Thermal fluctuations and critical current in twinned single crystals

We study theoretically the depinning processes in twin boundaries in high-Tc superconducting crystals. It is shown that due to the thermodynamic fluctuations of the vortex system pinned in a twin boundary, the critical current can drop to zero at a temperature less than the critical one. There are two contributions to such a process: one is the individual vortex depinning determined by the deformation of a potential well by the current, and the other is the multivortex depinning because of the short-range intervortex interaction.