Crossover from thermally activated flux creep to quantum tunneling of vortices in a YBa2Cu3O7-x film

We present the results of a study of flux creep in a ring-shaped epitaxial superconducting YBa2Cu3O7-x film at low temperatures. Measurements between 2 and 20 K have been made and it is confirmed that the flux creep is a ther mally activated process at temperatures exceeding 10 K. The low-temperature data are analyzed by assuming a crossover to quantum tunneling of the vort ex lines. Using the fact that the critical current in our sample is almost independent of temperature below 20 K, we establish the temperature depende nce of the Euclidian action S directly from the experimental data without a ny a priori assumptions. Our results imply that at temperatures below 8.5 K S(T) = S(0)(1 - T-2/T-qc(2)), With approximately the same value of T-qc ap proximate to 15 K for the case of the remnant magnetization as well as in a n external magnetic field of 1 kOe. (C) 2000 Elsevier Science B.V. All righ ts reserved.