Some Effects of Fluctuations and Disorder on Type II Superconductors

Thermal fluctuations alter the phase transitions in type-II superconductors. For strongly type-II materials (like the high-T sub c systems) there are two crossovers as T sub c is approached at zero magnetic field, first from gaussian to x-y model behavior and then, extremely near T sub c, to the universality class of a superconductor with strong gauge fluctuations. A vortex fluid phase intrudes between the Meissner and Abrikosov vortex lattice phases at H sub c1 as has been emphasized by Nelson. Also the transition at H sub c2 is suppressed and becomes the freezing transition of a vortex liquid. Quenched disorder destroys the long-range order of the vortex lattice, perhaps replacing it with a superconducting glass phase. Some physical properties of the systems in the various phases and near the various phase transitions will be discussed.