DIRECT OBSERVATION OF INTRINSIC PINNING IN YBCO THIN-FILMS

The force-displacement response of vortices pinned in epitaxial YBCO t hin films has been determined using an ac transport technique. The eff ective curvature of the pinning wells and the elastic limit of the dis placement of the vortices before depinning are obtained. For magnetic fields parallel to the c axis the elastic limit is about a coherence l ength, consistent with core pinning. When the field is applied paralle l to the ab planes structure appears in the force-displacement curve b efore the onset of flux flow. These ''steps'' appear at vortex displac ements which are closely related to the spacing between the supercondu cting layers, providing direct evidence for intrinsic pinning.