Evidence for vortex pinning by surface irregularities in untwinned YBaCuO crystals

A direct method of induction in a small pick up coil shows that in untwinne d YBaCuO crystals, the subcritical current flows close to the surfaces of t he sample. By studying the frequency dependence of the ac penetration depth , we have also shown that the two-modes through Mathieu-Simon (MS) model of pinning by surface irregularities is able to account for the observed spec trum. The Campbell model of weak bulk pinning can be completely ruled out. Above the first order transition line, the critical current is zero and the ac response is that of an ohmic system. This can be interpreted by the cur vature of the vortices close to the surface. This shows the irrelevance of weak bulk pinning models for clean untwinned YBaCuO crystals that were ofte n proposed as model systems for this case. (C) 2000 Elsevier Science B.V. A ll rights reserved.