Evidence for vortex surface pinning in YBa2Cu3O7-δ from the frequency dependence of the complex penetration depth

We have measured the high-frequency complex skin depth in the mixed state of 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 based on two-mode electrodynamics, free bulk vortex flow and surface slippage conditions. Above the first-order line, critical currents vanish and the ideal pinning-free response is recovered. This experiment points out the irrelevance of pinning by point defects in clean YBaCuO, and adds a piece to the physics of the first-order transition in high-T_c materials.