Thermally activated flux creep in YBa2Cu3O7-x film

We have studied the flux creep in a ring-shaped epitaxial YBa2Cu3O7-x (YBCO ) film. It is shown that in the temperature range between 10 and 60 K the f lux creep can be described in terms of a thermally activated hopping of vor tices over potential barriers. Taking into account the temperature dependen ce of the critical current, all voltage-current (V-I) characteristics in th e above temperature range may be merged onto a single curve. This procedure provides the current dependence of the activation energy. The data obtaine d at T < 10 K are not compatible with the assumption of thermal activation and we interpret this as a crossover to the quantum flux-creep regime. (C) 2000 Elsevier Science B.V. All rights reserved.