Thermal partial vortex depinning and channel formation in type-II superconductors

A model of flux pinning in type-II superconductors with randomly distributed point-like defects is suggested. In a vortex lattice thermal fluctuations are taken into account by adding entropy terms into the system free energy. By free energy minimization the equilibrium number of pinned vortices as a function of the applied magnetic field and temperature was calculated. Evidence of a thermal partial vortex depinning at relatively low temperatures was found in RE-123 superconductors. It is shown that the thermal partial depinning assists the channel formation. Using the percolation theory, the T(B) curves - at which the probability of the channel formation approaches unity - were calculated. Dependencies of the channel formation time on magnetic field and temperature are calculated for several sets of pinning and material parameters; there is a qualitative agreement with experimental data.