• Friday, January 24 1:30PM
  Herb Fertig
  (U. Kentucky)
  Deconfinement and Dissipation in Quantum Hall ``Josephson'' Tunneling
  Bilayer quantum Hall systems support an interlayer coherence that makes them analogous to XY ferromagnets. Interlayer tunneling introduces a magnetic field in the effective $XY$ problem, which has drastic effects on the vortices of the system: a single vortex/antivortex pair is connected by a string of overturned spins, leading to linear confinement. According to conventional wisdom, this should suppress vortex unbinding. We revisit this old problem, and argue that in fact vortex deconfinement can occur in this system, via a two-step process: with increasing fluctuations, strings first proliferate through the system, and then vortices unbind. Langevin dynamics simulations are shown to support the picture. Finally, we use the simulations to investigate the tunneling resistance in a quantum Hall bilayer system. Recent experiments have shown this to be reminiscent of the DC Josephson effect, although there is significant dissipation whose origin is not understood. The simulations suggest that disorder may cause string proliferation and vortex deconfinement even at low temperature. It is shown that unpaired vortices support low energy, localized excitations which may explain the dissipation.
  Host: Goldman


• Friday, January 31 1:30PM
  Peter Abbamonte
  (Cornell U.)
  A new probe of the local hole density in cuprate superconductors
  Doped cuprates are frustrated materials, with competition occuring between antiferromagnetism and the tendency of doped carriers to delocalize. It has been shown in a simple model that the expected ground state is a "stripe" phase, in which the carriers condense into domain lines separated by undisturbed antiferromagnetism. It is not clear if "stripes" exist in any real superconductor, which are three dimensional and contain impurities and multiple bands. However the proposition that there is a hidden length scale somewhere in the carrier liquid in the cuprates is compelling. This seminar will describe a new x-ray diffraction technique in which scattering from the carrier liquid in the cuprates is selectively enhanced by more than three orders of magnitude. This permits, for the first time, direct structural analysis of the superconducting ground state. First measurements have shown that optimally-doped La₂CuO_4+x is extremely homogeneous - whether modulations exist in underdoped materials has not yet been investigated. This study illustrates a new and very general approach to x-ray scattering, in which spectroscopy and diffraction are merged to probe new attributes of condensed matter.
  Host: Stephens


• Wednesday, February 12 12:30PM
  Paul M. Goldbart
  (University of Illinois at Urbana-Champaigne)
  Characterizing quantum entanglement geometrically
  The status of quantum entanglement has recently been upgraded to that of physical resource. A consequence of this promotion is that the task of quantifying entanglement has emerged as a central theme in the theory of quantum information processing. In this talk, I shall describe recent work (done in collaboration with Tzu-Chieh Wei) in which this task is approached from the perspective of geometry in Hilbert space. This geometric perspective, which is already present in a number of settings [1] and which shares features with the well-known Hartree approximation scheme, permits a unified analysis of entanglement for bipartite and multipartite pure and mixed quantum states [2]. I shall illustrate this approach with applications to a selection of entangled quantum states.
  [1] See A. Shimony, Ann. NY. Acad. Sci. 755, p. 675 (1995); H. Barnum and N. Linden, J. Phys. A: Math. Gen. 34, p. 6787 (2001).
  [2] T.-C. Wei and PMG, quant-ph/0212030.
  Host: Abanov


• Thursday, February 13, 4:00PM
  P. G. Silvestrov
  (Institut-Lorentz, Netherlands and Budker Institute of Nuclear Physics, Russia )
  Adiabatic Quantization of Andreev Levels
  The problem of finding a semi-classical spectrum of an Andreev billiard (ballistic chaotic cavity coupled to superconductor by an N-mode constriction) is considered. We identify the time T between Andreev reflections as a classical adiabatic invariant. Quantization of the adiabatically invariant torus in phase space gives a discrete set of periods T_n, which in turn generate a ladder of excited states. The largest quantized period is given by the Ehrenfest time, proportional to the logarithm of the Planck constant. The wave functions of Andreev levels fill the cavity in a highly non-uniform ``squeezed'' way, which has no counterpart in normal state chaotic or regular billiards. The theory is applied to the problems of calculating a hard gap in the semi-classical spectrum and crossover between semi-classical and random matrix description of Andreev billiards. Similar ideas may be used for the description of classical to quantum crossover in shot noise in a ballistic quantum dot.
  Host: Averin


• Friday, February 21, 1:30PM
  Anton Andreev
  (University of Colorado at Boulder and Lucent Technologies)
  Full charge counting statistics of a quantum charge pump
  Charge pumps can produce an electric current that is nearly quantized and are currently used as modern capacitance standards. The noise properties of the pumped current are important in this respect. I will discuss the pumping current noise for a charge pump based on a single electron transistor. The model is equivalent to a non-equilibrium g=1/2 Luttinger liquid with an impurity which has excitations with a fractional charge e^*=e/2. Despite that only integer transmitted charges can be observed. We obtain full charge counting statistics at arbitrary pumping strengths. At weak pumping the distribution function for the transmitted charge corresponds to a truncated Poisson process for e^*=e/2 particles from which all events with half-integer transmitted charge have been excluded.
  Host: Abanov


• Friday, February 28, 1:30PM
  Alexander Finkelstein
  (Weizmann Institute)
  Dilute electron gas near the metal-insulator transition in two dimensions
  In recent years, systematic experimental studies of dilute two dimensional electron systems have revived the fundamental question: could a metal-insulator transition exist in two dimensions? I will demonstrate how the metal-insulator transition may occur in a very low density system with strong electron-electron interactions. Based on the example of the Si-MOSFET, I will argue that the system-specific properties determine the nature of the transition.
  Host: Abanov


• Friday, March 7, 1:30PM
  Lev Bulaevskii
  (Los Alamos National Laboratory)
  Tunneling Measurements of Quantum Spin Oscillations
  We consider the problem of tunneling between two leads via a localized spin 1/2 or any other microscopic system (e.g. a quantum dot), which can be modeled by a two-level Hamiltonian. We assume that a constant magnetic field acts on the spin that electrons are in a voltage driven thermal equilibrium and that the tunneling electrons are coupled to the spin through exchange interaction. Using the non-equilibrium Keldysh formalism we find the dependence of the spin-spin and current-current correlation functions on the applied voltage, temperature and magnetic field. We find conditions under which current-current correlation function exhibits peak at the renormalized Larmor frequency. We compare our results with the quasiclassical approach and discuss the experimental results observed using STM dynamic probes of the localized spin.
  Host: Averin


• Friday, March 14, 2:00PM (note the time)
  Fabian Essler
  (Brookhaven National Laboratory)
  Weakly Coupled 1D Mott Insulators
  I consider a model of one-dimensional Mott insulators coupled by a weak interchain tunnelling. We first determine the single-particle Green's function of a single chain by exact field-theoretical methods and then take the tunnelling into account by means of a Random Phase like approximation. When the interchain tunnelling exceeds a critical value, a small Fermi surface develops in the form of electron and hole pockets. The metallic state close to the transition retains many features of the one-dimensional system in the form of strong incoherent continua.
  Host: Abanov


• Friday, March 28, 1:30PM
  Victor Yakovenko
  (University of Maryland)
  Andreev bound states in superconductors: Spontaneous soliton formation and fractional Josephson effect
  First [1], we demonstrate that a pi-soliton (kink) should spontaneously form in a 1D superconducting wire when the total number of electron is odd, because Andreev bound states in the soliton would decrease the total energy of the system. If such a wire is closed in a ring, the phase difference between the two sides of the soliton will generate a supercurrent detectable by a SQUID. The two degenerate states with the current flowing clockwise or counterclockwise can be utilized as a qubit. Second [2], we show that the contribution of the Andreev bound states to the Josephson effect between two Q1D p_x-wave or two Q2D d-wave superconductors has the period of 4p in the phase difference. Consequently, the ac Josephson current has the fractional frequency eV/h, a half of the conventional value. In the tunneling limit, the Josephson current is proportional to the first power of tunneling amplitude, thus the critical current is greatly enhanced and has an unusual temperature dependence.
  [1] H.-J. Kwon and V. M. Yakovenko, Phys. Rev. Lett. 89, 017002 (2002).
  [2] H.-J. Kwon, K. Sengupta, and V. M. Yakovenko, cond-mat/0210148.
  Host: Abanov


• Friday, April 4, 1:30PM
  David Khmelnitskii
  (Cambridge University)
  On phase transition from itinerant ferromagnet into normal metal at T=0
  We discuss the effects of magneto-dipole interaction on phase transition near the quantum critical point.
  Host: Abanov


• Friday, April 11, 1:30PM
  Robert Hwang
  (Brookhaven National Laboratory)
  Kinetics of nanostructures on surfaces
  Host: Mendez


• Friday, April 25, 1:30PM
  Sophie deBrion
  (Grenoble HMFL)
  Phase separation in the charge ordered compounds Nd(Pr)_1-xCa_xMnO₃: an ESR study
  High frequency (9.4 GHz - 475 GHz), high magnetic field ( 0 - 12 T ) Electro Spin Resonance has been used to study the magnetic excitations and possible phase separation effects in the charge ordered manganites Nd(Pr)1-xCaxMnO3. In the Nd compounds, in form of powders, we show [1] that the Nd ions are weakly coupled to the Mn ions via ferromagnetic exchange and are responsible for the peculiar ferromagnetic resonance observed in the FM phase of both compounds ( ground state below 120K for x=0.3, high field state for x=0.5). We also show that there is no trace of the FM state imbedded in the low field, CO phase in Nd 0.5Ca 0.5MnO3. On the contrary, in a 250 nm thick Pr0.5Ca0.5MnO3 film grown on LaAl2O3 substrate, we show evidences for the presence of a FM phase within the CO phase in form of very thin layers, with the ferromagnetic easy axis at 45° from the film plane. The coupling of this FM phase with the CO phase depends on the orientation of the applied magnetic field (parallel or perpendicular to the film plane).
  Host: Mihaly


• Thursday, May 1, 1:30PM
  Karoly Holczer
  (UCLA)
  Development of a Single Electron Spin Microscope
  The Single Electron Spin Microscope (SESM), under construction at UCLA, is based on using a single electron spin as a scanning probe. Small interactions have to be detected, while the probe - sample distance will be less than a nanometer. The choice of atomic scale probe (single spin)
  • Requires the use of high magnetic field (3.5 Tesla) to clearly define the functionality of the probe.
  • Allows a ~ 10-14 Newton force signal be generated by the inversion of single spin.
  • Results in a natural atomic scale resolution for SESM as a scanning probe microscope.
  • Imposes the use of stiff cantilevers (> 10 N/m) to achieve sub nanometer sample - probe distance control. As a consequence, the thermal noise contribution to the detection noise becomes insignificant next to photon shot-noise, even at room temperature.
  • The stiff cantilevers used allow for high enough frequency broadband detection to target spins with reasonable short (room temperature) lifetime.
  Conceptually, SESM is a quantum mechanical device. It works only in the single spin limit and it is inappropriate to study signals originating from a macroscopic set of spins, as opposed to the ferromagnet based, classical, incremental-sensitivity approach. The distinct set of original problems (millimeter wave excitation technology, distance control, cantilever fabrication, electronic SPM control, signal processing and computer control) defined as the road map of the SESM development will be discussed.
  Host: Mihaly


• Friday, May 2 , 1:30PM
  Adam Durst
  (Yale University)
  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. We perform a diagrammatic calculation and find radiation-induced resistivity oscillations with the correct period and phase. Results are explained via a simple picture of current induced by photo-excited disorder-scattered electrons. The oscillations increase with radiation intensity, easily exceeding the dark resistivity and resulting in negative-resistivity minima. At high intensity, we identify additional features, likely due to multi-photon processes, which have yet to be observed experimentally. Andreev, Aleiner, and Millis (cond-mat/0302063) have shown that the negative resistivity state is unstable, which leads to a domain structure in the current distribution. This causes the resistivity to saturate at zero rather than going negative, in agreement with experiment.
  Host: Abanov


• Friday, May 9, 1:30PM
  Paul Wiegmann
  (University of Chicago)
  Stochastic Conformal Maps - a new approach to critical phenomena in 2D
  I will discuss a new approach to study fractal geometry of objects which appear in 2D critical phenomena. The approach (Stochastic Loewner's Evolution) establishes an elegant relation between conformal field theory and stochastic evolution of conformal maps. The approach is developed in recent works of O. Schramm, et. al.
  Host: Abanov


• Monday, June 30, 2:00 pm
  Oleg Berman
  (Rochester University)
  Quaisparticle density-matrix representation of nonlinear time-dependent density-functional and current density-functional response functions
  The time-dependent density functional (TDDFT) equations may be written either for the Kohn-Sham orbitals in Hilbert space or for the single electron density matrix in Liouville space. A collective-oscillator, quasiparticle, representation of the density response of many-electron systems which explicitly reveals the relevant electronic coherence sizes is developed using the Liouville space representation of adiabatic TDDFT. Closed expressions for the nonlinear density-density response are derived, eliminating the need to solve nonlinear integral equations, as required in the Hilbert space formulation of the response. Closed expressions for the linear and quadratic conductivities, which are exact in the static limit, are derived by solving the adiabatic Time Dependent Current Density Functional (TDCDFT) equations for the single-electron density matrix in Liouville space.
  Host: Likharev

Send comments to Laszlo Mihaly; updated 2/5/2003.