Macroscopic magnetic properties of high temperature superconductors

The magnetic properties of YBaCuO crystals are strongly affected by thermally activated flux creep over pinning barriers. In this paper the effect of a power-law distribution of pinning barriers is explored. The critical current density is predicted to have a power-law dependence on the reduced temperature difference approaching the transition temperature Tc and a logarithmic dependence on the timescale of the measurement. The dependence of the critical current on the measurement time provides a possible explanation of the disparity between the critical current measured in thin film transport experiments and Jc obtained from magnetic data. Also, in contrast to some earlier theory, the normalized logarithmic time derivative of the remenent magnetization is shown to not diverge near Tc, irrespective of the pinning barrier distribution