CRITICAL CURRENTS APPROACHING THE DEPAIRING LIMIT AT A TWIN BOUNDARY IN YBA2CU3O7-DELTA

Interest in vortex matter has risen considerably since the discovery o f the high-temperature superconductors, which exhibit magnetic vortex states that are especially rich and complex(1). The global behaviour o f magnetic vortices in nearly perfect crystals-such as melting of the vortex lattice(2-5)-has been much studied, but of more technological r elevance is the influence on the vortex states of the various structur al defects present in most practical superconductors. An important exa mple of such a defect is the twin boundary present in twinned orthorho mbic crystals of YBa2Cu3O7-delta (YBCO). Studies of such samples using magnetic-field-sensitive probes(6-9) have suggested that the twin bou ndary plays an important role in pinning the vortices and so enhancing the currents that YBCO can support while remaining superconducting. B ut the low spatial resolution of these techniques does not permit thes e effects to be studied at the scale of the vortices or boundaries the mselves. Scanning tunnelling spectroscopy offers a means of circumvent ing these problems of resolution(10-12), as it directly probes the sup erconducting order parameter at nanometre length scales. Here we use t his technique to investigate the importance of twin boundaries in YBCO . In particular we observe an unexpectedly large pinning strength for perpendicular vortex flux across the boundary, which implies that the critical current that can be supported along the boundary approaches t he theoretical 'depairing' limit.