General Features of Quantum Creep in High-TcSuperconductors

Measurements of the relaxation rate $Q\left(T\right)$ of superconducting currents have been performed on a carefully selected set of dirty and clean high- $T_c$ superconductors for temperatures $T$ down to 100 mK and magnetic fields up to 7 T. The extrapolated relaxation rates $Q\left(0\right)$ 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 $Q\left(0\right)\cong 0.022$ is found at 1 T, implying that the number of superconducting charge carriers involved in the tunneling of a vortex segment is $\cong 14$.