Remanent flux creep in YBa 2 Cu 3 O 7 − δ films in the critical region

The remanent magnetic flux creep in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\mathrm{\ensuremath{\delta}}\mathrm{}}$ thin films was investigated in the vicinity of the critical temperature ${T}_{c}.$ The trapped magnetic flux was generated in the specimen by ramping the applied field to a finite value and then back to zero. The remanent magnetic field of persistent currents circulating in the sample was measured as a function of temperature and time. Data on the critical current density ${j}_{c}$ and the creep activation energy U were extracted. A sharp decrease of U at temperatures ${T}_{c}\ensuremath{-}T\ensuremath{\lesssim}1 \mathrm{K}$ was related to critical fluctuations. In this critical region a power-law time decay of the currents $j\ensuremath{\propto}{t}^{\ensuremath{-}p}$ was observed and explained by the logarithmic dependence ${U=U}_{0}\mathrm{ln}{(j}_{c}/j)$ for the current-assisted thermal unbinding of vortex-antivortex pairs generated by the critical fluctuations. In the critical region, the temperature dependence ${j}_{c}\ensuremath{\propto}{(T}_{c}\ensuremath{-}{T)}^{2}$ was observed. A possibility to relate this result to the critical behavior of the three-dimensional $\mathrm{XY}$ model is discussed.