Dependence of activation energy upon magnetic field and temperature in YBa 2 Cu 3 O 7 − δ epitaxial thin film

The broadening of the resistive transition in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ epitaxial thin film was measured in various applied magnetic fields. The irreversibility line and its angular dependence were obtained. The temperature and magnetic field dependence of the activation energy ${U}_{0}(H,T)\ensuremath{\propto}{U}_{0}[1\ensuremath{-}{T/T}_{c}(0)]{(H}_{0}/{\ensuremath{\varepsilon}}_{\ensuremath{\theta}}{H)}^{2/3}$ were obtained from the experimental results, which is consistent with the expression of the activation energy for thermally excited flux creep deduced from the irreversibility line and Arrhenius law except for low field data in which the prefactor of the Arrhenius law has little deviation from that in high fields.