Besides vanishing resistivity, superconductors exhibit further unusual properties when placed in an external magnetic field. Small fields are totally excluded from the superconductor (the ``Meissner phase'') while large fields totally destroy the superconductivity. For all technologically important superconductors, there is an intermediate ``mixed'' phase in which vanishing resistivity coexists with quantized filaments of magnetic flux which thread the sample.

These flux lines exhibit fascinating static and dynamic phase transitions in high temperature superconductors: The critical current can depend on the type of disorder, ``point,'' ``columnar,'' or ``splay.'' The Abrikosov lattice, a regular array of flux lines formed in certain circumstances, can be destroyed by randomness, inducing various glassy regimes, including ``moving glass'' phases with surprising response properties. The resistance of the vortices to motion can exhibit an anomalous enhancement near the melting temperature: the ``peak effect.'' Understanding of these effects constitutes some of the most challenging and important issues in the study of high temperature superconductivity today.

We published a paper which described numerical simulations in the London Langevin approximation, using a new realistic representation of the disorder. At low magnetic fields we found a disentangled and dislocation free Bragg Glass regime. Increasing the field introduced disorder--driven entanglement, leading to a Vortex Glass phase. Increasing temperature melted both glasses into a Vortex Liquid. The phase boundaries we found were in quantitative agreement with experimental data. We also have submitted a paper describing the first numerical observation of the peak effect.

Relevant Publications:

[103.] C.J. Olson, C. Reichhardt, R.T. Scalettar, G.T. Zimanyi, and N. Gronbech-Jensen, ``Disordering Transitions in Vortex Matter: Peak Effect and Phase Diagram,'' Physica C384, 143 (2002).
[110.] C. Reichhardt, G.T. Zimanyi, R.T. Scalettar, A. Hoffmann, and Ivan K. Schuller, ``Individual and Multi Vortex Pinning in Systems with Periodic Pinning Arrays,'' Phys. Rev. B64, 052503 (2001).
[111.] C. Reichhardt, C.J. Olson, R.T. Scalettar, and G.T. Zimanyi, ``Commensurate and Incommensurate Vortex Lattice Melting in Periodic Pinning Arrays,'' Phys. Rev. B64, 144509 (2001).
[116.] Mahesh Chandran, R. T. Scalettar, and G. T. Zimanyi, ``Metastability and uniqueness of vortex states at depinning'' Phys. Rev. Lett. 89, 187001 (2002).
[121.] Mahesh Chandran, R. T. Scalettar, and G. T. Zimanyi, ``Domain regime and peak effect in disordered vortex matter,'' Phys. Rev. B69, 024526 (2004).
[122.] Mahesh Chandran, R. T. Scalettar, and G. T. Zimanyi, "Dynamic Transition across the peak effect in type-II superconductors,'' Phys. Rev. B67, 052507 (2003).