Observation of quantum creep in oxygen deficient YBa 2Cu 3O 7−δ at fields up to 7 T

Abstract

Superconducting current densities j s and dynamic relaxation rates Q ≡ d ln j s/d ln (dB e/dt) in an oxygen deficient YBa 2Cu 3O 7−δ epitaxial film were measured as a function of temperature (1.5K < T < T irr ) in magnetic fields up to 7 T by means of high sensitivity capacitance torquemeters. Below 5 K, the relaxation rate remains high and does not extrapolate to Q = 0 at T = 0 K. This is associated to the quantum tunneling of vortices through the energy barrier separating two adjacent vortex configurations. In the case of ohmic dissipation the quantum creep rate is proportional to the normal state resistivity ϱ n (0) at T = 0 K. In order to check this theoretical prediction we measured Q as a function of the oxygen content of the sample. The quantum creep rate increases systematically with increasing ϱ n (0). However, Q remains finite even when ϱ n = 0.