Fluctuation, 'melting', depinning, creep, and diffusion of the flux-line lattice in high-Tc superconductors

The transition in crystalline high-Tc superconductors from irreversible to reversible magnetic behavior above a field-dependent temperature, first ascribed to 'glassiness' then to 'flux melting', is caused by thermally activated depinning of vortices. Due to the easiness of vortex cutting and to the presence of many pins per vortex, the nature of the three-dimensional strongly fluctuating 'vortex liquid' is not yet known. Thermal depinning is a complicated problem, too, since plasticity of the vortex lattice (dislocations, cutting and reconnection, vortex kinks in the CuO-layers) is crucial for collective flux creep. Very likely, due to vortex crossing and dislocation loops the resistivity is ohmic for current density J → 0, i.e., the activation energy is finite and thermally assisted flux flow (TAFF) and flux diffusion occur rather than a vortex glass state. (orig.)