Evidence for vortex surface pinning in YBa2Cu3O7-delta from the frequency dependence of the complex penetration depth - art. no. 054503

We have measured the high-frequency complex skin depth in the mixed state o f a large untwinned YBaCuO crystal. Our frequency range (0.1-3 MHz) covers the pinning crossover in the vicinity of the first-order transition (T = 85 -87 K). The spectrum deviates from that expected from bulk-pinning theories but closely follows that predicted by a surface pinning model which is bas ed on two-mode electrodynamics, free bulk vortex flow and surface slippage conditions. Above the first-order line, critical currents vanish and the id eal pinning-free response is recovered. This experiment points out the irre levance of pinning by point defects in clean YBaCuO, and adds a piece to th e physics of the first-order transition in high-T-c materials.