MIT | |
| Physics Building | “Quantum Computation with Josephson Devices” |
| Quantum computers offer exponential speedup over classical computers in solving certain tasks. Superconducting Josephson devices are promising candidates in realizing qubits for quantum computer due to the ease of scaling up. The main challenge of superconducting qubits is decoherence, arising from the coupling between the superconducting qubits and the environment. Here we directly measure the intrawell energy relaxation time \tau_d between macroscopic quantum levels in the double well potential of a Nb superconducting qubit. The qubit's decoherence time, estimated from \tau_d, is longer than 20 \micro s, indicating a strong potential for quantum computing employing Nb-based superconducting qubits. |
| Thursday, February 12, 2004 | Stergios Papadakis [Host: Yongsoon Yoon] | |
| 4:00 PM, Room 204 | UNC | |
| Physics Building | “Multi-walled Carbon Nanotube-based electromechanical oscillators” |
| I will describe our fabrication and characterization of torsional oscillators which use multi-walled carbon nanotubes (MWNTs) as the torsion springs. We have, through direct force measurements with a scanning probe microscope, measured the torsional properties of individual MWNTs. We discovered a surprising increase in the torsional stiffness of the MWNTs with repeated deflections. We have also actuated the devices electrostatically, demonstrating a self-contained electromechanical device. We use optical interferometry to measure the on-resonance behavior of the electrostatically-driven devices. |
| Thursday, February 19, 2004 | Eugene Mishchenko [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Harvard University | |
| Physics Building | “Fluctuations in electronic, spintronic and photonic systems” |
| I will present both the experimental results and the theory for the non-equilibrium fluctuations of electric current in mesoscopic systems (diffusive wires and chaotic quantum dots). Non-interacting electrons exhibit Poissonian statistics while correlations in mesoscopic systems lead to the universality of current fluctuations. I will consider the effects of spin-polarized transport on the noise and magnetoresistance of spintronic systems. Finally, the electron-optical analogies will be discussed in relation to the fluctuations of radiation from chaotic absorbing ('grey body') and amplifying ('random laser') optical media. |
|
Monday, February 23, 2004 Note Special Day |
Julia Meyer [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Univ. Minnesota | |
| Physics Building | “Electron transport in granular arrays” |
| Electron-electron interactions in low-dimensional systems have attracted a great deal of attention in recent years. We show that arrays of large, strongly coupled quantum dots present an analytically tractable, yet non-trivial model of such systems. A single dot strongly coupled to leads exhibits almost no Coulomb blockade (save for corrections that are exponentially small in the dot-lead conductance). In the array geometry with large inter-grain conductance g>>1, however, the interactions drive the system into an insulating state with a charge gap proportional to exp(-g). The latter reflects the energy cost to create a large-size, unit-charge soliton -- the only charged excitation the system supports. In 2d, such solitons bring about a Berezinskii-Kosterlitz-Thouless crossover at a certain (g-dependent) critical temperature. Upon changing the charge imbalance (e.g. by a gate voltage), the array undergoes a phase transition into the pinned Wigner crystal state. The model, thus, allows one to follow the system from a good metal (at high temperature) all the way to the Wigner crystal insulator (at low temperature) within a single framework. |
| Thursday, February 26, 2004 | Vladimir Butko [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Los Alamos | |
| Physics Building | “Nanoscopic Transport in the Ultrathin Metal Films and Molecular Organic Crystals” |
| A description of the properties of low dimensional electron gases remains one of the most important and challenging problems in condensed matter physics. I will present electron transport and tunneling measurements on ultrathin, metal films. I will show that in the strong disorder limit a gap emerges in the density of states that is solely attributable to fundamental many body electron-electron interaction effects, i.e. the Coulomb gap. Interestingly, a quantum metal state can be realized, in otherwise highly insulating films, by suppressing the Coulomb gap via a magnetic field. In the second part of my talk I will describe ongoing research into the properties of the two dimensional electron gas of Field Effect Transistors (FETs) fabricated on high quality organic molecular single crystals. Practical applications of the molecular organic materials, such as flexible, large-area electronic devices will also be discussed. |
| Thursday, March 4, 2004 | Igor Zutic [Host: Olivier Pfister] | |
| 4:00 PM, Room 204 | University of Maryland | |
| Physics Building | “Spintronics: Fundamentals and Applications” |
| Spintronics is an interdisciplinary field in which the central idea is the manipulation of spin degrees of freedom in solid state systems. The motivation to examine spintronics ranges from fundamental studies, where the changes of the spin degrees of freedom can be a sensitive probe for basic physical phenomena, to applications that are neither feasible nor effective with conventional electronics. This talk will focus on two examples: (1) spin-polarized transport in hybrid structures containing superconductors and (2) a proposal for magnetic p-n junctions. Our prediction that a superconducting response can be used to probe a novel class of ferromagnetic semiconductors has recently led to the first direct measurement of the spin polarization in these materials. In the second example, we develop a theory of inhomogeneously doped semiconductors. We predict the spin-voltaic effect, a spin-analogue of the photo-voltaic effect. We show that the direction of the charge current (which can even flow at no applied bias) can be switched by the reversal of an equilibrium magnetization or of a polarization of the injected spin. The same spin-voltaic effect can be used to develop a novel class of tunable magnetic transistors. |
| Thursday, April 8, 2004 | Jian Li [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | UVa | |
| Physics Building | “TBA” |
| Thursday, April 15, 2004 | Yoonseok Lee [Host: Jongsoon Yoon] | |
| 4:00 PM, Room 204 | UFL | |
| Physics Building | “TBA” |
| Thursday, April 22, 2004 | Junzong Yu [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Building a Magnetic Force Microscope” |
|
Wednesday, May 19, 2004 Note Special Day |
Charles Molhoek [Host: M. Reed] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “TBA” |
| Thursday, July 22, 2004 | John R. Tucker [Host: William A. Jesser] | |
| 4:00 PM, Room 204 | University of Illinois | |
| Physics Building | “Future Nanoelectronics: Materials Science and Physics” |
| A new nanoelectronics must be developed over the next two decades to sustain present rates of progress. The general outlines for what that technology should do are beginning to come into focus, but as yet there is no consensus on how to proceed toward these goals. In this talk, I will summarize research on nanometer MOS transistors and atom-scale silicon devices by my colleagues and myself over the last ten years. Our long-term strategy envisions increasingly higher levels of synergy between materials science and quantum physics in developing this new technology. |
| Thursday, July 29, 2004 | Stuart Wolf [Host: William A. Jesser] | |
| 4:00 PM, Room 204 | DARPA | |
| Physics Building | “Center for Spins in Nanotechnology -CeSpIN -a proposal for a new interdisciplinary center at UVa” |
| In this talk I will discuss the details of a proposed new center at UVa that will explore how electron spin can be utilized in electronics that will transcend Moore's law. There are two parallel paths that this center will explore. The first is the development of spin devices that will be nanoscale extensions of the spin transport electronics that will be introduced in the very near term as non-volatile random access memory and magnetic sensors. The second path is the exploration of spin coherent effects that will directly impact the development of a quantum information processor. There are challenges in both paths but the key to a new paradigm of electronics will be the development of a single technology that can provide both conventional logic and memory as well as quantum logic and memory and spins seem to fit the bill. |
| Thursday, September 30, 2004 | Erik Svedberg [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | Seagate | |
| Physics Building | “The Development and Analysis of Texture in Thin Metallic Films from Electrodeposition and Sputtering” |
| Thursday, November 11, 2004 | Zhixian Zhou [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | FSU | |
| Physics Building | “Magnetic, Transport, and Thermodynamic Studies of Weakly Magnetic Systems of Transition Metal Oxides” |
| LnBaCo2O5.5 (Ln=Gd, Eu) and Sr3Ru2O7 are examples of weakly magnetic systems of 3d and 4d transition metal oxides, respectively. The former is nonmetallic and exhibits magnetic properties of two sublattice magnetic systems with an in-plane ferromagnetic interaction and a relatively weak temperature dependent inter-plane magnetic coupling. The latter is a paramagnetic metal with strongly correlated electrons near a magnetic instability. The magnetization, resistivity and magnetoresistance (MR) of single crystals of GdBaCo2O5.5 and EuBaCo2O5.5 are measured over a wide range of dc magnetic fields (up to 30 T) and temperature. The data suggest an equal ratio of low spin (S=0) and intermediate spin (S=1) Co3+ ions below TMI, with no indication of additional spin state transitions. The low field magnetization shows a transition to a highly anisotropic ferromagnetic phase, followed by another magnetic transition to an antiferromagnetic phase at a slightly lower temperature. Significant anisotropy between the a-b plane and c axis was observed in magnetic and magnetotransport properties for both compounds. The specific heat and electrical resistivity of Sr3Ru2O7 single crystals are measured in several magnetic fields applied along the c-axis for temperatures below 2 K and at fields up to 17 T. Near the critical metamagnetic field at B1 *~7.8 T, the electronic specific heat divided by temperature increases logarithmically as the temperature decreases, over a large range of T, before saturating below a certain T* (which is sample dependent). This crossover from a non-Fermi Liquid to a Fermi Liquid state is also observed in the resistivity data near the critical metamagnetic field for I || c and B || c. At the lowest temperatures, a Schottky-like upturn with decreasing temperature is observed. |
|
Tuesday, November 16, 2004 Note Special Day |
Cristina Bena [Host: Eugene Kolomeisky] | |
|
3:30 PM, Room 204 Note Special Time |
UCLA | |
| Physics Building | “Spin Transport in One-Dimensional Systems” |
| Thursday, November 18, 2004 | L. D. Noordam [Host: Tom Gallagher] | |
| 4:00 PM, Room 204 | AMOLF - Amsterdam | |
| Physics Building | “Rydberg Atoms are Forever” |
| Thursday, December 9, 2004 | Feng Pan [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Colorado | |
| Physics Building | “Continuous Tuning of 2D-Superconductivity in Strontium Titanate Field Effect Transistors” |
| Understanding the doping mechanisms in metal-oxide superconductors is an important part of probing the superconductor theory. Modulation of the carrier concentration using electric fields to tune superconductivity is a good method for this approach for its ability to minimize changes in both chemical impurities and structure. This talk explores the electric field modulation of normal state properties and superconductivity in thin-film field-effect transistors based on lanthanum-doped strontium titanate channels and undoped strontium titanate gate insulation. Electric field tuning of normal-state sheet resistance and channel critical current are observed in enhancement (positive gate potential) and depletion (negative gate potential) modes, consistent with an electric field induced variation of channel depletion width. Detailed analysis of the non-linear channel current-voltage characteristics is consistent with electric field modulation of a 2-dimensional Kosterlitz-Thouless transition and shows the scaling of T_KT (Kosterlitz-Thouless transition temperature) with conduction channel thickness and normal conductance at different gate fields. Also, small signal modulation experiments were performed on these devices. Modulation of critical current, sheet resistance, and T_KT is found for frequencies out to at least 1 kHz, which indicates possible exciting superconductor electronics. |
| Thursday, January 20, 2005 | Aditi Mitra [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Columbia University | |
| Physics Building | “Non-equilibrium Transport Through a Phonon Coupled Molecule” |
| The theory of non-equilibrium many-body systems is only partially understood, and developing the theory further is of relevance now as nanostructures (such as single molecule and NEMS devices) are routinely operated in the non-equilibrium regime. In this talk I will present a comprehensive theory for non-linear transport through a single molecule device where the tunneling electrons are coupled to a vibrational mode of the molecule. I will present results for the current and the noise through the device and show their sensitivity to phonon equilibration rates and hence to the steady state phonon distribution function. I will also outline a method to tackle the regime of low temperatures and strong electron-phonon coupling where non-equilibrium conditions do not allow for a variational treatment. Instead the key issue is the formulation of rate equations for the reduced density matrix that can be solved in the semiclassical limit. The steady-state solutions lead to a multi-peaked density matrix, implying an absence of bistability in the current but structure in the noise. I will also demonstrate how departures from equilibrium produce decoherence that prevents the formation of characteristically quantum features such as the polaron peak in the spectral function, and present generalizations of this method to other strongly correlated systems such as the non-equilibrium Kondo model. |
| Thursday, January 27, 2005 | Anatoli Polkovnikov [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Harvard University | |
| Physics Building | “Superfluid-Insulator Transition in a Moving System of Interacting Bosons” |
| Cold atomic systems with their high tunability and nearly perfect isolation from environment became very attractive for studying strongly correlated systems. Particularly exciting is the possibility to address problems of quantum dynamics far from equilibrium, which are beyond the reach of conventional condensed matter systems. Most of theoretical studies of dynamics in these systems employ essentially classical (Gross-Pitaevskii) equations of motion. At the same time in quilibrium a lot is known about various quantum regimes, where the classical description does not work. In this talk I will discuss a problem where both quantum and dynamic effects are important. In particular, I will describe a moving system of interacting bosons in a periodic optical lattice potential and generalize the conventional superfluid-Mott insulator transition to this highly non-equilibrium situation. I will discuss implications of our results to recent and future experiments. |
| Thursday, February 3, 2005 | RESERVED FOR PHD CANDIDATES | |
| 4:00 PM, Room 204 | ||
| Physics Building | “TBA” |
| Thursday, February 10, 2005 | Victor Galitski [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Santa Barbara | |
| Physics Building | “Vortex Dynamics and Fluctuations Near the Magnetic Field Tuned Superconductor-Insulator Transition” |
| Superconductivity in two dimensions provides a unique area in which a fascinating variety of novel and fundamental phenomena occur. In this talk, I will review recent theoretical and experimental results on disordered films, which undergo a magnetic field tuned superconducting-insulator transition at low temperatures. I will focus on the unusual phases and fluctuation phenomena evident in the experimental studies of the field-tuned transition. First, I will explain how rare disorder fluctuations can enhance global superconductivity and increase the critical magnetic field at which samples become superconducting. Next, I will briefly summarize the recently developed theory of quantum superconducting fluctuations, which explains transport properties above the transition. At the end of my talk, I will focus on the low-temperature metallic phase observed in certain materials. This metallic state is truly mysterious and can not be explained by any conventional theory (involving bosonic vortices as basic excitations). I will argue that under certain circumstances the statistics of the vortices can change from bosonic to fermionic. Such a statistical transmutation may explain the nature of the metallic state. I will discuss possible experimental signatures of the resulting vortex Fermi liquid. |
| Thursday, February 17, 2005 | Paul Fendley [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Non-abelian Statistics” |
| In quantum physics in 2+1 dimensions, particles need not behave as a boson or fermion. With non-abelian statistics, the wave function depends not only on the particles being exchanged, but also on the order in which they are exchanged. Such behavior probably occurs in the fractional quantum Hall effect. I discuss ways of building models with this behavior, and note the connection to quantum computers. |
| Thursday, February 24, 2005 | RESERVED FOR PHD CANDIDATES | |
| 4:00 PM, Room 204 | ||
| Physics Building | “TBA” |
| Thursday, March 3, 2005 | Wenhui Li [Host: Tom Gallagher] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Dipole-Dipole Excitation and Ionization in an Initially Frozen Rydberg Gas” |
| In cold dense Rydberg atom samples, the dipole-dipole interaction strength is effectively resonant at the typical interatomic spacing in the sample, and the interaction has a 1/R3 dependence on interatomic spacing R. The dipole-dipole attraction leads to ionizing collisions of initially stationary atoms, which produces hot atoms and ions and initiates the evolution of initially cold samples of neutral Rydberg atoms into plasmas. More generally, the strong dipole-dipole forces lead to motion, which must be considered in proposed applications. |
| Thursday, March 17, 2005 | Yuki Motome [Host: Seung-Hun Lee ] | |
| 4:00 PM, Room 204 | RIKEN | |
| Physics Building | “How to Relax Frustrated Systems: Spin and Orbital Physics in V and Cr Spinels” |
| Geometrical frustration gives rise to a large number of degenerate ground states and suppresses a simple-minded long-range ordering in general. Here we discuss theoretically how the degeneracy is lifted in three-dimensionally frustrated pyrochlore systems whose lattice structure consists of corner-sharing tetrahedra. The pyrochlore systems are most typically found in the so-called B spinel oxides. Focusing on the spinels with B=V and Cr, we explore the low-temperature physics by the mean-field argument and Monte Carlo simulation for effective spin-orbital-lattice coupled models. For V spinels, we clarify the strong interplay between spin and orbital degrees of freedom. Orbital ordering reduces the magnetic frustration partially and realizes weakly coupled one-dimensional chains in the three-dimensional pyrochlore lattice. For Cr spinels, we show that the system exhibits novel phase transitions under the external magnetic field due to the spin-Peierls type mechanism. An emergence of a half-magnetization plateau is discussed in detail. |
| Thursday, March 31, 2005 | Pengcheng Dai [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | University of Tennessee/Oak Ridge National Laboratory | |
| Physics Building | “Antiferromagnetic Order as a Competing Ground State in Electron-doped High -Tc Superconductors” |
| In this talk, I will summarize recent progress on using neutron scattering to study electron-doped high-Tc superconductors. As a function of increasing doping, the as-grown antiferromagnetic (AF) insulator Pr0.88LaCe0.12CuO4 (PLCCO) can be transformed into a Tc=25 K superconductor without AF order by annealing at different temperatures. We find that underdoped PLCCO's are electronically phase separated with a three dimensional AF phase, a quasi-two-dimensional (2D) spin density wave, and a superconducting phase. An applied external field that suppresses superconductivity also enhances the quasi-2D spin density wave order, thus suggesting AF order is a competing phase for superconductivity in these materials. |
| Thursday, April 7, 2005 | Vittorio Celli | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Theory of Rydberg Gases” |
| Thursday, April 14, 2005 | Slade Culp | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Optimization of Complex MNiSn for High-Temperature Thermoelectric Power Generation” |
| Thursday, April 28, 2005 | Zhe Zhang | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Spin Order in Geometrically Frustrated System CdxZn1-xV2O4” |
| Thursday, May 5, 2005 | Tsuyoshi Kimura [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | Los Alamos National Laboratory | |
| Physics Building | “Control of Electric Polarization by Using Magnetic Field in Magnetic Oxides with Long-Wavelength Magnetic Structures” |
| Recent observations of gigantic magnetoelectric and magnetocapacitive effects in rare-earth manganites, TbMnO3 and DyMnO3 [1,2], provide a novel approach to the mutual control of magnetization and electric polarization in magnetic ferroelectrics. We can control the magnitude and/or direction of the electric polarization vector by the application of magnetic field in these manganites. In comparing the results from the both manganites, we noticed that a characteristic common to the both materials is that they possess modulated magnetic structures with long wavelengths (as compared to the chemical unit cell) which arise from competing magnetic interactions. Ferroelectricity in these materials appears to originate from the competing magnetic interactions which cause lattice modulations through magnetoelastic coupling. We extend our study to other modulated magnets with competing magnetic interactions. In this talk, I show magnetic control of electric polarization in several modulated magnets [3], which may provide new route to design magnetoelectrics. [1] T. Kimura, T.Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, Nature 426, 55 (2003). [2] T. Goto, T. Kimura, G. Lawes, A. P. Ramirez, and Y. Tokura, Phys. Rev. Lett. 92, 257201 (2004). [3] T. Kimura, G. Lawes, and A. P. Ramirez, Phys. Rev. Lett. (in press). |
| Thursday, May 26, 2005 | Oleg Tchernyshyov [Host: Paul Fendley] | |
|
11:00 AM, Room 313 Note Special Time |
John Hopkins | |
| Physics Building | “Topological Defects and Fractionally Quantized Vortices in Nanomagnets” |
| Thursday, July 7, 2005 | Dmitry Reznik [Host: Seunghun Lee] | |
|
11:00 AM, Room 204 Note Special Time |
Institute of Solid State Physics, Forschungszentrum Karlsruhe | |
| Physics Building | “Unusual Magnetic State in MnSi under Hydrostatic Pressure” |
| Thursday, August 18, 2005 | Petros Thomas [Host: George Hess] | |
|
3:30 PM, Room 204 Note Special Time |
University of California at Davis | |
| Physics Building | “Novel Optical Studies of Ion-erosion, Growth, and Diffusion on Metal Surfaces**” |
| The novel optical techniques of oblique-incidence reflectivity difference (OI-RD) and linear optical diffraction (LOD) are applied to several distinct surface physics problems. Studies of Ar and Ne-ion sputtering and thermal annealing of Nb(110) and Cu(111) using OI-RD will be discussed. The evolution of the average slope of the surface morphology of a moderately stepped Ni(111) surface under ion erosion will be presented. OI-RD studies of the adsorption and desorption of atomic hydrogen and deuterium on Nb(110) and Cu(111) will also be reported." **If time allows, the following topic will also be discussed: "The adsorption, growth and diffusion studies of Xe on Nb(110)". |
| Thursday, August 25, 2005 | Sambandamurhy Ganapathy [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Magnet Lab, Tallahassee and Princeton University | |
| Physics Building | “Novel, Collective Insulating Phase in 2D Superconductors in High Magnetic Fields” |
| Superconductor-insulator transition (SIT) in 2D films has been an active area in condensed matter physics research. The reason is the continuing theoretical focus to understand zero temperature quantum phase transitions (QPT) and the relative experimental ease with which QPT can be tuned successfully in 2D films. We present results from our study of the magnetic field (B)-tuned SIT in amorphous indium oxide films. We present three experimental evidences that suggest that a novel, collective insulating state emerges in the B-induced insulating phase. We argue that our results lend support to the central assumption of the QPT approach. |
| Thursday, September 22, 2005 | Maxim Vavilov [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Yale University | |
| Physics Building | “Current-Induced Spin Polarization of 2DEG in Perpendicular Magnetic Field” |
| An electric field applied to a two-dimensional electron gas with spin-orbit coupling generates spin magnetization. Based on a quantum kinetic equation, we study the dependence of the magnitude and direction of the spin magnetization on the strength of a perpendicular magnetic field. We show that as magnetic field increases, the magnitude of the magnetization decreases. However, in sufficiently strong magnetic fields, the magnetization acquires a giant oscillating component; the amplitude of this component may exceed the magnetization at zero magnetic field. The large amplitude of magnetization oscillations is related to the enhanced electron-hole asymmetry in quantizing magnetic fields. |
| Thursday, September 29, 2005 | Leonid Rokhinson [Host: Yongsoo Yoon] | |
| 4:00 PM, Room 204 | Purdue | |
| Physics Building | “Spin Separation in Cyclotron Motion” |
| We demonstrate spatial separation of carriers with opposite spin orientations in a non-magnetic semiconductor. An ability to manipulate spin of charge carries in a controllable fashion is central to the rapidly developing field of spintronics, as well as for the development of spin-based devices for quantum information processing. However, creation of spin-polarized currents is proven to be a formidable challenge and, previously, required either injection from magnetic materials or application of strong Zeeman magnetic field. We show that in a non-magnetic semiconductor with spin-orbit interactions spins can be spatially separated in a “spin spectrometer”, utilizing difference in momenta and, thus, cyclotron radii, for two spin polarizations. For holes in GaAs almost 100% bipolar spin filtering has been achieved in magnetic focusing geometry with spatial separation of polarized beams by 0.2 microns. We confirmed spin polarization of the injected currents by applying strong Zeeman field and using point contacts as spin filters. Spin-orbit interaction constant has been measured directly in these experiments. The new technique of spin injection/detection opens a possibility to investigate density and electric field dependence of spin-orbit interactions, spin dynamics at a few tenths of picoseconds without RF fields, and shed light on such outstanding problems as “0.7 anomaly” in quantum point contacts by measuring spin polarization of charge carriers. |
| Thursday, October 13, 2005 | Harry Kojima [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Rutgers | |
| Physics Building | “Spin Polarized Superfluid He-3” |
| In high magnetic field and at low temperatures, liquid He-3 condenses into a unique superfluid phase (A1) in which the condensate is spin-polarized. Owing to the broken relative symmetry in gauge and spin spaces, A1 phase hydrodynamics has unusual properties. To demonstrate the unusual hydrodynamics, two experiments on (1) magnetic fountain effect and (2) spin-entropy wave (SEW) propagation will be discussed. (1)Owing to the ability to respond to gradients in pressure, temperature as well as magnetic field, A1 phase should develop a steady pressure gradient across two chambers when a steady magnetic field gradient is applied across a superleak connecting the two chambers. Such magnetic fountain effect has been observed but with shorter relaxation time than expected. The observed relaxation is likely related to the important but not yet well-understood spin relaxation phenomenon occurring at the wall boundaries. (2)The superfluid-normal fluid counter-flow propagating mode in A1 phase is accompanied by oscillations in both entropy and spin densities. The large “spin stiffness” makes the velocity of this SEW is much greater that it would be for “entropy stiffness” alone. The measured SEW velocity gives the superfluid density directly. Anisotropy in the superfluid density may be studied with SEW. The liquid-crystal-like property of anisotropy “texture” may be probed and its kinetic anisotropy “texture phase transition” has been observed. |
| Thursday, October 20, 2005 | Jim Valles [Host: Joe Poon] | |
| 4:00 PM, Room 204 | Brown University | |
| Physics Building | “Exploring a New Route to a Two Dimensional Metal” |
| According to the scaling theory of localization (1979), simple metallic phases should not exist in two dimensional electronic systems. Experiments showing the divergence of the low temperature resistance in ultrathin films of metallic elements and 2d electron gases in semiconductor heterostructures tended to support this prediction for about 15 years. More recently, however, evidence of metallic transport has begun to emerge, popping up in thin normally superconducting films and in very low density, high mobility 2D electron gas systems. The physics behind these metallic behaviors is not known although it is generally agreed that explanations must go beyond the scaling theory paradigm and include electron-electron interactions. In an effort to uncover an understandable metallic phase in two dimensions, we are studying ultrathin films composed of superconducting (S) and normal metal (N) elements. Interactions are essential to their superconducting state and such "SN" systems have been predicted to undergo a quantum superconductor to metal transition (SMT) as N is increased. I will describe how our transport and tunneling experiments on SN (Pb/Ag) bilayer films exhibit deviations from standard superconductor proximity effect theories that are consistent with an impending SMT. For example, the quasiparticle density of states of superconducting bilayers acquires a hybrid superconductor-metal appearance. This characteristic suggests that coexisting but separate superconducting and metal quasiparticle populations develop in the approach to the metallic phase. |
| Thursday, October 27, 2005 | Eunseong Kim [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Penn State University | |
| Physics Building | “Observation of Superflow in Solid He-4” |
| We have observed superflow in both bulk solid He-4 and solid He-4 confined in porous media by torsional oscillator technique.[1] Below 0.25K non-classical rotational inertia appears as an abrupt drop in the resonant period of the torsional oscillator. The temperature dependence of the supersolid fraction is different from those found in liquid helium and weakly interacting alkali gases. The non-classical rotational inertia fraction is independent of the oscillation speed below an extremely small critical value of 10 um/s, above which strongly attenuated. The pressure dependence of the supersolid fraction in the low temperature limit reveals an intriguing maxium around 55 bars, whereafter the supersolid fraction monotonically decreases at least up to 136 bars and can be linearly extrapolated to zero at 170 bars. [1] E. Kim and M. H. W. Chan, Nature 427, 225 (2004); Science 305, 1941 (2004); J. Low Temp. Phys. 138, 859 (2005). |
| Thursday, November 3, 2005 | Chris Henley [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Cornell University | |
| Physics Building | “Overcoming Degeneracy in Highly Frustrated Antiferromagnets” |
| Thursday, November 10, 2005 | Wei Pan [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Sandia National Laboratories | |
| Physics Building | “New Results in Quantum Hall Effects ” |
| After a brief introduction to the quantum Hall effects and the composite fermion model, I will discuss recent results in the second Landau level (LL). The data were obtained at very low temperatures (sample temperatures as low as 9 mK) in a very high mobility 2D electron system. We observe well-quantized FQHE states at the LL filling nu= 2+1/2, 2+1/3, and nu= 2+2/3 in coexistence with the reentrant integer quantum Hall states, and a new FQHE state at nu= 2+2/5. The origin of the 2+2/5 state is not clear and the numerical results strongly suggest that it is not a conventional FQHE state but a parafermionic state. There is also evidence for a second even-denominator FQHE state at nu= 2+3/8. Most importantly, we discovered an unexpected quantization of the diagonal resistance, Rxx, at the edges of several quantum Hall states. Each quantized Rxx value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, Rxy. Surprisingly, we can trace this observation back to a small density gradient, about 1%/cm, in our sample. Under this scenario almost all Rxx features can be explained quantitatively by an electron density gradient. These findings have very important implications for any Rxx data taken on two-dimensional electron systems, since Rxx seems now to be solely determined by Rxy, while its relationship to the diagonal resistivity, rxx, is unclear. These findings are further corroborated by data from a different sample at a temperature of 1.2K. There, Rxx shows a strictly linear dependence on the magnetic field, except for sharp spikes at B-fields where the IQHE develops. Interestingly, this linear magnetoresistance can also be explained by the density gradient model. |
| Thursday, November 17, 2005 | Vito Scarola [Host: Victor Galitski] | |
| 4:00 PM, Room 204 | University of Maryland | |
| Physics Building | “Quantum Phases of the Extended Bose-Hubbard Hamiltonian: Possibility of a Supersolid State of Cold Atoms in Optical Lattices” |
| Cold atoms confined to optical lattices offer the unique opportunity for direct simulation of strongly correlated models in a pristine environment. Recent experimental progress in simulating the zero-range Bose-Hubbard model with bosonic atoms in an optical lattice provides a direct experimental probe of the superfluid to Mott phase transition. Another lattice model, the extended Bose-Hubbard model, yields a rich but contested phase diagram which includes the possibility of supersolid and density wave order. I will discuss a specific proposal designed to extend the range of interaction in these otherwise zero-range systems to effectively simulate an extended Bose-Hubbard model in a cold atom optical lattice. I will also discuss potential issues affecting experimental detection of supersolid and density wave order. |
| Thursday, November 24, 2005 | ****THANKSGIVING BREAK**** | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, December 1, 2005 | Anton Burkov [Host: Victor Galitski] | |
| 4:00 PM, Room 204 | Harvard University | |
| Physics Building | “Bosons on the Triangular Lattice: Mott Transition in the Presence of Geometrical Frustration” |
| Interest in the behavior of interacting bosons on two-dimensional lattices has recently been revived by the experimental realization of superfluid-insulator transition of cold atoms in an optical lattice. Such models are also interesting from a broader prospective since they provide a simpler context to explore general aspects of conducting-insulating transitions of electrons. I will talk about our recent study of the interplay between Mott localization and geometrical frustration in a system of interacting bosons on the triangular lattice. Frustration prevents localization into simple ordered states, that can be determined by minimizing only the interaction energy. A variety of unconventional states thus become possible, notably supersolids and valence bond ordered insulators. I will describe a general phenomenological theory of these states, based on a duality transformation from bosons to vortices and compare this theory with numerical simulations. |
| Thursday, January 19, 2006 | Frank Tsui [Host: Joe Poon] | |
| 4:00 PM, Room 204 | University of North Carolina, Chapel Hill | |
| Physics Building | “Spin-dependent Properties of Silicon-based Epitaxial Structures” |
| Thursday, January 26, 2006 | Donald Priour [Host: Victor Galitski] | |
| 4:00 PM, Room 204 | University of Maryland | |
| Physics Building | “The Low Temperature Phase Diagram of the RKKY Model: Competing Interactions and Magnetic Percolation” |
| We examine magnetic moments (e.g. Mn impuritiesin GaAs) coupled via the indirect exchange RKKY interaction. We obtain via Monte Carlo the T=0 phase diagram as a function of the Mn density n_{i} and the carrier concentration n_{c}. As evidenced by a diverging correlation length and the magnetic susceptibility, the boundary between the ferromagnetic (FM) and the paramagnetic (PM) phases represents a line of zero temperature critical points with behavior very similar to a percolation transition. In particular, the ferromagnetic clusters increase in size and ultimately coalesce to span the system as the phase boundary is approached from the PM side. We have found this "ferromagnetic percolation" behavior in a variety of disordered magnetic systems with competing interactions, including the Edwards-Anderson model. In the RKKY system, we find that in the dilute limit, bulk ferromagnetism vanishes for n_{c}/n_{i} > 0.1. We also examine the impact of a strong local antiferromagnetic superexchange coupling between magnetic impurities, and we discuss the impact of a finite mean free path, which we include as a damping factor in the RKKY range function. |
| Thursday, February 2, 2006 | Daniel Phelan [Host: Despina Louca] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Emergence of Magnetism from a Non-magnetic Mott Insulator” |
| The parent compound of La1-xSrxCoO3 has a non-magnetic, Mott insulating ground state. As temperature is increased, the parent undergoes a non-magnetic to paramagnetic (PM) transition, and with hole-doping, static ferromagnetic (FM) correlations develop. The mechanisms by which the PM and FM states emerge from the non-magnetic state will be discussed with reference to our neutron scattering measurements. In the PM state, dynamic FM and antiferromagnetic (AFM) correlations likely result from the super-exchange interactions in a dynamically orbitally ordered state. With doping, static isotropic FM droplets are formed due to double exchange, and the FM correlation length increases with hole concentration. |
| Thursday, February 23, 2006 | Vladimir Gritsev [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Harvard University | |
| Physics Building | “Full Counting Statistics From Interference Between Interacting Bose Liquids in One Dimension” |
| In my talk first I will give a general introduction in physics of one-dimensional Bose condensates. Because of the absence of phase coherence, the interference between one dimensional condensates gives rise to quantum noise in the form of fluctuating fringe patterns. I describe the possible experiments for measuring the probability distribution function of interference fringe amplitudes. This probability distribution can be related to the boundary sine-Gordon model which is also known in mesoscopic physics to describe an impurity in a Luttinger liquid. The probability distribution function demonstrates an interesting crossover from broad Poissonian distribution for the case of impenetrable bosons to universal narrow asymmetric distributions at weak interactions. Finally we argue that by measuring such distributions experimentally, one can study the properties of conformal field theories with negative central charge, that appear in a variety of contexts ranging from stochastic growth models to two dimensional quantum gravity. |
| Thursday, March 2, 2006 | Aldo Migone [Host: George Hess] | |
| 4:00 PM, Room 204 | Southern Illinois University | |
| Physics Building | “Adsorption of Gases on Carbon Nanotubes” |
| Thursday, March 9, 2006 | ****SPRING RECESS***** | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, March 16, 2006 | Matthew Wingate [Host: Hank Thacker] | |
| 4:00 PM, Room 204 | University of Washington | |
| Physics Building | “Field Theory Studies of a Unitary Fermi Gas” |
| Fermion pairing leads to interesting phenomena in many physical systems, whether the constituents are atoms, nucleons, electrons, or quarks. The newfound ability to trap and cool fermionic atoms has provided a versatile avenue for experimental exploration of fermion pairing. I have been working on using field theory techniques to describe and calculate properties of these gases. In this talk, I focus on a dilute gas of 2-component, nonrelativistic fermions whose scattering length is much larger than the average interparticle spacing. This is a setup which describes atomic gases tuned to a Feshbach resonance, as well as neutron matter. I discuss how Monte Carlo methods constitute an ab initio method for theoretical study of this gas. As an example, I show an exploratory calculation of the critical temperature separating the normal and superfluid phases. Next, I outline how effective field theory enables one to study the low temperature properties of the gas beyond superfluid hydrodynamics. It turns out that this system possesses a great deal of symmetry which tightly constrains the form of next-to-leading order behavior. |
| Thursday, March 23, 2006 | Zhibin Lin [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | UVA | |
| Physics Building | “Ultrafast Phase Transformation in Metals Induced by Laser Irradiation: a Molecular Dynamic Study” |
| Short (pico- and femtosecond) laser irradiation has the ability to bring materials into a highly non-equilibrium state where the electron temperature is high while the lattice is still cold. By using Molecular Dynamic (MD) simulation, we can study the kinetics and mechanisms of laser melting and ablation phenomena at the atomic level. In this talk, I will talk about our simulation results on ultrafast melting induced by strong laser irradiation in metals and coherent acoustic phonon generation by laser excitation. In addition, I will relate to recent experimental observation in time-resolved diffraction experiments in probing the structure dynamic under extreme conditions. |
| Thursday, March 30, 2006 | Gil Refael [Host: Victor Galitski] | |
| 4:00 PM, Room 204 | Caltech | |
| Physics Building | “Fate of the Josephson Effect in Thin-film Superconductors” |
| The dc Josephson effect is a probe of the fundamental nature of the superconducting state. In this talk, I will analyze the case of two superconducting thin films connected by a point contact. Remarkably, the Josephson effect is absent at nonzero temperature, even below the K-T transition of the films, and the resistance across the contact is nonzero. Moreover, the point contact resistance is found to vary with temperature in a nearly activated fashion, with a universal energy barrier determined only by the superfluid stiffness characterizing the films, an angle characterizing the geometry, and whether or not the Coulomb interaction between Cooper pairs is screened. As will be shown, this should be testable even in finite systems at a proper range of length-scales and temperatures. |
| Thursday, April 6, 2006 | Don Gubser [Host: Stu Wolf] | |
| 4:00 PM, Room 204 | Naval Research Laboratory | |
| Physics Building | “SUPERCONDUCTIVITY - An Emerging Technology for Power Systems” |
| As the world becomes more “electrified”, efficient distribution and use of electrical power becomes increasingly important. The use of superconducting materials significantly reduces electrical energy loss in the distribution and use of electrical power as well as producing significant reductions in size and weight of power components and machinery. Although superconductivity was first discovered in 1911 the requirement of an extreme “cryogenic” environment (near absolute zero temperature) limited its utility. With the discovery in 1986 of a new class of “high temperature superconductors (HTS)” that operate at substantially higher temperatures (although still cryogenic), remarkable progress has been made in advancing a broader use for superconducting technology. Full scale demonstrations are now being built to develop engineering skills required for systems implementation of this new HTS technology and to better quantify system benefits. This talk will briefly review some of the fundamental attributes of superconductivity before turning to the main focus of the talk describing ongoing power demonstration projects (transmission lines, transformers, motors/generators, etc.). I will end with thoughts on what it will take to realize the full potential of these emerging superconducting technologies. |
| Thursday, April 13, 2006 | Igor Zaliznyak [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Brookhaven National Laboratory | |
| Physics Building | “Quasiparticle Breakdown in a Quantum Spin Liquid” |
| Quasiparticles are the elementary excitations carrying energy and momentum quanta in consensed matter, much like photons and elementary particles carry energy and momentum in the Universe surrounding us. Recent neutron scattering experiments demonstrate how quasiparticle description of energy spectra fails in magnetic crystals with non-magnetic ground states that can be identified as "quantum spin liquids" (QSL). The elementary excitations in such systems can be identified with massive (i.e. having non-zero rest energy) quasiparticles, called magnons, which obey Bose statistics, forming a Bose liquid. In a Bose quantum liquid, however, the single-particle dispersion can terminate at an energy where quasiparticle break-ups into two excitations become allowed, i.e. where the single-particle dispersion enters a continuum of two-particle states. Such spectrum endpoint was originally predicted by L. Landau for the superfluid helium-4, where it was subsequently extensively studied both theoretically and experimentally. A manifestation of this peculiar phenomenon in the case of a quantum spin liquid was found in the spin dynamics of the Haldane-chain S=1 antiferromagnet CsNiCl3, where it was initially identified as a crossover from the single quasiparticle to a spin-continuum response. More recently, signatures of the quasiparticle spectrum termination were observed in the excitation spectrum of the two-dimensional (2D) quantum spin liquid existing in the organo-metallic material piperazinium hexachlorodicuprate (PHCC), indicating a failure of the Bose-quasiparticle description of the QSL state of the 2D S=1/2 Heisenberg antiferromagnet in an extended region of its phase space. These findings are of great current interest as they might have important implications for the type of high-temperature superconductivity found in cuprates. |
| Thursday, April 20, 2006 | Steve Thornton, Michael Timmins, Rob Watkins [Host: University of Virginia Physics Department] | |
|
6:00 PM, Room 203 Note Special Time |
UVA | |
| Physics Building - McCormick Road | “National Physics Day Show” |
| This is a family-oriented event. Come see an hour of exciting and intriguing demonstrations! For more information on this free public event, call 434 924 3781. Free public parking is available in nearby Scott Stadium lots. |
| Thursday, April 27, 2006 | Leah Chock [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Naval Research Laboratories | |
| Physics Building | “Desynchronization and Spatial Effects in Multistrain Diseases” |
| Dengue fever, a multi-strain disease, has four distinct co-existing serotypes (strains). The serotypes interact by antibody-dependent enhancement (ADE), in which infection with a single serotype is asymptomatic, but contact with a second serotype leads to serious illness accompanied by greater infectivity. It has been observed from serotype data that outbreaks of the four serotypes occur asynchronously (Nisalak et al., Am. J. Trop. Med. Hyg. 68: 192). We present a compartmental model for multiple serotypes with ADE, and consider autonomous, seasonally driven, and stochastic versions of the model. For sufficiently small ADE, we find that the number of infectives of each serotype synchronizes, with outbreaks occurring in phase. However, when the ADE increases past a threshold, the system becomes chaotic, and infectives of each serotype desynchronize. Spatial effects are included in a multipatch model. We observe desynchronization between spatially distinct regions. |
| Thursday, August 31, 2006 | Dwight Whitaker [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Williams College | |
| Physics Building | “Rapid Motion in the Plant Kingdom: Nature's Weapons of Mass Reproduction.” |
| Plants and fungi have developed a number of remarkable methods to bring about rapid motion. The most rapid movements rely on stored elastic energy and take place in a timescale that is shorter than a single enzymatic reaction (~1 ms). Because these processes are entirely mechanical we can describe the motion with a straightforward biomechanical model based on classical mechanics. High-speed video lets us test the predictions of our models, which enable us to isolate the key features required for each type of movement and to assess how effectively the system performs. This information, when combined with field observations, helps us to understand the adaptive significance of the motion and put it into evolutionary context with similar species. In this talk we will present a number common plants that exhibit a variety of uncommon methods to disperse seeds and spores. |
| Thursday, September 14, 2006 | Ganapathi Myneni and Geoff Kraft [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Applications of several MeV CW Superconducting Radio Frequency Accelerators” |
| Thursday, September 28, 2006 | Qimiao Si [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Rice University | |
| Physics Building | “Quantum Criticality” |
| Quantum criticality describes the strong fluctuations of a second order phase transition at zero temperature. There is growing evidence that it is relevant to a large part of the phase diagram of a variety of strongly correlated materials. In this talk, I will describe some of the basic issues that challenge the theoretical description of quantum criticality. I will go on to make the case for a new class of quantum critical point, with critical fluctuations that are genuinely quantum-mechanical. In the case of heavy fermion metals, the theory of local quantum criticality characterizes such quantum fluctuations in terms of the collapse of a Kondo entanglement scale. Finally, I will summarize some of the experimental evidences for this theory, particularly those concerning the evolution of the Fermi surface. |
| Thursday, October 5, 2006 | Collin Broholm [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Johns Hopkins | |
| Physics Building | “Ferro-electricity in Frustrated Magnets” |
| While electrostatics and magnetostatics are disparate phenomena in a vacuum, no symmetry forbids materials from responding magnetically to an electric field or vise versa. Materials with a strong magneto-electric response are of interest for applications and challenge our understanding of magnetic dielectrics. I discuss specific examples of magneto-electricity in metal oxides with triangular or kagomé lattices of spins with competing antiferromagnetic interactions [1-3]. It is shown that inversion symmetry breaking magnetic order can act as an effective electric field through magneto-elastic distortions that relieve frustration. The results presented in this talk are based on magnetic neutron scattering experiments. |
| Thursday, October 12, 2006 | Doron Bergman [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | UC Santa Barbara | |
| Physics Building | “Magnetic ordering in a half--polarized magnetization plateau of the pyrochlore antiferromagnet” |
| The classical pyrochlore antiferromagnet (AFM) is considered the ``most'' geometrically frustrated system. Classically, this leads to the absence of any ordering transition at non-zero temperature, even in an applied magnetic field. Recent experiments on the spinel chromites, ACr2o4(A=Cd,Hg) show the existance of a very robust magnetization plateau in a strong magnetic field, and a simultaneous magnetic ordering. We describe a model of spin-lattice coupling that explains both the plateau formation and the observed ordering on the plateau. The predictions are confirmed by recent neutron scattering and x-ray scattering experiments (S. H. Lee et al.). The same model applied to zero magnetic field predicts a reduced but still large ground state degeneracy, including the states observed in both the Cd and Hg materials. This is consistent with the dominance of spin-lattice interactions, with weak additional effects determining the low field magnetic ordering. |
|
Wednesday, October 18, 2006 Note Special Day |
Ricardo Schwarz [Host: Vittorio Celli] | |
| 4:00 PM, Room 204 | Los Alamos National Laboratory | |
| Physics Building | “Low Energy Vibrational Excitations in Metallic Glasses” |
| The specific heat and elastic constants of metallic glasses show anomalies not seen in crystals. These anomalies result from low-energy vibrational excitations in the glass. We have measured the low-temperature heat capacity, the elastic constants, and the phonon density of states of both glassy and single-phase crystalline Pd40Cu40P20. The specific heats of both alloys, plotted as CP/T3 vs. T, show different humps (commonly known as "Boson Peaks"). The elastic constants of the crystal and glass have different T-dependence: the shear modulus of the glass varies as C'(T) = C'(0)[1 - AT], whereas that of the crystal varies as C'(T) = C'(0)[1 - BT2 - DT4]. This suite of low-temperature measurements enabled us to identify the low-energy vibrational excitations responsible for these anomalies |
| Thursday, October 19, 2006 | Igor Herbut [Host: Seunghun Lee] | |
|
2:00 PM, Room 203 Note Special Time |
Simon Fraser University | |
| Physics Building | “Graphene: symmetries, phase transitions, Hall effect” |
| A single sheet or graphite, or graphene, in its natural state is a semimetal with two Fermi points in its Brillouin zone. The low-energy excitations near those points are equivalent to massless relativistic fermions, with the Fermi velocity assuming the role of the speed of light. I will discuss the effects of Coulomb interaction between electrons in such a system at zero and finite magnetic field. The integer Hall effect in graphene will be explained as the combination of the Landau level quantization and spontaneous breaking of the emerging "chiral" symmetry in such a quasi-relativistic system. |
| Thursday, October 26, 2006 | Andrey Zheludev [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | Oak Ridge National Laboratory | |
| Physics Building | “Quantum phase transitions and magnon stability in gapped spin chains and ladders” |
| The lowest energy excitations in quantum disordered spin chains and ladders are a triplet of massive magnons. The stability of these quasiparticles depends critically on the symmetries of the actual spin Hamiltonian. When the spin system undergoes a quantum phase transition, for example one induced by a strong external magnetic field, the symmetry of the ground state changes, and so do the excitations. I will present a comparative inelastic neutron scattering study of three distinct one-dimensional disordered spin systems: the S=1 quasi-1D bond-alternating antiferromagnet NTENP, the uniform anisotropic S=1-chain Haldane-gap compound NDMAP and the uniform isotropic ``composite'' Haldane spin chain IPA-CuCl3. For each material I will discuss the field-induced condensation of magnons, and analyze the spectra measured below, at and above the transition point. The high-field phase will be characterized either as a Bose-Einstein magnon condensate, with a cartelistic gapless Goldstone mode, or as a gapped "quantum spin solid". The issue of magnon stability in each phase will be addressed in detail. |
| Thursday, November 2, 2006 | Veerle Keppens [Host: Joe Poon] | |
| 4:00 PM, Room 204 | University of Tennessee | |
| Physics Building | “To tunnel or not to tunnel - or why clathrates are so fascinating” |
| Motivated by the search for improved thermoelectric materials, several compounds have attracted attention that combine the high electron mobilities found in crystals with a low thermal conductivity approaching values typical for glasses. The common structural feature of these “Electron Crystal Phonon Glasses” (ECPG) is that they contain loosely bound atoms that reside in a large crystalline “cage”. These “rattlers” scatter phonons and greatly reduce the thermal conductivity of the material. One family of ECPGs is formed by the Ge-clathrates Sr 8 Ga 16 Ge 30 and Eu 8 Ga 16 Ge 30 . The presence of the rattler significantly softens the elastic behavior. Combined with results from low-temperature ultrasonic attenuation, neutron-scattering, thermal conductivity and microwave absorption measurements, it provides clear evidence for the existence of a new type of four-well tunneling states. |
|
Wednesday, November 15, 2006 Note Special Day |
Prof. D.D. Sarma [Host: Bellave Shivaram] | |
|
3:30 PM, Room 204 Note Special Time |
Indian Institute of Science | |
| Physics Building | “Unusual features in magnetism and magnetoresistance of double perovskite oxides” |
| Sr2FeMoO6 is a magnetic metal with an unusually high Curie temperature (~ 420 K) and belongs to the double perovskite family of compounds. It shot in to fame a few years ago due to its remarkable magnetoresistive properties. [1] We discuss the origin of ferromagnetism in this and related compounds based on a mechanism driven by the kinetic energy, [2-4] establishing these as members of a new class of magnetic materials; this mechanism also explains [4,5] the occurrence of ferromagnetism in dilute magnetic semiconductors, such as Mn-doped GaAs and in pyrochlore, Tl2Mn2O7. [6] We also show [7] that the magnetoresistance in Sr2FeMoO6 arises from a magnetically triggered nearly-resonant tunnelling condition in contrast to other mechanisms discussed so far in the context of CMR and GMR materials. |
| Thursday, November 16, 2006 | Daniel Khomskii [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Universitaet zu Koeln, Germany | |
| Physics Building | “Charge ordering instead of Jahn-Teller distortion” |
| Due to Jahn-Teller effect the systems with orbital degeneracy usually have an orbital ordering with corresponding lattice distortion. However this distortion weakens and finally disappears when the electrons become itinerant. We show that in the intermediate regime between localized and itinerant electrons there exist a new possibility: orbital degeneracy may be lifted not by Jahn-Teller distortion, but by charge ordering, or charge differentiation. We demonstrate, by theoretical calculations and experimentally, that this happens in particular in rare earths nickelates such as YNiO3 or LuNiO3. The same physics apparently operates also in several other systems, such as ferrates (e.g. CaFeO3) and some others. I will also discuss a possible role of small charge transfer gap and oxygen holes in this phenomenon. |
| Thursday, November 23, 2006 | Thankgiving Recess [Host: N/A] | |
| 4:00 PM, Room 204 | N/A | |
| Physics Building | “N/A” |
| Thursday, November 30, 2006 | Syed Qadri [Host: Stu Wolf] | |
| 4:00 PM, Room 204 | U. S. Naval Research Laboratory | |
| Physics Building | “Phase Transitions in Nano-dimensional Materials” |
| Research interest in the field of nanodimensional materials is growing rapidly due to their interesting size-dependent electronic, magnetic, optical and mechanical properties that have many potential industrial applications. Metals and semiconductors with dimensions in the nanometer realm exhibit novel optical, electrical, and chemical properties which are closely associated with their structural characteristics. In addition, metastable phases and size-dependent phase transitions are observed when the particle sizes are reduced to nanodimensions. Several examples from our previous work will be presented to illustrate the existence of these unusual structural characteristics. X-ray diffraction is a non-destructive characterization tool that plays an important role in the synthesis and in understanding the physical properties of the nanoparticles. |
| Thursday, February 1, 2007 | Predrag Nikolic [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Harvard | |
| Physics Building | “ Mutual influence of vortices and quasiparticles in d-wave superconductors ” |
| Vortices in clean d-wave superconductors at low temperatures can behave as quantum particles. Their quantum dynamics is made possible by the smallness of their cores, due to short coherence length in typical cuprates, and by the presence of massless fermionic quasiparticles, which give rise to certain universal effects. We calculate a small finite renormalization of vortex mass by the nodal quasiparticles, and demonstrate the absence of Bardeen-Stephen damping of vortex motion in the limits of zero temperature, no disorder and vanishing core size. Being liberated from strong friction, light vortices can experience significant quantum fluctuations that can explain several phenomena observed in cuprates, including the Nernst effect and density waves. We also show that quantum fluctuations of localized vortices can significantly affect quasiparticle spectra. The local electronic density of states (LDOS) near a quantum fluctuating vortex shows no zero-energy peak, but has satellite features at energies set by the vortex trapping potential. These are proposed to be the origin of the sub-gap LDOS peaks observed in recent STM experiments near the vortex cores. |
|
Monday, February 5, 2007 Note Special Day |
Maria D'Orsogna [Host: Paul Fendley] | |
|
3:30 PM, Room 204 Note Special Time |
UCLA | |
| Physics Building | “PATTERNS, STABILITY AND COLLAPSE FOR TWO-DIMENSIONAL BIOLOGICAL SWARMS” |
| One of the most fascinating biological phenomena is the self-organization of individual members of a species moving in unison with one another, forming elegant and coherent aggregation patterns. Schools of fish, flocks of birds and swarms of insects arise in response to external stimuli or by direct interaction, and are able to fulfill tasks much more efficiently than single agents. How do these patterns arise? What are their properties? How are individual characteristics linked to collective behaviors? In this talk we discuss various aspects of biological swarming by investigating a non-linear system of self propelled agents that interact via pairwise attractive and repulsive potentials. We are able to predict distinct aggregation morphologies, such as flocks and vortices, and by utilizing statistical mechanics tools, to relate the interaction potential to the collapsing or dispersing behavior of aggregates as the number of constituents increases. We also discuss passage to the continuum and possible applications of this work to the development of artificial swarming teams. |
|
Wednesday, February 7, 2007 Note Special Day |
Austen Lamacraft [Host: Paul Fendley] | |
|
3:30 PM, Room 204 Note Special Time |
Oxford | |
| Physics Building | “Phase ordering in atomic gases” |
| The ordering of matter into different phases is a central preoccupation of many areas of physics, from condensed matter to cosmology. Hand in hand with the existence of different phases goes the question of which dynamical processes are responsible for = their formation, which may be equally important in determining what is observed in a given situation. Recent experimental advances in the creation of degenerate atomic gases have begun to realize the prospect of a rich variety of new phases in atomic matter, involving the hyperfine degrees of freedom, mixtures of different species, or spatial order on optical lattices. With each new phase comes the issue of how that phase will appear under laboratory conditions.=C2=A0 In this talk I'll discuss the theoretical treatment of phase ordering = in=C2=A0 several recent experiments, and the possibility of observing new dynamical phenomena in the future. |
| Thursday, February 15, 2007 | Israel Klich [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Caltech | |
| Physics Building | “Perfect Pulses” |
| Generic perturbations of a Fermi liquid create noise in the form of numerous particle-hole pairs. In the talk I will show that it is possible to excite particles from a Fermi sea in a noise-free fashion by electromagnetic pulses of a special form. The resulting many-body states are characterized by a finite number of particles excited above the Fermi surface accompanied by no disturbance below it. The excitations can be regarded as realization of vertex operators. I will also discuss potential applications in creating entangled electron hole pairs, and effects of interactions. |
|
Monday, February 19, 2007 Note Special Day |
Alexander Seidel [Host: Paul Fendley] | |
|
3:30 PM, Room 204 Note Special Time |
NHMFL/Florida State | |
| Physics Building | “A unifying perspective on charge fractionalization” |
| Condensed matter physicists are generally faced with the task of solving problems involving some 1024 particles that interact strongly. Amazingly, in many cases this task of seemingly hopeless complexity is amenable to the following simple strategy: Try to find a way to (almost) switch of the interactions in a manner that preserves all the fundamental properties of the system. If this is possible, one says that the system is "adiabatically connected" to a non-interacting system. In the past 20 years, however, much focus has been on problems where the traditional approach does not seem feasible. In particular, a new paradigm has surfaced which applies to certain novel incompressible quantum liquids that are said to have "topological order". This new paradigm encompasses the fractional quantum Hall liquids, as well as some theoretically proposed scenarios for novel magnetism in materials similar to the parent compounds of high transition temperature superconductors. The phenomenology of topologically ordered states is very exotic, including fractionally charged excitations and fractional braiding statistics. This fact seems to preclude the possibility that these states have simple non-interacting limits. In spite of this, it will be shown in this talk that such a trivial limit does exist for fractional quantum Hall systems. In fact, by studying quantum Hall states on cylinders with varying circumference, these states can be adiabatically evolved into simple one-dimensional charge-density-wave systems. This point of view provides simple, intuitive pictures for some of the exotic properties of fractional quantum Hall systems. In particular, the principles of charge fractionalization in two spatial dimensions and in one spatial dimension are completely unified by this approach. The potential usefulness of this adiabatic continuity for some unresolved problems will also be discussed. |
|
Monday, March 12, 2007 Note Special Day |
Bella Lake [Host: Seunghun Lee] | |
|
3:30 PM, Room 204 Note Special Time |
Hahn-Meitner Institut | |
| Physics Building | “Neutron Scattering Investigation of Excitations Spectrum in Doped and Undoped Spin-Ladders” |
| The ‘telephone number compound’ family, (La,Sr,Ca) 14 Cu 24 O 41 , is composed of copper oxide planes which form alternating layers of ladders and chains. In the absence of Sr these materials are intrinsically hole doped with a valence of +2.25 per Cu.In contrast, for La 4 Sr 10 Cu 24 O 41 the hole doping is much reduced and magnon heat conductivity measurements show that the ladders are free of holes [1]. A third member of the family, Sr 2.5 Ca 11.5 Cu 24 O 41 , is hole-doped with ~2 holes per seven rungs of the ladder (the exact doping is debated). This compound is of particular interest because it shows a number of phenomena in common with high-Tc cuprates, including linear temperature dependence of resistivity above 130K, charge ordering below 60K and superconductivity under applied pressure [2]. Our neutron scattering measurements on the undoped ladder La 4 Sr 10 Cu 24 O 41 reveal gapped one-magnon and multi-magnon excitations. The results have been modelled to find accurate values of the exchange constants [3]. The doped ladder Sr 2.5 Ca 11.5 Cu 24 O 41 has also been measured. The holes give rise to a number of changes in the excitation spectrum which will be described and discussed. |
| Thursday, March 22, 2007 | John Mather [Host: Brad Cox] | |
|
7:30 PM, Room 402 Note Special Time |
Goddard Space Center | |
| Chemistry Building | “From the Big Bang to the Nobel Prize” |
| The history of the universe in a nutshell, from the Big Bang to now, and on to the future – John Mather will tell the story of how we got here, how the Universe began with a Big Bang, how it could have produced an Earth where sentient beings can live, and how those beings are discovering their history. Mather was Project Scientist for NASA’s Cosmic Background Explorer (COBE) satellite, which measured the spectrum (the color) of the heat radiation from the Big Bang, discovered hot and cold spots in that radiation, and hunted for the first objects that formed after the great explosion. He will explain Einstein’s biggest mistake, show how Edwin Hubble discovered the expansion of the universe, how the COBE mission was built, and how the COBE data support the Big Bang theory. He will also show NASA’s plans for the next great telescope in space, the James Webb Space Telescope. It will look even farther back in time than the Hubble Space Telescope, and will look inside the dusty cocoons where stars and planets are being born today. Planned for launch in 2013, it may lead to another Nobel prize for some lucky observer. |
| Thursday, March 29, 2007 | Andras Janossy [Host: Stu Wolf] | |
| 4:00 PM, Room 204 | Budapest University of Technology and Economics | |
| Physics Building | “Search for stripes in lightly hole doped antiferromagnetic YBCO” |
| Stripes, a periodic spin and charge modulation in superconducting and antiferromagnetic cuprates are a manifestation of an instability of the two dimensional hole system. A multifrequency electron spin resonance study of the antiferromagnetic domain structure in undoped and lightly Ca doped antiferromagnetic Y(Gd)Ba2Cu3O6 single crystals will be discussed. Gd substituting for Y serves as a non-perturbing localised ESR probe of the antiferromagnetic CuO2 layers. Hole doping, variation of temperature and magnetic fields change the antiferromagnetic structure. ESR and infrared transmission experiments in high magnetic fields show that the stripe structure is not an array of hole rich lines in YBCO. |
| Thursday, April 5, 2007 | Taner Yildirim [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “From fundamental understanding to predicting new nanomaterials for high capacity hydrogen storage and fuel cell technologies” |
| Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel-cell technologies. The current state-of-the-art is at an impasse in providing any materials that meet a storage capacity of 6wt% or more required for practical applications. The main obstacles in hydrogen storage are slow kinetics, poor reversibility and high dehydrogenation temperatures for the chemical hydrides, and very low desorption temperatures/energies for the physisorption materials such as metal-organic frameworks (MOF). Recently we have proposed a novel concept to overcome these obstacles. From accurate quantum mechanical calculations, we show that light transition metals (TM) such as a Ti-atom affixed to several nanostructures such as nanotubes/C60 and small organic molecules (C2 H4 ) strongly bind up to five hydrogen molecules. The first H 2 adsorption is dissociative with ~0.25 eV energy barrier while other adsorptions are molecular with significantly elongated H-H bonds. The metal-hydrogen interaction is found to be very unique, lying between chemi and physisorption, with a binding energy of 0.4 eV compatible with room temperature desorption and absorption. Simulations at high temperature indicate that such hybrid systems of transition metals affixed to nanostructures are quite stable and exhibit associative desorption upon heating, a requirement for reversible storage. These results not only advance our fundamental understanding of dissociative adsorption of hydrogen on transition metals in nano-structures but also suggest new routes to better storage and catalyst materials. Finally, time permitted, we will discuss the possibility of dimerization, polymerization, and incorporation of the predicted TM-nanostuctures in nanoporous materials such as MOF to improve the life-cycle and kinetics of the predicted storage materials. |
| Thursday, April 26, 2007 | Sophia Economou [Host: Eddy Barnes/Paul Fendley] | |
| 4:00 PM, Room 204 | Naval Research Lab | |
| Physics Building | “Coherence and optical spin rotations in quantum dots” |
| Optically controlled quantum dots, sometimes referred to as artificial atoms, have been the subject of intense research in recent years. This is due both to their potential role as qubits for quantum computing, and to novel emerging physics, as a result of the interplay of confinement and the semiconductor environment. In the first part of the talk I will present such an effect, Spontaneously Generated Coherence (SGC), in which spontaneous emission of an excited level results in a coherent superposition of two lower levels. This phenomenon, predicted in the context of atomic physics, is so far unobserved in atoms. I will sketch our theory of SGC in quantum dots and present the experimental results of the first observation of this effect. In the second part of my talk the focus will be on the design of arbitrary optical spin rotations, which are necessary operations for quantum computing. Simulations show that our approach yields high quality gates, up to two orders of magnitude faster that existing proposals. |
| Thursday, August 30, 2007 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 6, 2007 | Sungdae Ji [Host: Seunghum Lee] | |
| 4:00 PM, Room 204 | UVA/NIST | |
| Physics Building | “Resonant X-ray Scattering Study of Quadrupole-Strain Interactions in Rare-Earth Tetraborides ” |
| Thursday, September 20, 2007 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 27, 2007 | Jeff Lynn [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | National Institute of Standards and Technology | |
| Physics Building | “Recent Results on CMR and Multiferroic Manganese Oxides” |
| A few results from recent neutron and x-ray scattering results will be presented on both CMR and multiferroic systems. For the multiferroics, we have been investigating the magnetic structure of hexagonal HoMnO 3 as a function of temperature and field [1], which is a commensurate antiferromagnetic (T N =72 K) ferroelectric (T C =875 K). Three different chiral symmetries describe the zero field magnetic phases, with strong dielectric anomalies associated with the phase transitions. The spin dynamics are well described by a Heisenberg model in two dimensions. Orthorhombic TbMnO 3 develops a longitudinally polarized spin density wave state below 41 K, with a change in magnetic structure at 28 K that permits the development of ferroelectricity, while the magnetic structure remains incommensurate [2]. The magnetism is particularly sensitive to Na doping [3]. For the RMn 2 O 5 system (R=Tb, Dy, Ho) strong anomalies in the specific heat, thermal expansion, and dielectric constant are a manifestation of the magnetic coupling to the ferroelectricity [4]. Strong magnetoelastic coupling is also found in the triangular antiferromagnetic multiferroic CuFeO 2 [5]. For the Kagome staircase system Co 3 V 2 O 8 the rich variety of magnetic phases and lock-in transitions is a signature of competing interactions [6], and is quite different from ferroelectric Ni 3 V 2 O 8 . For the CMR systems, we will review recent results for the polaron dynamics in optimally doped La-BaMnO 3 and La-SrMnO 3 , and compare these results with La-CaMnO 3 [7] and the bilayer system [8]. The overall behavior observed in the CMR regime of the manganites is quite similar to that observed in the relaxor ferroelectrics as well as the spin and charge stripes found cuprate oxides, demonstrating a commonality of many of the underlying physical concepts of these perovskite oxides. |
| Thursday, October 4, 2007 | Igor Mazin [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | Naval Research Laboratory | |
| Physics Building | “Superconductivity in graphene-based structures” |
| MgB2 and CaC6 are some of the most interesting new stars on the superconducting skies. The former, with Tc=39K is the most high-temperature conventional superconductor, and it is by far superior technologically to the cuprate-base high-Tc materials. The latter has Tc >13K, nearly an order of magnitude higher than that of old intercalated graphites. Theory says that despite both materials being, essentially, doped graphenes, superconductivity comes from two different bands, one existing in MgB2 but not in CaC6 and the other in CaC6 but not in MgB2. In this talk I will explain what are this bands and why they are responsible for superconductivity in the respective compounds, and will discuss whether or not it is possible to invent a new material that would combine superconducting advantages of both. |
| Thursday, October 11, 2007 | Giti Khodaparast [Host: Keith Williams] | |
| 4:00 PM, Room 204 | Virginia Tech | |
| Physics Building | “Narrow Gap Semiconductors: spin splitting with no magnetic fields and more,…..” |
| In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors (NGS). NGS offer several unique features such as small effective masses, large effective g-factors, high intrinsic mobilities, large spin-orbit coupling effects and zero-field spin splitting. For example, In InSb quantum wells (QWs), spin-resolved cyclotron resonance (CR) caused by the non-parabolicity of the conduction band and electron spin resonance have been observed. In order to increase our understanding of the dynamics of carriers and spins in NGS, we performed several time-resolved measurements such as magneto-optical Kerr effect. In this talk, magneto-optical studies on InSb QWs and ferromagnetic semiconductors such as InMnSb will be discussed. Our results are important for understanding the electronic and magnetic states in NGS. |
| Thursday, October 18, 2007 | S. Bandopadhyay [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | VCU | |
| Physics Building | “Single Spin Logic” |
| Electronic devices have traditionally relied on charge to encode, store, process and transmit information. A well-known example is the celebrated Field Effect Transistor, which is the workhorse of all modern digital electronic chips. When a transistor channel is filled with charge, the transistor is “on” and encodes the binary bit 1. When the channel is depleted of charge, the transistor is “off” and encodes the binary bit 0. Switching between bits is therefore accomplished by moving charge within the transistor which causes current flow and associated power dissipation. This is a fundamental shortcoming of all charge based devices. Since charge is a scalar, and has only a magnitude, logic bits 0 and 1 must be demarcated by a difference in the magnitude of the stored charge. As a result, switching between bits always involves changing the magnitude of charge and therefore causing a current flow and I2R dissipation. This cannot be avoided. An electron’s spin, on the other hand, is a pseudo vector with a polarization. If spin polarization is made “bistable” by placing the electron in a static magnetic field, then the two allowed polarizations (parallel and anti-parallel to the field) can encode bits 0 and 1. Switching would require simply flipping the spin without moving charge in space and causing a current flow (I2R = 0). This can result in tremendous energy saving which is currently the most important goal in electronics. The Single Spin Logic (SSL) idea1 is based on this paradigm. Spins of single electrons in quantum dots encode digital bits. Logic gates are configured by placing the dots in suitable layouts to engineer the spin-spin interactions between them. Inputs are provided by aligning the spins in input dots along desired orientations using locally generated magnetic fields. The arrival of a new input string takes the system to an excited state. When the spins ultimately relax to the many body ground state, the spin polarizations in “output” dots represent the result of a computation in response to the input bits. I will show how the universal NAND gate is configured in this way. With the NAND gate, any arbitrary circuit can be built. Detailed quantum mechanical calculations show that switching in these circuits dissipate the minimum energy allowed by thermodynamics (the Landauer-Shannon limit), which is kTln(1/p) where p is the bit error probability. With p = 10-9, the energy dissipated is ~ 21 kT, whereas modern transistors dissipate 40,000 – 50,000 kT. The SSL is incomparably superior to spin based transistors (Spin Field Effect Transistors, Spin Junction Transistors, etc.) which do not compare well with traditional transistors. The reason is that they still rely on charge for encoding information and do not exploit the advantage of spin. I will conclude by showing that SSL type constructs are best realized with organic nanostructures where the spin relaxation time can be extremely long. We have measured a spin relaxation time of 1 second at 100 K in a nanostructure of the -conjugated organic tris(8-hydroxyquinolinolato aluminum) popularly known as Alq3 2. |
| Thursday, October 25, 2007 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, November 1, 2007 | Yong Baek Kim [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | University of Toronto | |
| Physics Building | “Classical and Quantum Frustrated Magnets ” |
| We will discuss the properties of classical and quantum Heisenberg models on the lattices with corner sharing triangles; namely two dimensional Kagome and three-dimensional Hyper-Kagome lattices. The roles of thermal and quantum fluctuations, and the resulting ground states will be the focus of the discussion. Connection to recent experiments on Volborthite, Na4Ir3O8, and Zn-paratacamite will also be discussed. |
| Thursday, November 8, 2007 | Yeong-Ah Soh [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Dartmouth College | |
| Physics Building | “Mesoscopic physics in a quantum magnet chromium” |
| Cr is a simple metal and quintessential spin density wave antiferromagnet that has a quantum critical point. The Hall number collapses on entering the antiferromagnetic state because the Fermi surface is reduced as some of the carriers are localized to produce the magnetic order. Our studies of the Hall effect in Cr[1] were the first to measure the Hall effect in any metal near an antiferromagnetic quantum critical point and showed that it can be a very sensitive probe to study quantum phase transitions. We recently extended our studies to films to explore a two-dimensional system. By growing extremely high quality Cr films we observe quantization of spin density waves and scattering from antiferromagnetic domain walls, effects which can be easily tuned by the film thickness. |
| Thursday, November 15, 2007 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, November 29, 2007 | Dr. Thomas Proffen [Host: Despina Louca] | |
| 4:00 PM, Room 204 | Los Alamos National Laboratory | |
| Physics Building | “Understanding short- and medium range order in materials using total neutron scattering” |
| Determination of the atomic structure is mainly based on the measurement of Bragg intensities and yields the average structure of the infinite crystalline material. However, this approach ignores any defects or local structural deviations that manifest themselves as diffuse scattering. It also fails in case of disordered materials, badly crystalline such as many nano-materials, or not crystalline at all, such as glasses. In some cases crystalline and amorphous phases coexist making the traditional crystallographic structure refinement difficult or incomplete. The total scattering pattern or the derived atomic pair distribution function (PDF), however, contains structural information over all length scales [1] and can be used to obtain a complete structural picture of complex materials. One of the great advantaged of the PDF is the fact that one can limit the range on atom-atom distance over which the structural model is refined. Focusing on small distances up to a few Angstroms will illuminate the local structure where as refinements over a wide range will yield the medium and long range structure. It is interesting to consider, that instruments such as the high resolution neutron powder diffractometer NPDF located at the Lujan Neutron Scattering Center at Los Alamos National Laboratory allows the measurement of PDFs up to distances in excess of 200Å or 20nm. As a result one can obtain a ‘complete’ structural fingerprint of nanoparticles that are frequently smaller in size as demonstrated in a recent study of gold nanoparticles [2]. [1] Th. Proffen, S.J.L. Billinge, T. Egami and D. Louca, Z. Krist. 218, 132-143 (2003). [2] K.L. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri and A.K. Cheetham, Chem. Phys. Lett. 393, 385-388 (2004). |
| Thursday, December 6, 2007 | Dr. Randy Fishman [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | Oak Ridge National Laboratory | |
| Physics Building | “Giant Negative Magnetization in a Class of Layered Molecular-Based Magnets” |
| Bimetallic oxalates are a class of layered molecular-based magnets with transition metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Energy, structure, and symmetry considerations are used to construct a reduced Hamiltonian, including exchange and spin-orbit interactions, that explains the magnetic compensation and giant negative magnetization in some of the ferrimagnetic Fe(II)Fe(III) compounds. By shifting the Fe(II) ions with respect to the oxalate molecules, the organic cation between the magnetic layers alters the C_3 -symmetric crystal field and the orbital angular momentum of the ground-state doublet at the Fe(II) sites. We provide new predictions for the spin-wave gap, the effects of uni-axial strain, and the optical flipping of the negative magnetization in Fe(II)Fe(III) bimetallic oxalates. |
| Thursday, January 17, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, January 24, 2008 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, January 31, 2008 | Bruce Gaulin [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | McMaster University | |
| Physics Building | “Frustrated and Satisfied Ground States in Pyrochlore Magnets” |
| Geometrical frustration arises quite generally when pairwise interactions in magnetic materials are incompatable with their local geometry. This often involves magnetic materials made up of assemblies of triangles or tetrahedra. The frustration is manifest by disordered low temperature states for the magnetic material - some of which are described by spin liquids, spin glasses, and spin ice. I will discuss (mostly) neutron scattering work using DCS at NIST on two magnetic pyrochlores Tb2Ti2O7 and Ho2Ti2O7, which can be thought of as Ising-like moments decorating a network of corner-sharing tetrahedra. Tb2Ti2O7 displays a spin liquid, or cooperative paramagnetic ground state, but can be brought to order in an applied magnetic field. Ho2Ti2O7 displays a static disordered "spin ice" state at low temperatures. |
| Thursday, February 7, 2008 | Taku Sato [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | University of Tokyo | |
| Physics Building | “Neutron scattering study on static and dynamic spin structures in quasicrystalline magnets” |
| Quasicrystals have distinct spatial symmetry characterized by highly-ordered but non-periodic (quasiperiodic) atomic structure, which differs both from the periodic and random structures. Ordering and excitations of quasiperiodically arranged magnetic moments (spins) are yet fundamental open problems, despite the intensive efforts continuously made since the discovery of the quasicrystal. In this talk I will present recent development of understanding on the static and dynamic spin structures in quasicrystalline magnets, using the extensively studied Zn-Mg-RE systems as typical examples. In magnetization measurements, the Zn-Mg-RE quasicrystals all show spin-glass-like behavior, indicating random freezing of spins at low temperatures, however, well-defined short-range order have been observed in the neutron scattering. The inelastic response of Zn-Mg-RE falls into two classes: For RE = Tb and Dy, a broad inelastic peak has been observed with very weak Q dependence, suggesting an existence of the strongly localized collective excitation modes. On the other hand, for RE = Ho, temperature-independent S(Q, h ω ) was observed in the neutron-energy-gain side (h ω >0) for an incredibly large temperature range (up to 200 K!). The anomalous spin fluctuations may be related to criticality of electron wave functions in the quasiperiodic lattice. |
|
Monday, February 11, 2008 Note Special Day |
Gia-Wei Chern [Host: Seunghun Lee] | |
|
3:30 PM, Room 204 Note Special Time |
Johns Hopkins University | |
| Physics Building | “Order by distortion and chiral magnetism in CdCr2O4” |
| Thursday, February 14, 2008 | Kazuma Hirota [Host: Seung-Hun Lee] | |
| 4:00 PM, Room 204 | University of Tokyo | |
| Physics Building | “Frustrations in Relaxors” |
| Relaxors have been extensively studied for a variety of applications as ideal dielectric materials because they often exhibit extremely large dielectric and piezoelectric constants while the dielectric loss and temperature dependence are small. Although it is now widely believed that heterogeneity embedded and appearing in relaxors are relevant to various phenomena specific to relaxors, there is still no established microscopic theory for relaxors. The prototypical relaxors Pb(Mg1/3Nb2/3)O3 (PMN) consists of Pb2+ on the A site of the ABO3 Perovskite structure and Mg2+ and Nb5+ on the B site. To keep the charge neutrality, Mg2+ and Nb5+ have to form a solid solution with a ratio of 1:2 so as to have an average valence of 4+. However, it is likely that a large difference between the ionic radius of Mg2+ and that of Nb5+ prefers the 1:1 solid solution resulting in an alternating arrangement of Mg2+ and Nb5+. The 1:2 state and the 1:1 state are mutually exclusive, thus the system falls in a state of frustration. The concept of frustration has been studied almost exclusively in magnetism, e.g., a geometrical frustration in an antiferromagnetic triangular lattice, though phenomena related to frustration are widely seen in nature. Since there is no unique ground state in a frustrated system, the system becomes unstable among various different states, which may cause large fluctuations leading to a extremely large susceptibility against an external field and to a novel exotic phase. We now consider that heterogeneous structures appearing in relaxors can originate from such a frustration. In this presentation, a review is given on a series of neutron and x-ray scattering experiments on spatial structures and dynamics of polar nano regions in relaxors. We then would like to discuss how such experimental results can be understood in the frame work of frustration. We also discuss what we will be able to study the microscopic mechanism of relaxors by controlling the frustration through lattice distortion, charge imbalance and dimensionality. |
| Thursday, February 21, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, February 28, 2008 | Jack Chan [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Random Telegraph Signal in Carbon Nanotube Device” |
| Due to the low dimensionality of carbon nanotubes (CNTs), charging of a single defect site near a CNT may have a profound effect on modifying carrier transport mobility in a long CNT channel. Random Telegraph Signals (RTS) have been studied in carbon nanotube field effect transistors (CNT-FETs). If the energy of the defect center is close to the Fermi level of the CNT-FET, trapping and detrapping of carriers would modify the carrier transport in the channel, and give rise to RTS. RTS is observed as a switching between discrete current levels, representing a carrier being trapped and detrapped successively in the defect center. We speculate that RTS spectra could provide a characteristic signature of specific adsorbates or adducts on the nanotube channel. This capability is of interest not only for potential sensing technology but also provides a way to introduce controllable quantum interference resonances in the channel transport. |
| Thursday, March 13, 2008 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, March 20, 2008 | Zhiqiang Mao [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Tulane University | |
| Physics Building | “Fascinating Exotic Phenomena in Layered Ruthenates” |
| Perovskite ruthenates (Sr,Ca) n+1 Ru n O 3n+1 exhibit a rich variety of fascinating ordered ground states. Spin-triplet superconductivity, metamagnetic quantum criticality, itinerant ferromagnetism, antiferromagnetic Mott insulating, and half-metallic behavior were all found in close proximity to one another. These diverse ground states originate from the strong interplay of charge, spin, lattice, and orbital degrees of freedom. They offer a unique opportunity to tune the system and study the physics of novel quantum phases. In this talk, I will first give a brief overview on studies in this area, and then present our recent work on double layered ruthenates (Sr 1-x Ca x ) 3 Ru 2 O 7 (0 ≤ x ≤ 1). We have established a magnetic phase diagram for this system using the high quality single crystals grown by the floating-zone technique; this phase diagram exhibits significant new phenomena. We find a very unique magnetic state in close proximity to a two-dimensional ferromagnet with T c =0 K for 0.1 < x < 0.4. This state exhibits a surprisingly large Wilson ratio RW (e.g. R W ≈ 700 for x = 0.2); it freezes into a cluster glass (CG) at low temperatures. Furthermore, we observe evidence of non-Fermi liquid behavior as the frozen temperature of the CG phase approaches zero near x = 0.1. The origin of such a state will be discussed. |
| Thursday, March 27, 2008 | Jack Simonson [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Investigation of high temperature TE compounds” |
| Thursday, April 3, 2008 | Peter Gehring [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “Effects of Short-Range Order on the Structure and Dynamics of Relaxor Ferroelectrics” |
| Relaxors are disordered perovskite (ABO3) oxides, typically characterized by the presence of mixed valence B-site cations, that have found widespread use in numerous device applications because they exhibit low hysteresis and record-setting piezoelectric coefficients. Relaxors derive their name from an unusually frequency-dependent dielectric susceptibility, but they also display a rich variety of unusual physical phenomena including simultaneously soft zone-center and zone-boundary phonons, temperature dependent diffuse scattering, and an anomalous thermal expansion where a transition to a low-temperature invar-like behavior is observed. Recent neutron elastic and inelastic scattering results on the lead-based relaxors PbMg1/3Nb2/3O3 (PMN),PbZn1/3Nb2/3O3 (PZN), and their solid solutions with PbTiO3 will be discussed that indicate the development of static, short-range polar order at high temperatures is central to these phenomena. These results can be understood by analogy with random-field models in which just two temperature scales are required to describe the essential features of relaxor compounds. |
| Thursday, April 10, 2008 | Yize (Stephanie) Li [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “The Mysterious Metallic Phase in 2D Superconductors and the Resulting Phase Diagram” |
| Conventional treatments of electronic transport predict that no metallic phase exists in two-dimensional (2D) superconducting materials at zero temperature (T=0). This view has been challenged by the observation of magnetic field (B) induced metallic behavior in amorphous MoGe and Ta thin films. We have demonstrated that the metallic phase in Ta thin films has an intrinsic origin and associates with nonlinear voltage-current (I-V) characteristics that are qualitatively different from those of superconducting and insulating phases. Based on transport measurement, we can map the phase diagram of Ta thin films in B-T-Disorder space and study the physics it reveals. We also investigated the nature of the B-induced insulating phase. We found that the peak structure of differential IV traces displayed a non-monotonic change as a function of B, which might be a signature for the localized Cooper pairs. |
| Thursday, April 24, 2008 | Wei Chen [Host: Jongsoo Yoon] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Exploration of Novel Tunnel Barrier Materials for STT-RAM ” |
| The basic structure of magnetic tunnel junctions (MTJs) consists of two ferromagnetic(FM) layers sandwiched by an ultra thin insulating barrier, and it shows high or low resistance depending on the relative direction of the the magnetization of two FM layers. Conventional Magnetic random access memory (MRAM) using MTJs as storage units are switched using external filed, so it has the scaling problem beyond 65nm node. A new switching mechanism called Spin Torque Transfer (STT) has been proposed and experimentally confirmed. In our work, new tunnel barrier materials are being explored to enhance the performance of this new STT-RAM technology. One of the particular tunnel barrier materials VO2 has the metal-insulator transition close to room temperature, and we're trying to incorporate VO2 into MTJs stack as the smart barrier for the STT switching so that the MTJs could be switched in low resistance state and read at high resistance state. |
| Thursday, August 28, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 4, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 11, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 18, 2008 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 25, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, October 2, 2008 | Parsa Bonderson [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | Microsoft / UCSB | |
| Physics Building | “Measurement-Only Topological Quantum Computation” |
| The topological approach to quantum computing derives intrinsic fault-tolerance by encoding qubits in the non-local state spaces of non-Abelian anyons. The original prescription required topological charge measurement for qubit readout, and used braiding exchanges of anyons to execute computational gates. We present an anyonic analog of quantum state teleportation, and use it to show how a series of topological charge measurements may replace the physical transportation of computational anyons in the implementation of computational gates. |
| Thursday, October 9, 2008 | Kit Matan [Host: Seunghun Lee] | |
| 4:00 PM, Room 313 | ISSP, U of Tokyo | |
| Physics Building | “Neutron scattering and magnetization studies of the kagome lattice antiferromagnet” |
| The collective behavior of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting spin dynamics and chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. I will talk about our studies of the spin-wave excitations and spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single crystal samples of KFe3(OH)6(SO4)2, an ideal kagomé lattice antiferromagnet. The spin-wave excitations have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted "zero energy mode," verifying a fundamental prediction for the kagomé lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin-wave data. The antisymmetric Dzyloshinskii-Moriya interaction is the primary source of anisotropy and explain the low-temperature magnetization and spin structure. In addition, combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures. The transition indicated by the sudden increase in the magnetization arises as the spins on alternating layers, which are previously oppositely canted due to the ferromagnetic interplane coupling, rotate 180° to align the canting moment along the c-axis. These observations are consistent with the ordering induced by the Dzyloshinskii-Moriya interaction. Elastic neutron scattering measurements in high field verify the 180° spin rotation at the transition. |
| Thursday, October 16, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, October 23, 2008 | James Kushmerick [Host: Keith Williams] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “Study of Silicon Devices by Inelastic Tunneling Spectroscopy” |
| Thursday, October 30, 2008 | Reserved [Host: Dinko Pocanic] | |
| 4:00 PM, Room 204 | ||
|
Monday, November 3, 2008 Note Special Day |
Kai Sun [Host: Paul Fendley] | |
|
3:30 PM, Room 204 Note Special Time |
University of Illinois | |
| Physics Building | “Time-reversal symmetry breaking and spontaneous anomalous Hall effect in Fermi fluids” |
| We study the spontaneous non-magnetic time-reversal symmetry breaking in a 2D Fermi liquid without breaking either the translational symmetry or the U(1) charge symmetry. Using a Berry phase approach, we found that for a large class of models, including all one- and two-band models, the time-reversal symmetry breaking states can be classified into two classes, dubbed type I and II, depending on the accompanying spatial symmetry breaking patterns. The properties of each class are studied. In particular, we show that the states breaking both time-reversal and chiral symmetries (type II) are described by spontaneously generated Berry phases and exhibit anomalous Hall effect in the absence of magnetic fields and magnetic impurities. We also show examples of the time-reversal symmetry breaking phases in several different microscopically motivated models and calculate their associated Hall conductance within a mean-field approximation. In particularly, we found a simple lattice structure in which the time-reversal symmetry breaking phases can be stabilized by infinitesimal interactions. |
| Thursday, November 6, 2008 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, November 13, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, November 20, 2008 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, December 4, 2008 | Melissa Commisso [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “A Study of the Group-IV Diluted Magnetic Semiconductor GeMn” |
| Thursday, January 15, 2009 | Layla Hormozi [Host: Israel Klich ] | |
| 4:00 PM, Room 204 | NIST | |
| Physics Building | “Topological Quantum Computing with Read-Rezayi States ” |
| A topological quantum computer is a hypothetical device in which intrinsic fault-tolerance is embedded in the hardware of the quantum computer. It is envisioned that in these devices quantum information will be stored in certain "topologically ordered" states of matter, and quantum computation is carried out by braiding the world-lines of quasiparticle excitations that obey non-Abelian statistics, around one another, in specific patterns. Certain fractional quantum Hall states are among the prime candidates for realizing non-Abelian quasiparticles that can be used for topological quantum computation. I will review some of the properties of these states, and describe a method for finding braiding patterns which can be used to carry out a universal set of quantum gates on encoded qubits based on non-Abelian quasiparticles that can be realized as excitations of the Read-Rezayi series of fractional quantum Hall states. |
| Thursday, January 22, 2009 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
|
Monday, January 26, 2009 Note Special Day |
Diyar Talbayev [Host: Despina Louca] | |
|
3:30 PM, Room 204 Note Special Time |
LANL | |
| Physics Building | “Combining ferroelectricity and magnetism: the low-energy electrodynamics” |
| Multiferroics, materials with coexisting magnetic and ferroelectric orders, hold the promise of implementing multifunctional devices, for example, an electric-write magnetic-read memory element. The success of this venture depends on our fundamental understanding of magnetoelectric coupling – the interaction that mixes magnetism with ferroelectricity. Although the electrical switching of a magnetic domain has yet to be achieved, various degrees of electric control of magnetism and magnetic control of ferroelectric state have been demonstrated. Magnetic and lattice vibrations and their mixing by the magnetoelectric coupling play a central role in the properties of multiferroics. This low-energy electrodynamics can help unravel the fundamental interactions between magnetic and lattice degrees of freedom. I will present a study of long-wavelength magnetic excitations in several important classes of multiferroics and demonstrate the relationship between the magnetic excitations and the materials’ magnetoelectric functionality. |
| Thursday, January 29, 2009 | Derek Stein [Host: Keith Williams] | |
| 4:00 PM, Room 204 | Brown University | |
| Physics Building | “Nanopores and nanofluidics for single DNA studies” |
| Lab-on-a-chip fluidic technology takes inspiration from electronic integrated circuits, from which its name, its fabrication methods, and its ?smaller, cheaper, faster? paradigm are derived. For silicon-based electronics, miniaturization eventually gave rise to qualitatively different behavior, as quantum mechanical phenomena grew increasingly important. As we shrink fluidic devices down to the nanoscale to probe samples as minute as a single molecule, what physical phenomena will dominate in this new regime, and how might we take advantage of them? This talk will focus on our studies of single DNA molecules using nanofluidic devices and solid-state nanopores. We are studying how nanofluidic structures, whose critical dimensions are tens to hundreds of nanometers, can manipulate long DNA molecules by a variety of nanoscale phenomena, including electrokinetics, hydrodynamics, Coulomb interactions, and the statistical properties of polymers. Our work also focuses on solid-state nanopores, single-nanometer-scale devices that can not only manipulate single molecules, but also detect them electronically. The basic principle behind this is that when DNA is electrophoretically driven through a nanopore, it blocks a measureable fraction of the ionic current that is transmitted through the pore. Thanks to its size, the nanopore also forces each base along the DNA to pass through in sequence, suggesting intriguing possibilities for genetic analysis. |
|
Wednesday, February 4, 2009 Note Special Day |
Tinyong Chen [Host: Despina Louca] | |
|
3:30 PM, Room 204 Note Special Time |
Johns Hopkins University | |
| Physics Building | “Mind the Gap of Fe Superconductors” |
| A family of new Fe superconductors of SmFeAsO1-xFx and others have been discovered in 2008 that contain the puckered FeAs planes instead of the hallmark CuO2 planes in the cuprate superconductors. Central to any superconductor is the nature of its superconducting gap, its value, its structure if any, its temperature dependence. We used Andreev reflection spectroscopy to investigate the gap of these new (1111) Fe superconductors and its temperature dependence. Although the gap values and transition temperatures are different for the Fe superconductors with different F doping, the values of 2 Δ /kBTC are all close to 3.53 that of a BCS s-wave superconductor. The gap is nearly isotropic with temperature dependence close to that of an s-wave superconductor. We did not find evidences of pseudogaps, but the spin density wave transition can induce spurious “pseudogaps” in the system. These characteristics of the Fe superconductors are dramatically different from those of the cuprate superconductors. |
| Thursday, February 5, 2009 | Cristian Batista [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | Los Alamos National Laboratory | |
| Physics Building | “Dimensional Reduction at a Quantum Critical Point” |
| Competition between ground states near a quantum critical point is expected to lead to unconventional behavior in low dimensional systems. New phases of matter have been predicted, and explanations proposed for unsolved problems including non-Fermi liquid behavior and high temperature superconductivity using two-dimensional (2d) theories. In this talk, I will present a theory that describes the Bose-Einstein condensate (BEC) quantum critical point (QCP) in layered systems with a frustrated inter-layer coupling. I will demonstrate that the main effect of this geometric frustration is to reduce the dimensionality of the QCP (its critical exponents are the ones expected for a 2d system). In addition, I will present the first experimental evidence of dimensional reduction at a QCP observed in the Mott insulator BaCuSi 2 O 6 (Han Purple). |
| Thursday, February 12, 2009 | Susumu Takahashi [Host: Despina Louca] | |
|
3:30 PM, Room 204 Note Special Time |
University of California, Santa Barbara | |
| Physics Building | “Quenching spin decoherence in diamond and single-molecule magnets” |
| Overcoming spin decoherence is critical to spintronics and spin-based quantum information processing devices. For spins in the solid state, an interaction with fluctuations of the surrounding spin bath is a major source of spin decoherence. One approach to reducing spin bath fluctuations is to bring the spin bath into a well-known quantum state that exhibits little or no fluctuations. A prime example is the case of a fully-polarized spin bath. We present our recent demonstrations of significant suppression of spin decoherence measured with high-field electron paramagnetic resonance (EPR). One example is nitrogen-vacancy (NV) centers in diamond [1]. Another is S=10 Fe8 single-molecule magnets [2]. We will also present the development of UCSB free-electron laser (FEL)-based pulsed EPR spectrometer which aims for nano-second time resolution.[1] S. Takahashi et al., Phys. Rev. Lett. 101, 047601 (2008). [2] S. Takahashi et al., Phys. Rev. Lett. in-press; arXiv: 0810.1254. |
| Thursday, February 19, 2009 | Yuanbo Zhang [Host: Despina Louca] | |
|
3:30 PM, Room 204 Note Special Time |
University of California, Berkeley | |
| Physics Building | “Imaging Dirac Fermions in a Two-dimensional Sheet of Carbon” |
| It has been four years since the first successful isolation of graphene, a single atomic sheet of carbon, and enthusiasm for this material is still growing. Part of this excitement arises from the fact that electrons in graphene behave as massless "relativistic" particles (Dirac fermions) with an effective speed of light equal to c/300. Microscopic disorders in graphene, such as ripples in the carbon sheet or static "puddles" of charge, profoundly alter the behavior of these electrons. I will describe our recent experiments aimed at directly probing these perturbations and imaging their influence on Dirac fermions down to the atomic scale. Our measurements, performed using the techniques of scanning tunneling microscopy and spectroscopy, reveal unexpected electronic interference patterns that vary as a function of both electron energy and applied electric field (via a gate electrode). I will show that electron interference in graphene nanodevices arises from scattering off of static puddles of electrons, and that these puddles are caused by charged impurities underneath the graphene sheet. |
| Thursday, February 26, 2009 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, March 12, 2009 | Liviu Movileanu [Host: Keith Williams] | |
| 4:00 PM, Room 204 | Syracuse University | |
| Physics Building | “Single-molecule biophysics with a protein nanopore” |
| Advances in rational protein design and single-molecule technology allow for biochemical sampling at high temporal and spatial resolution and for the detection, manipulation, and exploration of individual molecules. We have developed a methodology for examining single biopolymer dynamics within a protein nanopore, a simple system that is highly pertinent to several more complex biological processes such as the translocation of nucleic acids and polypeptides through transmembrane pores. The ionic current through a single protein nanopore was determined by single-channel electrical recordings in lipid bilayers. The results revealed unprecedented details of biopolymer behavior at single-molecule resolution. These examples demonstrate an unusual control of single biomolecules and pore-based nanostructures by using simple principles learned from physics and modern biology. |
|
Friday, March 13, 2009 Note Special Day |
Takuro Katsufuji [Host: Seunghun Lee ] | |
|
11:00 AM, Room 313 Note Special Time |
Waseda University | |
| Physics Building | “Orbital degrees of freedom on triangle-based lattice” |
| Orbital degrees of freedom arising from the degeneracy of d states give rise to intriguing behaviors in various transition-metal oxides. For example, spinel vanadates (ZnV2O4, MnV2O4, FeV2O4, AlV2O4), where two d electrons occupy the triply degenerate t2g orbitals, exhibit various types of orbital ordering. In this talk, orbital ordering of V ions on triangle-based lattice (SrVxGa12-xO19, BaV10O15) is discussed. In these compounds, V trimers in a spin-singlet state are formed, but spins on a part of the V ions survive, which are frustrated or ordered at low temperatures. Such a spontaneous segregation into spin-singlet clusters and magnetic ions could be a characteristic of the spin-orbital system on triangle-based lattice. |
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Tuesday, March 17, 2009 Note Special Day |
Hussain Zaidi [Host: Genya Kolomeisky] | |
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3:30 PM, Room 204 Note Special Time |
University of Virginia | |
| Physics Building | “Geometrical Interpretation of the Non-universal Casimir Energy of An Infinite Cylindrical Wedge” |
| The majority of calculations in the literature on the Casimir energy of curved bodies focus on the universal part of the energy, implicitly assuming that the non-universal terms subtracted from the formal expression for the energy have no physical consequence. We explicitly calculate the non-universal terms for the particular case of an infinite cylindrical wedge and show that these terms are important quantities that arise out of the dependence of the surface tension and the bending/rigidity coefficients of a body on the energy cut-off. This lends support to a recent phenomenological argument in favor of a geometrical interpretation of the non-universal parts of the Casimir energy. |
| Thursday, March 19, 2009 | Guoguang Wu [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Six-Port Reflectometer: an Alternative Network Analyzer for THz Region” |
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Monday, March 23, 2009 Note Special Day |
Micky Holcomb [Host: Stu Wolf] | |
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11:00 AM, Room 313 Note Special Time |
University of California, Berkeley | |
| Physics Building | “Understanding Magnetism in Multiferroics” |
| Magnetoelectric multiferroics are materials that exhibit multiple order parameters, such as ferroelectricity and ferromagnetism. The potential for coupling between such order parameters and their manipulation through external stimuli (electric or magnetic fields) allows the exploration of novel physics and device functionalities. Among a large number of materials systems, the BiFeO3 system has emerged as an attractive model system, mainly because both the ferroelectric Curie temperature and the antiferromagnetic Neel temperature are well above room temperature. In order to understand magnetoelectric coupling, the individual order parameters must first be understood. While the ferroelectric order can be probed using conventional capacitive measurements as well as by scanned probe techniques (such as piezoforce microscopy) probing the antiferromagnetic order requires the use of optical probes, such as SHG and x-ray photoemission spectromicroscopy. Angle and temperature dependent absorption measurements using a state-of-the-art high-resolution photoemission microscope allowed imaging and direction determination of the order parameters in this multiferroic. These studies reveal the first observation of electrical control of antiferromagnetism and the extension to electrical control of ferromagnetism through exchange bias. Though this study, a generic method for separating order parameters in complex systems has been developed and applied. |
| Thursday, March 26, 2009 | Michael Stone [Host: Paul Fendley] | |
| 4:00 PM, Room 204 | University of Illinois | |
| Physics Building | “Continuum Calogero-Sutherland Models” |
| The Calogero-Sutherland family of models consist of point particles moving on a line or circle and interacting with a repulsive inverse-square potential. Both the classical and quantum versions are completely integrable, and are the subject of an extensive literature. I will describe their origin, their connection with the Hall effect and the Benjamin-Ono equation, and some of the subtleties that arise from the existence of two inner products. |
| Thursday, April 2, 2009 | Juan Yu [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “A study on the spin-state transition and complex magnetic coupling in Perovskite Cobaltite” |
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Monday, April 6, 2009 Note Special Day |
Junghwa Kim [Host: Seunghun Lee] | |
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3:30 PM, Room 204 Note Special Time |
University of Virginia | |
| Physics Building | “Neutron and X-ray scattering studies of frustrated, quantum, and multiferroic transition metal oxides” |
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Tuesday, April 7, 2009 Note Special Day |
Shanti Deemyad [Host: Despina Louca] | |
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3:30 PM, Room 204 Note Special Time |
Harvard University | |
| Physics Building | “Simple Elements at high densities: En-Route to metallic hydrogen and insulating lithium” |
| Thursday, April 9, 2009 | Brian Burke [Host: Bellave Shivaram ] | |
| 4:00 PM, Room 204 | University of Virginia | |
| Physics Building | “Characterization of Doped Silicon and Endofullerenes by Raman Spectroscopy for Sensing and Transport Devices” |
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Wednesday, April 15, 2009 Note Special Day |
Jiawan Tian [Host: Despina Louca] | |
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3:30 PM, Room 204 Note Special Time |
University of Tennessee | |
| Physics Building | “Mechanical Behavior of a Ni-based Crystalline and a Zr-based Amorphous Materials Subjected to Surface Severe Plastic Deformation ” |
| A surface-treatment process, surface-severe-plastic deformation (S2PD), is developed and applied on both crystalline and amorphous materials to introduce plastic deformation in the near-surface layer. A S2PD-processed crystalline component is expected to have enhanced fatigue properties because the refined grains in the near-surface layer and the coarse grains in the interior have good resistance to the crack initiation and propagation, respectively. The microstructures and mechanical properties of the processed specimens were systematically investigated. It is shown that the S2PD process has the capability of simultaneously creating (a) a work-hardened surface layer, (b) a nanocrystalline (nc) surface layer, (c) a surface region with compressive-residual stresses, and (d) a grain-size gradient with a nc surface and a coarse-grained interior for the polycrystalline superalloy. Improved yield strength and fatigue strength were found after the process. For the amorphous material, thermal properties of the processed near-surface layer were characterized by means of the differential-scanning calorimetry (DSC). Effects of the treatment on the microhardness were studied by the nanoindentation. After the treatment, the plastic-flow deformation in the unconstrained sample edge was observed. In the sub-surface layer, the impact-induced shear-band operations generate the extrusion and intrusion marks on the side surface. XRD and highenergy synchrotron diffraction techniques were used to inspect the possible crystalline phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Four-point-bending-fatigue behavior has been studied and related to the modified surface structure and the compressive-residual stress induced by the process. |
| Thursday, April 16, 2009 | Gil Refael [Host: Israel Klich ] | |
| 4:00 PM, Room 204 | Caltech | |
| Physics Building | “The thin film Giaever transformer - vortex drag in a superconductor thin-film bilayer ” |
| The normal-field induced superconductor-insulator transition in amorphous thin films, and the possible intervening metallic state, can be qualitatively explained within two paradigms: as vortex condensation, or as a percolation transition between the competing normal and superconducting phases. An experiment that may qualitatively distinguish these two paradigms is a drag experiment on a bilayer system, consisting of two parallel films, where a voltage response in one layer to current in the other is measured. The drag due to vortices is expected to dramatically exceed the Coulomb drag that may arise from the charge carriers in the percolation scenario. In my talk, I will present recent results estimating the drag response within the vortex and percolation paradigms, in the limit of no tunneling between the layers. |
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Monday, April 20, 2009 Note Special Day |
Luke Langsjoen [Host: Genya Kolomeisky] | |
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3:30 PM, Room 204 Note Special Time |
University of Virginia | |
| Physics Building | “The Effect of Thermal Fluctuations on the Problem of Euler Buckling Instability” |
| Renormalization techniques are used to study the effect of thermal fluctuations on the classical Euler buckling instability of a homogenous rod in 2 spatial dimensions. It is discovered that temperature has the effect of exponentially stiffening the rod, yielding a non-zero minimum critical force as a function of the length of the rod. |
| Thursday, April 23, 2009 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, May 7, 2009 | Yonghang Pei [Host: Bellave Shivaram] | |
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3:30 PM, Room 204 Note Special Time |
University of Virginia | |
| Physics Building | “Vertical nano-composite heteroepitaxial thin films with manganites and ferroelectrics” |
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