General Features of Quantum Creep in High-Tc Superconductors

Measurements of the relaxation rate QsTd of superconducting currents have been performed on a carefully selected set of dirty and clean high-Tc superconductors for temperatures T down to 100 mK and magnetic fields up to 7 T. The extrapolated relaxation rates Qs0d for the dirty compounds indicate that the viscosity experienced by a tunneling vortex segment is grossly underestimated by the standard Bardeen-Stephen theory. For the clean compounds a universal value Qs0d > 0.022 is found at 1 T, implying that the number of superconducting charge carriers involved in the tunneling of a vortex segment is >14.