Overview

This research program focuses on vortex matter, the electrodynamic system of quantified flux lines created by the application of a magnetic field to superconductors. There are three main directions of research:

• The equilibrium phase diagram of liquid, lattice, and glassy states of vortex matter
• Driven dynamics of vortices across a pinscape
• High performance technological materials for applications

The Superconductivity and Magnetism Group makes in-depth experimental and theoretical investigations of specific compounds and general issues important for applications and fundamental physics of novel materials. We maintain leading programs in both experiment and theory, with each deriving strong benefit through close mutual cooperation. We have extensive materials characterization facilities including sophisticated probes of the equilibrium and dynamic behavior of materials based on state-of-the-art magneto-transport, magnetization, and magnetic imaging apparatus. On the theoretical side, we emphasize the analytical derivation of measurable properties from simple assumptions, the phenomenological description of complex behavior such as occurs in disordered systems and at phase transitions, and the numerical simulation of systems that cannot be treated analytically because of strong non-linear effects or unusual boundary conditions.

The physics of vortices in superconductors is currently undergoing a major revision stimulated by new discoveries. In the last five years, many of the traditional concepts of vortex physics have been overthrown or found to be too limited to describe the new behavior. As vortex behavior is probed at ever deeper levels, novel phenomena are continuously discovered, which lead to new physical pictures of vortex behavior, to new experimental tools for probing this behavior, and to new theoretical concepts which often apply generally in condensed matter physics, well beyond their vortex origins. The issues in vortex physics fall into three categories: equilibrium phase diagrams, dynamics of driven phases, and technological applications. We develop forefront research in all these areas.

• Equilibrium phase diagrams

We explore the novel vortex phase diagram with transport, magnetization, and field imaging measurements, focusing on the order of the phase transition, the critical point suggested by resistivity data at high field, the quantitative thermodynamic characterization of the entropy and magnetization jumps at the transitions, the nature of the high field phases above the critical point, and the effect on melting of artificially induced disorder. We focus on the major question of the nature of the liquid and solid phases and the role of vortex entanglement in determining their properties.

• Dynamics of driven phases

We establish the principles governing the behavior of vortices in motion, a problem which is central to improving the performance of superconducting materials for applications and is of strong fundamental interest for the physics of nonlinear dynamical systems. It is now being recognized that there are many distinct dynamic vortex phases: liquid motion describable by hydrodynamics and solid motion falling into two classes: elastic, where the neighbors of a given vortex do not change during the motion, and plastic, where the neighbors do change. We use numerical simulations and analytic theory to characterize the salient features of each dynamic state, looking especially for symmetry in the positions and velocities of the vortices which define each kind of motion. Our experimental program motivates and validates the theoretical progress.

• Technological applications

We focus on the entangled vortex state as a novel means of controlling energy dissipation due to vortex motion. This state has been suggested theoretically and observed in some numerical simulations. A major obstacle to exploring this promising direction is the absence of experimental probes of entangled behavior. We will develop three such probes, based on three expected dynamic features of the entangled state: long relaxation times, high sheer viscosity, and finite longitudinal correlation. These probes require exploring new geometries for driven vortex states such as Corbino disks and controlled current pseudo-transformers.

We continue to develop magneto-optical imaging as a tool to characterize the local performance of composite BiSrCaCuO (BSCCO) wires and yttrium barium copper oxide (YBCO) thin and thick films on the scale of a few microns. We are developing shielding methods, which characterize the local pinning strength by examining the field penetration, and transport methods, which directly image the path and magnitude of transport currents within the superconductor. Both techniques are applied to commercially produced superconducting wires and coated conductors, as well as to the superconducting materials themselves in order to improve their performance.