• Monday, January 14, 4:00PM
  Rosario Fazio
  (Scuola Normale Superiore, Pisa)
  Interplay between Exchange and Pairing in Small Grains


• Friday, January 25 1:30PM
  Alex Kamenev
  (Department of Physics, University of Minnesota)
  New Jobs for Old Replica
  I shall explain and review the replica formalism for systems with quenched disorder. The method was recently advanced to handle various symmetry classes of the random matrix theory. It opens the door to treat some non--perturbative problems of disordered interacting fermions in normal metals and superconductors. I'll try to expose subtle points and hidden assumptions of the technique along with the prospects of its new applications.
  Host: Aleiner


• Friday, February 1 1:30PM
  Igor Zalizniak
  (Brookhaven National Laboratory)
  Quantum Spin Dynamics in 1D Antiferromagnets
  A peculiar feature of the spin-liquid singlet ground states found in quantum spin chains is a decoherence of the triplet excitations. Instead of a coherently propagating magnon quasiparticles, common in classical magnets, a spin excitation continuum is formed for some wavevectors. The most spectacular example is a des Cloiseaux-Pearson continuum in a S=1/2 antiferromagnetic chain, which is understood in terms of the fractional, S=1/2, spinon excitations. I will present a recent observation (by neutron scattering) of a spinon continuum in a 1D Mott-Hubbard insulator SrCuO₂, where these fractional excitations are a signature feature of the spin-charge separation. In a S=1 Haldane spin chain, spectrum is dominated by a coherent quasiparticle in the vicinity of q=p, but at small wavevectors it also turbs into a continuum feature, as was revealed in our recent neutron scattering measurements on the model Haldane compound CsNiCl₃.
  Host: Abanov


• Friday, February 15, 1:30PM
  Charles Ahn
  (Yale University)
  Epitaxial Ferroelectric Heterostructures: Writing Electronic Nanofeatures and Electrostatic Modulation of High Tc Superconductivity
  Using scanning probe microscopy and atomically smooth, epitaxial ferroelectric oxides, we have written electronic nanofeatures in ferroelectric/metallic oxide heterostructures. This approach allows one to achieve reversible, nonvolatile local electronic doping without lithographic processing or permanent metallic electrical contacts. We have also used the polarization field of a ferroelectric to shift the doping level of the high temperature superconductors in ferroelectric/high Tc heterostructures, modulating reversibly the transition temperature without introducing chemical or structural disorder.
  Host: Allen


• Friday, February 22, 1:30PM
  Clare P. Grey
  (Chemistry Department, Stony Brook)
  Solid state NMR Studies of Motion and Structure in Disordered Solids: Battery Materials and Ionic Conductors
  Host: Allen


• Friday, March 1, 1:30PM
  Konstantin Efetov
  (Bochum and Princeton)
  Metal-Insulator Transition at not very low Temperatures in Disordered Metals
  We consider interaction effects in a granular normal metal at not very low temperatures. Assuming that all weak localization effects are suppressed by the temperature we replace the initial Hamiltonian by a proper functional of phases and study the possibility for a phase transition depending on the tunneling conductance g. It is demonstrated for any dimension that, while at small g the conductivity decays with temperature exponentially, its temperature dependence is logarithmic at large g. The formulae obtained are compared with an existing experiment and a good agreement is found.
  Host: Aleiner


• Friday, March 8, 1:30PM
  Paul M. Goldbart
  (University of Illinois at Urbana-Champaign)
  Seeking simplicity in complex media: A universal picture of vulcanized matter, glass, and other random solids
  Host: Aleiner


• Friday, March 15, 1:30PM
  Leo Radzihovsky
  (University of Colorado at Boulder and Harvard University)
  Phase transitions in bilayer quantum Hall ferromagnets: a self-charging capacitor
  After an introduction to the quantum Hall effect in bilayers, I will discuss a number of phase transitions that can take place in such state of bilayers in the presence of a weak in-plane magnetic field. The most interesting of these is the interlayer charge-balanced to charge-unbalanced bilayers Ising-like transition, which can be either classical or quantum and is expected to be reentrant. The striking experimental signatures are the universal nonlinear charge-voltage and in-plane field relations, and the divergence of the differential bilayer capacitance at the transition, resulting in a bilayer capacitor that spontaneously charges itself, even in the absence of an applied interlayer voltage.
  Host: Abanov


• Wednesday, April 17, 4:00PM
  Victor Gurarie
  (Oxford University)
  "Random bosons, spin chains and pinned charge density waves."
  We consider bosonic excitations in disordered systems, focusing on generic features of quadratic Hamiltonians in the absence of Goldstone modes. We establish the symmetry classification for such Hamiltonians and discuss its relation to the random matrix ensembles which usually arise for disordered fermionic systems. Specializing to the case of 1D spin random field spin chains and the models of pinned charge density waves, we show how their density of states can be obtained.
  Host: Abanov


• Friday, April 12, 1:30PM
  A. J. Millis
  (Columbia University)
  "Why you should be interested in 'CMR' manganites?"
  The rare earth manganese-based perovskites have attracted substantial recent attention for their large magnetoresistance, which was thought to be technologically useful. This talk will argue that the materials are an important model system for basic condensed matter physics because they exhibit a 'tunable kinetic energy' which allows controlled investigation of the relevance of theories to actual materials, and because they exhibit a new mechanism for sensitivity of properties to perturbations, involving a 'first-order' free energy landscape in which long range 'accomodation strain' plays a crucial role.
  Host: Aleiner


• Friday, April 19, 1:30PM
  Meigan Aronson
  (University of Michigan, Ann Arbor)
  "Magnetism in the Hexaborides"
  Host: Mihaly


• Friday, May 3, 1:30PM
  Oleg Starykh
  (Hofstra University)
  "A `one-dimensional' view of geometrically frustrated magnets."
  Geometrically frustrated magnets have attracted strong interest from condensed matter community in recent years, primarily because of their close connection with an actively sought ``spin liquid'' phases of various two- and three-dimensional strongly correlated electron systems. In my talk I present a novel ``one-dimensional'' approach to one of the most frustrated magnet known to date - quantum pyrochlore antiferromagnet. I show that in a wide range of parameters this two-dimensional system exhibits truly one-dimensional (Luttinger liquid) ground state with deconfined spinons as elementary excitations. In the applied magnetic field this critical state transforms into an incommensurate long range ordered one which is stabilized below field-dependent transition temperature.
  Host: Abanov


• Friday, May 10, 1:30PM
  Sumathi Rao
  (Penn State University)
  "Transport in quantum wires"
  We propose a model to explain the flat and gate voltage independent renormalisations of the conductance quantisation in quantum wires. We model the contact regions themselves as short Luttinger liquids and allow for different Luttinger liquid parameters in the wire and contact regions. By introducing small barriers between the contacts and the leads and using the standard techniques of bosonisation and renormalisation group, we are able to qualitatively reproduce all of the experimental results including the temperature and length dependence. In the presence of a magnetic field, the theory also gives rise to an interesting odd-even effect, with bands with spins parallel to the magnetic field getting renormalised differently from the bands with spins anti-parallel to the magnetic field. This has also been experimentally seen. [Phys. Rev. Lett. 87, 026801 (2001) and cond-mat/0104402].
  Host: Allen


• Tuesday, May 14, 9:30AM
  Sokrates T. Pantelides
  (Vanderbilt University)
  "Molecular ELectronics by the numbers"
  Host: Allen

Send comments to Laszlo Mihaly; created 11/25/2001.