The surface impedance $Z(f)$ of conventional isotropic materials has been carefully measured for frequencies f ranging from 1 kHz to 3 MHz, allowing a detailed investigation of the depinning transition. Our results exhibit the irrelevance of classical ideas to the dynamics of vortex pinning. We propose a new picture, where the linear ac response is entirely governed by disordered boundary conditions of a rough surface, whereas in the bulk vortices respond freely. The universal law for $Z(f)$ thus predicted is in remarkable agreement with experiment, and tentatively applies to microwave data in YBaCuO films.
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Lütke-Entrup et al. (1997) studied this question.
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