Flux Creep and AC Measurements in Superconductors

Abstract

As discovered by Abrikosov, a magnetic field B can penetrate a type-II superconductor in form of flux lines or vortices which carry a quantum of flux each and arrange to a more or less regular triangular lattice. The flux-line lattice has interesting elastic and fluctuation properties, in particular in the highly anisotropic high-Tc superconductors (HTSC) with layered structure. Under the action of an electric current density J > J c the flux lines move and dissipate energy, but for J < J c they are pinned by material inhomogeneites. In HTSC thermally activated depinning causes a finite resistivity p even at current densities J < J c. At sufficiently high temperature T ohmic resistivity p(T,B) is observed down to J → 0. This indicates that the flux lines are in a “liquid state” with no shear stiffness and with small depinning energy. At lower T, p(T, B, J) is non-linear since the pinning energy of an elastic vortex lattice or “vortex glass” increases with decreasing J. In the extremely anisotropic Bi-and Tl-based HTSC short vortex segments (“pancake vortices” in the CuO layers) can depin individually with very small activation energy.