ARRAYS OF FLUXOIDS IN THE HIGH-TC SUPERCONDUCTORS AND THEIR PUTATIVE PHASE-TRANSITIONS

The static and dynamic properties of the magnetic vortices which penetrate films and crystals of the high-TC superconductors in fields above Hc1 have been examined in a variety of experiments. Direct visualization at low fields has been augmented with frequency dependent ac susceptibility, penetration depth, magnetization relaxation (using high-Q mechanical oscillators), and resistance measurements at high fields in an effort to obtain a complete understanding of the vortex dynamics. The various models for the vortex phase diagram, including isolated and collective vortex pinning, vortex lattice melting, and thermodynamic glass transitions are compared to the data. The bulk of the data presently available are consistent with strong pinning of independent vortices. Deviations from this model suggest where more complicated dynamics may become important.