VORTEX MOVEMENTS PARALLEL AND PERPENDICULAR TO TWIN BOUNDARIES SEEN THROUGH THE NERNST AND SEEBECK EFFECTS OF YBA2CU3O7-DELTA

Twin boundaries are the cause of important intrinsic pinning effects i n the mixed state of high temperature superconductors. However, adjace nt twin planes form channels which can guide the vortices with a never theless restricted motion. These phenomena have been studied in detail with a specially prepared sample of YBa2Cu3O7-delta (YBCO). The latte r is a heavily twinned single crystal containing a single domain i.e. the twin boundaries are parallel throughout the sample. Thus with a te mperature gradient applied parallel and then perpendicular to the twin boundaries, one can observe vortex motion in these two directions by measuring the Seebeck and Nernst effects as a function of temperature for both magnetic field polarities. Field inversion is necessary for t he separation of the odd and even components of the measured signals t o exploit a hitherto unexplored property of the Seebeck and Nernst eff ects.