Direct observation of intrinsic pinning in YBCO thin films

The force-displacement response of vortices pinned in epitaxial YBCO thin films has been determined using an ac transport technique. The effective curvature of the pinning wells and the elastic limit of the displacement of the vortices before depinning are obtained. For magnetic fields parallel to the c axis the elastic limit is about a coherence length, consistent with core pinning. When the field is applied parallel to the ab planes structure appears in the force-displacement curve before the onset of flux flow. These ``steps'' appear at vortex displacements which are closely related to the spacing between the superconducting layers, providing direct evidence for intrinsic pinning.