Frequency dependence of the ac susceptibility in a Y-Ba-Cu-O crystal: A reinterpretation of Hc2

We report a small roughly logarithmic increase with frequency of the apparent transition temperature in ac susceptibility experiments on a Y-Ba-Cu-O superconducting crystal. Magnetic field enhances the effect. A flux creep model predicts the correct order of magnitude and implies that the transitions previously used to determine the upper-critical field Hc2 actually represent an "irreversibility" line.