Flux creep in the critical state of a high-temperature superconductor

We have measured the field and temperature dependences of the logarithmic flux-creep relaxation in an oriented-grain sample of YBaCuO in the critical state. The data indicate that the effective activation energy ${\mathit{U}}_0$ increases continuously with increasing T between 6 and 30 K, and that ${\mathit{U}}_0$(T) tends to increase rather than decrease with increasing applied field. These results are compared to predictions of the thermally activated flux-creep model.