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

A model of flux pinning in type-II superconductors with randomly distribute d 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. Ev idence 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 un ity-were calculated. Dependencies of the channel formation time on magnetic field and temperature are calculated for several sets of pinning and mater ial parameters; there is a qualitative agreement with experimental data.