• Friday, January 26, 1:30PM
  Andrey Lopatin
  (Maryland)
  "Barriers in spin-glass models. The dynamical approach."
  Existence of large barriers separating metastable states is one of the most important features of the spin-glass systems. The barrier heights cannot be easily found by the standard spin-glass methods. Indeed, the replica theory describes only the thermodynamical properties and therefore contains information only about the free energy minima. The dynamical theory, in principle, does contain information about the barriers, but the probabilities of transitions between the metastable states are exponentially small and are neglected by the mean field theory which is usually used. I will present a method of calculation of the barriers between the metastable states developed in the framework of the dynamical theory. The transitions over the barriers are represented by instantons in the Lagrangian formulation of the Langevin dynamics. I will discuss the application of this method to different spin glass models. I will also show how the instanton method allows to find the distribution function of the height of the largest barrier in 1D random classical system.


• Friday February 2, 1:30PM
  Walter Schirmacher
  (Physics Dept., U. of Oregon)
  "Vibrational properties of disordered solids"
  on leave from Phys. Dept of Technical Univ. of Munich, in collaboration with Gregor Diezemann, Dept. of Chemistry, Univ. of Mainz
  The low-temperature thermal properties of disordered solids and the related low-frequency vibrational spectrum puzzles physicists already for several decades. Near the frequency, where the mean free path can be estimated to become equal to the phonon wavelength an anomalous increase in the density of states is observed in most materials, which has been called "boson peak". This vibrational anomaly is reflected in a characteristic maximum in the temperature dependence of the specific heat if plotted as C(T)/T³.
  We have performed calculations based on the coherent-potential approximation (CPA) as well as on numerical diagonalization on a model consisting of coupled harmonic oscillators on a cubic lattice with statistically distributed spring constants. These calculations (which are in excellent agreement to each other) show that the "boson peak" is a characteristic feature of a disordered solid. It is produced by the presence of configurations with particular soft force constants. It appears in the frequency range, where the concept of a propagating wave loses its meaning, i. e. where the disorder-induced mean free path approaches the wave length. By analyzing the energy level statistics of our models we show that the vibrational states associated with the "boson peak" are not localized in the Anderson sense. This corroborates the findings of Allen and coworkers that these states are able to transport energy in a diffusive way.


• Friday February 9, 1:30PM
  Steven L. Richardson
  (Howard University)
  "Using Supercomputers to design and model novel materials and molecules"
  Recent advances in computational materials science and theoretical condensed matter physics, coupled with the power and speeds of modern supercomputers, have enabled scientists and engineers to design and study novel materials from a first-principles or ab initio viewpoint. Such calculations have become increasingly useful in their ability both to explain experimental properties of materials and to predict the existence of new materials and molecules. As an example of the progress in this exciting field, we will discuss some of our recent work on the unusual structural and dynamical properties of the highly-energetic molecule, octanitrocubane, and on the possible stability of the polynitrogen ionic salt, (N₅⁺)(N₃^-).


• Friday February 16, 1:30PM
  Andrey V. Chubukov
  (University of Wisconsin, Madison)
  "Large Quantum-critical pairing in cuprates"
  I analyse the pairing problem in 2D fermionic systems assuming that the pairing is mediated by spin fluctuations. I argue that near antiferromagnetic instability this pairing involves fully incoherent fermions and diffusive spin excitations. I show that the competition between fermionic incoherence and strong pairing interaction yields the pairing instability temperature T₀ which increases and saturates as the magnetic correlation length diverges to infinity. I present the full solution of the Eliashberg-type equations below T₀ and argue that in this quantum-critical regime, the actual superconducting instability occurs only at T_c < T₀ leaving a pseudogap regime between T_c and T₀.
  Host: A.G. Abanov


• Friday, February 23, 1:30PM
  Igor Altfeder
  (Harvard University)
  "Subsurface Imaging with STM (Quantum Mirages in Self-assembled Nanostructures)"


• Friday, March 3, 1:30PM
  Nick Reed
  (Yale University)
  "Disordered superconductors, paired quantum Hall states, and random bond Ising models in two dimensions."
  Host: V.J. Goldman


• Friday March 9, 1:30PM
  Dmitri Ivanov
  (ETH Zurich)
  "Vortices in superconductors with triplet pairing: new symmetry classes and nonabelian statistics"
  Superconductors with triplet pairing are predicted to exhibit many remarkable effects arising from the symmetries of odd pairing. One such effect is the appearance of zero-energy quasiparticle levels in vortex cores, which makes physics of such vortices different from that in conventional superconductors. First, the presence of a zero-energy level changes the symmetry class of vortex excitations. In the framework of random-matrix theory, a Hamiltonian of a disordered mesoscopic system may, under certain conditions, be described by a random matrix ensemble depending on its symmetries. In such a case the level statistics is universal and classified into one of the twelve symmetry classes. While quasiparticles in a disordered vortex in a conventional (s-wave) superconductor belong to the symmetry class C, in a triplet (p-wave) superconductor excitations inside the vortex belong to the symmetry class B if the time-reversal symmetry is broken or to the symmetry class DIII-odd if the time-reversal symmetry is preserved [for spin vortices in the order parameter without time-reversal symmetry breaking]. These are the first known mesoscopic realizations of the symmetry classes B and DIII-odd. Second, the ground state of a system of several isolated vortices is degenerate, and vortices may possess nonabelian braiding statistics when they move around each other. We consider a system of 2n isolated half-quantum vortices in the order parameter of the A phase of ³He. Each such vortex has a zero-energy Majorana fermion level in its core, and the ground state of the system of vortices is 2ⁿ-degenerate. From the general properties of Bogoliubov-deGenens equations, we explicitly derive the unitary transformations generated by vortex braiding. The vortex statistics is found to be identical to that for the Moore-Read (Pfaffian) quantum Hall state, which supports the conjecture by Read and Green about the topological equivalence between the Pfaffian and the BCS p-wave states.
  Host: A.G. Abanov


• Friday, March 30, 1:30PM
  V. M. Kenkre
  (Univ. of New Mexico)
  "Nonlinear Response Theory and Resolution of a Mobility Puzzle in Organic Solids"
  The transport of quasiparticles in organic materials presents difficult and interesting challenges to the theorist. An example is the explanation of the nonlinear dependence (Poole-Frenkel) of photoinduced carriers in molecularly doped polymers. This talk will present an explanation of striking observations which apply over decades of field strength in terms of a nonlinear response theory developed recently. The experimental background will be briefly described, followed by a general prescription to address a large variety of forms of the nonlinear dependence of the mobility on applied electric field. The prescription goes beyond Kubo's linear response theory within its own restricted domain of applicability. The prescription will then be applied to explain, in a natural way, the remarkable square root field dependence of the logarithm of the mobility of photoinjected charge carriers in molecularly doped polymers on the basis of correlated dipole-induced disorder in these materials.
  (Work supported in part by the National Science Foundation under grant nos. DMR-0097210 and DMR-0097204)
  Note:- This talk deals with an application of nonequilibrium statistical mechanics to condensed matter physics. I had the good fortune of learning both these fields (as a graduate student at Stony Brook) from the greatest teacher I have ever known: Max Dresden. In particular, work to be described in this talk is based on a generalization of the the first paper I ever published, which was with Dresden, in PRL in 1971.
  References:-
  1. D. H. Dunlap, P. E. Parris, V. M. Kenkre: Charge-Dipole Model for the Universal Field-Dependence of Mobilities in Molecularly-doped Polymers, Phys. Rev. Letters 77, 542-545 (1996).
  2. P. E. Parris, D. H. Dunlap, V. M. Kenkre: Dispersive Aspects of the High-Field Hopping Mobility of Molecularly Doped Solids with Dipolar Disorder, J. Polymer Science B: Poly. Phys. 35, 2803-2809 (1997).
  3. P. E. Parris, M. Kusī, D. H. Dunlap,V. M. Kenkre: Nonlinear Response Theory: Transport Coefficients for Driving Fields of Arbitrary Magnitude, Phys. Rev. E 56 , 5295-5305 (1997).
  4. V. M. Kenkre, M. Kusī, D. H. Dunlap, P. E. Parris: Nonlinear Field Dependence of the Mobility of a Charge Subjected to a Superposition of Dichotomous Stochastic Potentials, Phys. Rev. E 58, 99-106 (1998).
  5. S. Novikov, D. Dunlap, V. M. Kenkre, P. E. Parris, A. Vannikov: Essential role of Correlations in Governing Charge Transport in Disordered Organic Materials, Phys. Rev. Lett., 81, 4472-4475 (1998).
  6. P. E. Parris, D. H. Dunlap, V. M. Kenkre: Energetic Disorder, Spatial Correlations, and the High Field Mobility of Injected Charge Carriers in Organic Solids, phys. stat. solidi (b) 218, 47 (2000).
  Host: P.B. Allen


• Friday, April 6, 1:30PM
  Boris Altshuler
  (Princeton)
  "Two Channel Kondo from Two Level Systems? Forget it!"
  Host: I. Aleiner


• Friday, Apr. 13, 1:30PM
  Mark R. Pederson
  (Naval Research Lab)
  "Application of Density-Functional Theory to Molecular Nanomagnets"
  Host: P.B. Allen


• Friday, Apr. 20, 1:30PM
  Andras Janossy
  (Technical University, Budapest)
  "C₅₉N:C₆₀: A magnetic fullerene embedded inC₆₀ "
  Host: L. Mihaly


• Friday, May 4, 1:30PM
  Natalya Zimbovskaya
  (CUNY)
  "Deformed Fermi Surface Theory of Magnetoacoustic Anomaly in Modulated Quantum Hall System Near Half Filling"
  Host: V.J. Goldman

Send comments to Laszlo Mihaly; filed 8/12/2001.