Temperature and magnetic-field dependence of quantum creep in various high-Tc superconductors

To investigate the quantum tunneling of almost macroscopic vortex segments we measured the normalized relaxation rate Q of superconducting currents for various high-T-c superconductors (HTS's) down to 100 mK in magnetic fields up to 7 T. At fields below congruent to 0.5 T, Q is essentially independent of T in the temperature regime between thermally activated and quantum motion as theoretically expected. However, at higher fields, we find an unexpected linear T dependence of Q persisting down to the lowest temperatures in all investigated samples. Since these compounds were chosen to represent the distinct classes of dirty and clean HTS's, the extrapolated Q(T = 0) values are used to discuss the theoretical models for quantum creep in the clean and dirty limits. Based on this discussion we propose a semiempirical interpolation formula for Q(T = 0) which is also valid for compounds in between the dirty and clean limits. [S0163-1829(99)00806-1].