UCSC Physics Department
Condensed Matter Seminar Schedule - Fall 2003

Sriram Shastry
Physics Department, UC Santa Cruz

September 26, 2003

Triangular Lattice Transport, Magnetism and superconductivity in Na_x Co O_2

The recently discovered Sodium Cobalt Oxide system provides a fascinating platform for the study of strong correlations coupled with frustration. We review some recent experiments, and suggest that these can be modelled by a triangular lattice t-J model. A study of this model suggests that it supports a time reversal violating superconducting state, a ferromagnetic metallic state and possibly a spin gap phase in a narrow composition range. We predict an unsaturated T linear dependence of the Hall constant in this system which seems to be corroborated by recent data.


Joan Adler
Physics Department
Technion Israel Institute of Technology

WEDNESDAY, October 1, 2003

Visualizing Bulk and Surface Melting

Melting is difficult to observe on an atomic scale; hence atomistic simulations with interactive visualization have a useful role.

We have simulated both bulk and surface melting in fcc and bcc metals and visualized these processes with the Atomistic Visualization software AViz, developed in the Computational Physics Group at the Technion. Features such as layer mixing and a thin liquid surface layer at a free surface were observed.

Applications of AViz to other atomistic systems and to spin visualization will also be briefly introduced.

Gergely Zimanyi
Physics Department
UC Davis

October 10, 2003

Fingerprinting disordered magnets

We review hysteretic properties of disordered magnets: the Sherrington-Kirkpatrick and the Eadwards-Anderson spin glasses, and the Random Field Ising model. A new and powerful diagnostic tool is introduced, the method of First Order Reversal Curves (FORC). FORC diagrams are used to characterize non-equlibrium phase transitions, memory effects, and refined parameter searches. Finally, a hysterestic optimization method is described.

Theodore H Geballe
Department of Applied Physics
Stanford University

October 17, 2003

What can we learn by paying attention to superconducting transition temperatures?

Historically plenty of new physics has been discovered simply by observing the variation of the superconducting transition temperature in a given set of materials. Tc is a conveniently measured and sensitive signal. Unexpected behavior sometimes signals new physics, and at other times new materials science.(*) Comparison of Tc's of hole-doped high-temperature superconducting cuprates demonstrates a great disparity in comparable compounds which shows there must be an extra enhancement in some or an extra depression in other ones, or both enhancement in some and depression in others. Current theories of superconductivity which assume that the pairing interactions are confined to the CuO2 layers are incapable of explaining enhancement, however a simple ionic approach can. Further consideration using an ionic approach leads to a new model for understanding transport and superconductivity in the hole doped cuprates.
(*) Of course we must exclude the not-too-infrequent reports of spurious signals which are incorrectly attributed to superconductivity.

Jim Eisenstein
California Institute of Technology

October 24, 2003

The Quantum Hall Effect Meets Bose Condensation: Long-Sought Superfluid Found?

Surprisingly enough, when two layers of electrons are close enough together they can take on properties reminiscent of both superfluid helium and superconducting Josephson junctions and at the same time display a quantized Hall effect. This exotic state of affairs arises when Coulomb interactions join forces with a magnetic field to convert the bilayer electron gas into a Bose condensate of excitons. In this talk I will discuss recent experiments on this fascinating many-body system and speculate on what it might have still in store.


October 31, 2003

Gerald Seidler
Department of Physics
University of Washington

November 7, 2003

Sand, Foam, and Entropy

The physics of granular and cellular mesoscale materials (henceforth sand and foam) have independently held the interest of a significant portion of the statistical mechanics community for several years. Most recently, there has been a growing appreciation of the similarities and differences between the various theoretical approaches to these materials. In this talk, I'll outline my group's experimental and simulational work on sand and foam. Specifically, I'll discuss in detail our microtomographic studies of their structure, and our continuing attempts to define the most efficient statistical description of their inherent disorder. This will directly lead to the computational challenge: "Given a large statistical sampling of a disordered structure, how do you calculate the configurational entropy?"


November 14, 2003

Leo Radzihovsky
Physics Department
University of Colorado

November 21, 2003

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 these systems in the presence of a weak in-plane magnetic field. The most interesting of these is the interlayer charge-balanced to charge-unbalanced transition.[1] The striking experimental signatures are the universal nonlinear charge-voltage and in-plane field characteristics, 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.
[1] L. Radzihovsky, Phys. Rev. Lett., 87 (2001) 236802


November 28, 2003

Stuart Brown
Department of Physics and Astronomy
University of California, Los Angeles

December 5, 2003

Charge order and the ground states of molecular conductors

Many molecular conductors are charge-transfer salts with the empirical formula M2X, with M a planar donor molecule and X a singly charged anion. Examples include (TMTTF)2X and (BEDT-TTF)2X. The constitutents are arranged in alternating layers, and depending on the configuration within the donor layers, can be considered as 1/4-filled or 1/2-filled systems. The 1/4-filled systems are susceptible to charge order, rendering the system an insulator and also influencing whether the ground state is magnetic or non-magnetic. The charge order can be suppressed through the application of mechanical or chemical pressure, tuning the system through a sequence of ground states and eventually resulting in high conductivity and sometimes superconductivity. There are indications from electronic model systems that include the interactions producing charge order that the resulting charge fluctuations can serve as a mechanism to induce superconducting pairing. Possible examples include not just some molecular systems but also the newly discovered cobaltate superconductor.