| 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” |