• Friday, February 6, 1:30PM
  Diyar Talbayev
  Stony Brook
  "Electron spin resonance in LaMnO₃"
  Host: Mihaly
• Friday, February 13, 1:30PM
  Gun Sang Jeon
  Department of Physics , Pennsylvania State University
  Composite Fermions in Quantum Dots: Real or Illusory?
  We consider interacting electrons in semiconductor quantum dots under high magnetic fields, which, with varying confinement, form a strongly correlated liquid as well as crystal states of various symmetry. We show that the microscopic composite-fermion theory provides an excellent account of these states. Surprisingly, the quantum mechanical formation of composite fermions, earlier associated with liquid states, automatically generates crystallites in a finite quantum dot at large angular momenta. A perturbative scheme is developed which allows further, systematic improvement of the wave functions and the energies of low-lying eigenstates. We find that the first-order theory gives practically exact answers for the ground state wave function, ground state energy, excitation gap, and the pair correlation function. In an anomalous case, the higher-order perturbation turns out to be useful in capturing the subtle physics of competing orders.
  Host: Abanov
• Monday, February 16, 1:30PM
  Vadim Oganesyan
  Princeton University
  Incommensurate valence bond phases of quantum magnets
  Quantum dimer models on bipartite lattices exhibit Rokhsar-Kivelson (RK) points with exactly known critical ground states and deconfined spinons. We examine generic, weak, perturbations around these points. In d=2+1 we find a first order transition between a "plaquette" valence bond crystal and a region with a devil's staircase of commensurate and incommensurate valence bond crystals. In the part of the phase diagram where the staircase is incomplete, the incommensurate states exhibit a gapless photon and deconfined spinons on a set of finite measure, almost but not quite a deconfined phase in a compact U(1) gauge theory in d=2+1!
  Host: Allen
• Wednesday, February 18, 1:30PM
  Mahn-Soo Choi
  Korea University
  Kondo Effect and Josephson Current through a Quantum Dot between Two Superconductors
  We investigate the supercurrent through a quantum dot for the whole range of couplings using the numerical renormalization group method. We find that the Josephson current switches abruptly from a p - to a 0 - phase as the coupling increases. At intermediate couplings the total spin in the ground state depends on the phase difference between the two superconductors. Our numerical results can explain the crossover in the conductance observed experimentally by Buitelaar et al. [Phys. Rev. Lett. 89, 256 801 (2002)].
  Host: Averin
• Friday, February 20, 1:30PM
  Ilka Bischofs
  Max Planck Institute of Colloids and Interfaces, Potsdam, Germany
  Elastic interactions of mechanically active cells with soft materials
  In condensed matter materials elastic interactions of classical defects are well known and there exists an established theoretical description in terms of force multipoles. Recent experiments in cell biology suggest that anchorage-dependent cells respond to the mechanical properties of their environment by adjusting cell orientation and position. We discuss the interaction of active cells with an elastic environment and compare it to case of physical force dipoles. Despite marked differences, both cases can be described in the same theoretical framework. We exactly solve the elastic equations for anisotropic contraction dipoles in different geometries and with different boundary conditions. The results are then used to predict optimal position and orientation of cells in soft materials in nice agreement with experiments for fibroblasts. The presented computational method could be applied for rational design of tissue equivalents in the near future.
  Host: Allen
• Monday, February 23, 1:30PM
  Andrey Shytov
  Harvard University
  "Can Electric Current Shake the Crystal? (Electromechanical noise in diffusive conductors)"
  Electrons moving in a conductor can transfer momentum to the lattice via collisions with impurities and boundaries, giving rise to a fluctuating mechanical stress tensor. Driving electrons out of equilibrium by applying the voltage across the conductor, one may control this electromechanical noise. The root-mean-squared momentum transfer per scattering event in a disordered metal of dimension greater than the mean free path and screening length is found to be reduced below the Fermi momentum. The excitation of an elastic bending mode by the electromechanical noise is estimated to fall within current experimental sensitivity for a nanomechanical oscillator.
  Host: Abanov
• Friday, February 27, 1:30PM
  Julia Meyer
  University of Minnesota, Department of Physics
  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 D ~ 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.
  Host: Allen
• Monday, March 1, 1:30PM
  Adam Durst
  Yale University, Department of Physics
  Thermal Hall Conductivity in the Vortex State of d-Wave Superconductors
  Experiments have now established that the order parameter (gap) in the high-Tc cuprate superconductors exhibits d-wave symmetry, vanishing at four nodal points on the Fermi surface. Near each of these four gap nodes, quasiparticles are easily excited and behave more like massless relativistic particles than electrons in a metal. An excellent probe of these nodal quasiparticles is the transport of heat in the presence of a magnetic field. In the weak-field regime, the thermal conductivity tensor can be expressed in terms of the cross section for quasiparticle scattering from a single magnetic vortex. We calculate this cross section and thereby obtain both the longitudinal thermal conductivity and the thermal Hall conductivity in surprisingly good qualitative agreement with the measured data. The transparent nature of our calculation allows us to obtain a physical understanding of the features seen in experiments.
  Host: Abanov
• Friday, March 5, 1:30PM
  Jian-Xin Zhu
  Theoretical Division, Los Alamos National Laboratory
  Nanoscale Physics in High-T_c Superconductors
  The local electronic properties around a vortex and a single impurity can provide useful information on the interplay between the antiferromagnetic (AF) and d-wave superconducting orderings in high-Tc superconductors. We carry out a self-consistent study of an effective model Hamiltonian with both types of interactions. We show that, depending on the strength of the on-site Hubbard repulsion, various interesting spin density wave as well as associated charge density wave can be induced around magnetic vortices. These results are consistent with recent observations of scanning tunneling microscopy (STM). We also study the effects of spin resonance mode on the electronic local density of states (LDOS). The coupling between the electrons and the resonce mode produces high-energy peaks in the LDOS, which displays a two-unit cell periodic modulation around a nonmagnetic impurity. It suggests that the momentum dependence of the magnetic mode might be detected with STM.
  Host: Allen
• Monday, March 8, 1:30PM
  Andy Lau
  Department of Physics and Astronomy , University of Pennsylvania
  Microrheology and Stress Fluctuations in Living Cells
  One of the major challenges for modern biology is to understand how cells sense and produce force to respond to their environment in a directed manner. As a prerequisite, an accurate physical picture of the viscoelasticity and active behaviors of the cytoplasm requires powerful experimental techniques and theoretical modelling. Recently, microrheology has emerged as a new experimental tool to probe active cytoskeleton dynamics. In this talk, we provide a theoretical framework for interpreting passive microrheology experiments on non-equilibrium active systems such as living cells, demonstrate that microrheology can be used to sensibly quantify the power spectrum of cytoskeletal stress fluctuations due to molecular motor activity in vivo, and propose a plausible microscopic model that explains the observed 1/f ² spectrum.
  Host: McCoy
• Friday, March 12, 1:30PM
  Ivan Bozovic
  (BNL)
  Long-lived coherent acoustic waves generated by femtosecond light pulses
  We report on photoexcitation of coherent longitudinal acoustic phonons in single-crystal cuprate thin films on epitaxialy-matched substrates. The photo-induced reflectance oscillations are unusually long-lived; in some samples we could easily resolve them for hundreds of periods. We studied the effect of varying a number of parameters, including the film doping level, thickness and temperature, as well as the pump and probe beam wavelength, power, polarization, and incidence angle. We account quantitatively for the oscillation period, dispersion, phase, and decay of amplitude with time. Some ideas for continuation of this work and on possible applications will also be presented.
  Work done in collaboration with M. Schneider and M. Onellion (U. Wisconsin-Madison), Y. Xu and R. Sobolewski (U. Rochester) Y. H. Ren and G. Lüpke (College of William and Mary, Williamsburg), J. Demsar and A. J. Taylor (LANL).
  Host: Allen
• Friday, March 19, 1:30PM
  Chang-hua Zhang
  (U. Arizona)
  Shell Effect and the Stability of Metal Nanowires
  Classically, a wire longer than its' circumference is unstable agaist the long wavelength perturbation, the well known Rayleigh instability. But experimental findings show that long metal nanowires can be fabricated and are very stable. In this talk, I will focus on the analysis of the new mechanism to stabilize the metal nanowires at equilibrium and nonequilibrium cases.
  Host: Likharev
• Friday, April 16, 1:30PM
  Susan L. Dexheimer
  (Washington State University^*)
  Ultrafast Physics in One Dimension
  Low dimensional materials, and in particular, quasi-one-dimensional systems have attracted considerable interest owing to their unusual electronic properties. In many of these systems, strong electron-phonon couplings can lead to the formation of nonlinear excitations such as self-trapped excitons, in which an electronic excitation is localized as a result of lattice distortions that occur in response to the excitation. I will present the results of femtosecond time-resolved experiments in which we have studied the electronic and vibrational dynamics associated with the formation and evolution of the self-trapped exciton in quasi-one-dimensional mixed-valence molecular solids.
  *Currently on sabbatical leave: Department of Chemistry, University of California, Berkeley
  Host: Allen
• Monday, April 19, 1:30PM
  Andrea Gauzzi
  (Institute of Materials for Electronics and Magnetism, National Research Council, Parma, Italy )
  NaMn₇O₁₂: a model system for studying charge, spin and orbital ordering in mixed-valence manganites
  We report on the structural, magnetic and transport properties of NaMn₇O₁₂, a mixed-valence manganese oxide with double perovskite structure prepared under high-pressure. Thanks to the peculiar AA'₃B₄O₁₂ crystal structure, the average valence of the Mn ions in the octahedrally coordinated B sites can be varied without any chemical substitution, contrary to the case of the classic compounds with simple ABO₃ perovskite structure, such as La_1-xCa_xMnO₃. Neutron diffraction data show that, similarly to the above system, at low temperature, also NaMn₇O₁₂ exhibits a charge ordering of the Mn³⁺ and Mn⁴⁺ ions in the B sites, followed by a CE-type ordering of these ions. Though, NaMn₇O₁₂ is characterized by three unique features: 1) the charge, spin and orbital order of the Mn³⁺ 3d electrons is full within the experimental uncertainty. 2) The orbital order involves the x²-y², instead of 3z^2-r2, orbitals owing to the different crystal symmetry. 3) By lowering further the temperature, the Mn³⁺ ions in the A' sites order antiferromagnetically, thus forming a second magnetic sublattice in addition to the CE one. As a result, the zig-zag pattern of 3z²-r² orbitals characteristic of La_1-xCa_xMnO₃ and related compounds is absent in NaMn₇O₁₂. Instead, a peculiar corner-sharing buckled chains of x²-y² orbitals are formed along the b-axis. The peculiar structural, magnetic and transport properties that arise from the above unusual crystal symmetry and orbital ordering are illustrated and discussed. Thanks to the absence of structural inhomogeneities and of local distortions inherent of doped compounds, the data presented provide direct information about the changes of crystallographic and physical properties occurring at the charge- and spin-ordering transitions. In conclusion, it is shown that NaMn₇O₁₂ offers a unique opportunity to study charge, orbital, and spin ordering, to test the validity of current theoretical models and to differentiate intrinsic from extrinsic phenomena in Jahn-Teller systems.
  Host: Mihaly
• Friday, April 23, 1:30PM
  E.V. Tsiper
  (George Mason University, Fairfax, VA & Naval Research Laboratory, Washington, DC )
  Polarization interactions between molecules, in molecular aggregates and solids.
  Weakly bound molecular systems appear in many areas of modern science ranging from the structure of water to molecular crystals in organic electronics to molecular aggregates in biophysics. Such systems pose significant difficulties to conventional computational methods. Long range Coulomb forces, small transfer integrals and localized nature of charges make polarization a major effect, with energy scale of about 1 eV. Our recent approach treats individual molecules rigorously as quantum-mechanical systems subject to classical fields of other molecules. Nonuniform molecular fields are described with minimal atomic multipole expansion (MAME) using a carefully-chosen set of atomic charges or higher multiples. Atom-atom polarizabilities are introduced to account for self-consistent charge redistribution within individual molecules. In molecular crystals we compute transport gaps and positions of charge-transfer states, dielectric tensors, polarization contributions to lattice energies. I will also address the problem of intermolecular forces, focusing on the example of water, where the approach reproduces hydrogen bonding and other features of the well-studied water pair potential to fine detail.
  Host: Goldman
• Friday, April 30, 1:30PM
  Valerii Vinokour
  (Argonne National Laboratory )
  Bose Glass Transition and Vortex Localization
  We investigate two-dimensional Bose system with the long range interactions in the presence of disorder. Formation of the bound states at strong impurity sites gives rise to an additional depletion of the superfluid density. We demonstrate the existence of the intermediate superfluid state where the condensate and localized bosons present simultaneously. We find that interactions suppress localization and that with the increase of the boson density the system experiences a sharp delocalization crossover into a state where all bosons are delocalized. We map our results onto the three dimensional system of vortices in type II superconductors in the presence of columnar defects; the intermediate superfluid state maps to an intermediate vortex liquid where vortex liquid neighbors pinned vortices. We predict the depinning transition within the vortex liquid and depinning induced vortex lattice/Bose glass melting.
  Host: Averin
• Friday, May 7, 1:30PM
  Helmut Strey
  ( Biomedical Engineering and Center for Biotechnology )
  DNA diffusion observed in a two-dimensional cavity array
  Host: Allen
• Friday, May 14, 1:30PM
  Henk Stoof (Simons lecturer)
  (University of Utrecht)
  The bosonic Kondo effect
  The Kondo effect is associated with the formation of a many-body ground state that contains a quantum-mechanical entanglement between a (localized) fermion and the free fermions. We show that a bosonic version of the Kondo effect can occur in degenerate atomic Fermi gases near the Feshbach resonance. We also discuss how this bosonic Kondo effect can be observed experimentally.
  Host: Abanov
• Monday, May 24, 1:30PM
  Rusko Ruskov
  (University of California, Riverside )
  QND squeezing of a nanomechanical resonator via continuous stroboscopic measurement and feedback
  A nanomechanical resonator with fundamental vibrational frequency w₀ < 1GHz at milli-Kelvin temperatures may reach the region of quantum behavior under weak continuous measurement with a solid-state detector. Particular experimental record from the detector (noisy detector current in case of a quantum point contact) allows us to monitor the quantum state of the resonator via quantum Bayesian equations. We show that squeezing in the resonator position (or momentum) is possible considerably below the limit of zero-point fluctuations via continuous weak measurement of its position if the measurement strength of the detector is modulated with a frequency w close to w₀/n (n=1,2,3...). This procedure can be considered as a continuous analog of the QND stroboscopic measurement scheme of Braginsky et al. and Thorne et al. By monitoring the quantum state of the resonator it is possible to organize a closed feedback loop that suppresses the heating of the resonator due to measurement back action (simplest physical realization would be a feedback force proportional to monitored average momentum). We discuss how to distinguish experimentally such a squeezed state from a non-squeezed (e.g., ground) state.
  Host: Averin

Send comments to Laszlo Mihaly; updated 30/8/2003.