Flux dynamics in high-temperature superconductors

Abstract

Magnetic flux penetration, flux pinning and depinning are examined. The induced electric field and the resistivity due to the thermally activated hopping of the flux lines are analysed. Depending on the relative magnitude of the transport or the induced current density, J, relative to the critical current density, Jc,, three flux regimes are identified: (i) the thermally activated flux flow (or TAFF) for J << Jc; (ii) the flux creep for J approximately= Jc; and (iii) the flux flow when J >> Jc. The critical state and its relation to flux creep is defined. The flux creep, TAFF and flux flow are contrasted in terms of the current-voltage characteristics and in terms of the vortex lattice. A phase diagram, in the magnetic field versus temperature plane, is identified for a typical high-temperature superconductor. Vortex fluid (TAFF) and vortex solid (flux creep) phases are defined. Flux relaxation and the flux profile inside a superconducting sample are examined. The characteristic time frame for the flux hopping is contrasted with a characteristic time of measurement in an AC field situation and frequency effects are explained. Finally, a basic outline of pinning and the methods which have been attempted to increase pinning are discussed.