• Wednesday, January 19, 4:00 PM
  Jonathan Miller
  (NEC Research Institute)
  "Non-fermi liquid as passive scalar fluid"


• Monday January 24, 4:00PM
  Jeongnim Kim
  (Department of Physics, Ohio State University)
  "Stability of Si-interstitial defects: Atomistic simulations for improved macroscopic models for transient enhanced diffusion"
  Transient enhanced diffusion (TED) in boron-implanted silicon is the limiting factor in controlling dopant profiles for submicron Si-based devices. Interstitial defects in bulk Si generated during implantation have been identified as the sources for boron TED. A class of macroscopical interstitial defects, namely {311} defects, was suggested to emit interstitials that can contribute to the enhancement of boron diffusion under typical implantation conditions. We study the stability and electronic structure of interstitial clusters and interstitial chains by performing tight-binding and first-principle total energy calculations. Accurate parameterization of the defect-formation energy on the number of interstitials and interstitial chains, together with the anisotropy of the interstitial capture radius, enables macroscopic defect-growth simulations.


• Friday, January 28, 1:30PM
  Shivaji Sondhi
  (Princeton)
  "Ising Models of Quantum Frustration"
  The interplay between frustration and quantum mechanics has been a question of considerable interest since the work of Fazekas and Anderson on the triangular lattice antiferromagnet. I will discuss a family of model systems in which one can make considerable progress in studying this interplay: these are Ising models where competing interactions lead to a macroscopic entropy at zero temperature and quantum mechanics is introduced by switching on a magnetic field transverse to the easy axis. Results on a small zoo of lattices will illustrate the elegant phenonmenon of "order by disorder" as well as the competing possibility of "disorder by disorder". I will also discuss reformulations of the quantum dynamics in terms of height and dimer representations that provide interesting connections to other physical problems, such as that of high temperature superconductivity.


• Monday, January 31, 4:00 PM
  Leonid Pryadko
  (Institute for Advanced Study)
  "Stripes and High Temperature Superconductors"
  Mean field (MF) models can give a surprisingly deep insight in physics of many systems. I will review some experimental data on phase diagram of underdoped cuprate materials, and demonstrate a set of very general exact constraints which any effective mean-field theory must satisfy to describe the stripe phases. In particular, the existence of antiphase stripes (domain walls) in the ground state requires a competition between interactions at a short distance scale; this is impossible near a second order phase transition where the correlation length diverges. I will also discuss even more general Landau models of incommensurate ordering in these systems.


• Friday February 4, 1:30 PM
  Alexander Abanov
  (MIT)
  Tunneling into the n=1/2 Quantum Hall state. Microscopic theory.


• Monday, February 7, 4:00PM
  Victor Barzykin
  (National High Magnetic Field Laboratory)
  "Ferromagnetism, superstructure, and unconventional superconductivity in Ca_1-xLa_xB₆"
  A qualitatively new low temperature state was recently discovered in Ca_1-xLa_xB₆ in the pioneering paper by Young et al. [Nature 397, 412 (1999)]. Pure CaB₆ is a semiconductor; doping it with La introduces carriers into the system, which leads to a sharply lower resistivity. It was found that Ca_1-xLa_xB₆ is a ferromagnet at low La doping ( x~0.05) with a small magnetic moment (~0.07mm_B), yet very large Curie temperature (T_c ~ 900K), which is comparable to the Fermi energy for doped carriers. On the other hand, pure LaB₆ is a metal which develops superconductivity at low temperatures ~ 100 mK. In this talk I will discuss the explanation of the metal-insulator transition and ferromagnetism based on the excitonic model. I will demonstrate that, in addition to ferromagnetism, the model leads to phase separation and stripes, and propose that the pairing state in LaB₆ is very exotic, with broken time-reversal symmetry.


• Friday February 11, 1:30 PM
  Dmitri Chklovski
  (Cold Spring Harbor)
  "Theoretical Neurobiology: a Physicist's View"
  The success of theoretical physics in the last century was largely based on the discovery of fundamental laws, or principles. In the same time, biology was mostly a technique driven science with less reliance on theory and few unifying principles. I believe that this situation is changing in one subfield of biology. Neurobiology today presents a unique opportunity to uncover fundamental principles and develop novel theoretical approaches to understand brain function. To demonstrate this, I formulate the principle of wiring economy and apply it to understand the organization of the human brain.


• Friday February 18, 1:30 PM
  Igor Aleiner
  (SUNY, Stony Brook)
  "Mesoscopic Fluctuations of the Coulomb Drag"
  I will consider mesoscopic fluctuations of the Coulomb drag coefficient in the system of two separated two-dimensional disordered electron gases. I will show that at low temperatures sample to sample fluctuations of the Coulomb drag coefficient exceed its ensemble average. It makes questionable all previous theroretical conclusions about the low-temperature behavior of the Coulomb drag in disordered systems. At low temperatures the Coulomb drag coefficient is found to grow as temperature decrease which, however, has nothing to do with the breakdown of Fermi liquid description.
  (work done together with B.N. Narozhny)


• Friday February 25, 1:30 PM
  Phil Adams
  (Louisiana State University)
  "Reentrance, and State Memory: The Extraordinary Behavior of Ordinary Superconductors in High Magnetic Fields"
  Over the past few years we have been conducting a series of experiments in which a parallel magnetic field is used to drive the superconductor-normal (insulator) transition in ultra-thin Al and Be films. We find that parallel field transition in intrinsically hysteretic. Recently we have used electron tunneling measurements of the density-of-states to show that the hysteresis is associated with a robust and essentially static "state memory effect". We show that state memory can be erased by raising the temperature thus producing reentrant behavior. Finally, we have also discovered a Zeeman anomaly in the normal state electron tunneling spectrum that is clearly associated with a non-perturbative superconducting fluctuation mode deep in the paramagnetic normal phase.


• **** CANCELLED **** Friday, March 3, 1:30PM
  Evgenii G. Maksimov
  (Lebedev Institute)
  "Relaxation and Pairing in High Tc Systems"


• Wednesday, March 29, 1:00PM
  Chris Ford
  (Cavendish Laboratory, Cambridge)
  "Coulomb blockade of tunnelling through compressible rings formed around an antidot: an explanation for h/2e Aharonov-Bohm oscillations"
  We consider the detailed charging mechanism of an antidot and give a full explanation for the h/2e Aharonov-Bohm oscillations observed when both spin orientations of the lowest Landau level form bound states around the antidot. Experimental data show that the resonance is only through states of one spin. An incompressible region forms between the compressible regions corresponding to each spin direction. Simple electrostatics coupled with screening in the compressible states, and Coulomb blockade, can explain the h/2e oscillations. At lower fields, the oscillations show complicated behaviour, which we interpret in terms of the breakdown of the incompressible strip and dynamical screening.


• Friday, March 31, 1:30PM
  D.Menashe and B.Laikhtman
  (Hebrew University of Jerusalem)
  "Transport in a 1D chain with random fluctuating energies"


• Friday, April 7, 1:30PM
  H.-K. Ho
  (MagiQ Technologies)
  "From Quantum Cheating to Quantum Security"


• Friday, April 14, 1:30PM
  Richard Berkovits
  (Bar-Ilan University, Israel and NEC Research Institute, Princeton)
  "No metal-insulator transition for interacting spinless electrons in two dimensions"
  Recently, much interest is focused on the influence of electron-electron interaction (EEI) on the localization properties of two dimensional electrons in disordered systems. It is motivated by the experimental observations of a crossover in the behavior of the conductance of low density two dimensional electrons from an insulating like temperature dependence at low densities to a metallic one at higher densities. In many numerical studies of 2D spinless electrons with EEI numerous equilibrium properties such as many particle energy level statistics, two-electron localization length high above the Fermi energy, persistent current flow patterns, and charge density response to an external perturbation were studied. As function of the interaction strength, some of these quantities undergo qualitative changes. It might then be tempting to interpret it as an evidence for a 2DMIT. However, the properties listed above are not directly related to zero temperature transport properties of the electron system, which are naturally measured in experiments. We therefore carefully examine some quantities which are directly related to transport properties of the system and find that EEI systematically attenuates the transport through the system and enhances its insulating features. Thus, there is no numerical evidence that EEI can drive a transition in the transport properties of spinless electrons.


• Friday, April 28, 1:30PM
  Harold Baranger
  (Duke)
  "Interactions and Interference in Quantum Dots: Coulomb-Blockade Peak Heights and Positions"
  The interplay of electron-electron interactions and quantum interference in quantum dots leads to new effects which are currently being intensively investigated. I will discuss two aspects of this interplay.
  (1) The dynamics in a quantum dot leads to an oscillatory dependence of the Coulomb blockade peak height on peak number. Corrections to the standard statistical theory of peak heights are non-universal and can be expressed in terms of periodic orbits for both integrable and chaotic systems.
  (2) The residual interactions between electrons in a quantum dot cause kinks in the dependence of the Coulomb blockade peak position on an external parameter such as the magnetic field. The kinks signal the rearrangement of electrons among the single particle levels due to the interaction, and are connected with the turning on or off of "Hund's rule". Recent experiments provide evidence for both of these effects.


• Wednesday, May 10, 1:30PM; Joint Condensed Matter - YITP Seminar
  Alexei Tsvelik
  (Oxford)
  "Particles with fractional quantum numbers; Abelian and non-Abelian statistics"

Send comments to Laszlo Mihaly; last updated 1/7/2000.