HELICAL CURRENT FLOW ALONG 90-DEGREES TWIST BOUNDARIES IN YBCO THIN-FILMS

Current may flow in helical paths along the boundary between two tilte d anisotropic conductors in circumstances when current flow in either conductor alone is restrained by the extreme anisotropy. We are able t o explain number of features of the electrical properties of (103)/(01 3) oriented YBa2Cu3O7 thin films deposited onto (110) SrTiO3 substrate s in terms of helical current paths involving the 90 degrees twist bou ndaries. These include the absence of weak link behaviour in the super conducting state for current flow along the substrate (1(1) over bar 0 ) direction, the temperature dependence of the normal state resistivit y for current flow in that direction, and the normal state electrical anisotropy. We predict that there are circumstances where currents wil l preferentially circulate around the boundaries of a tilt domain. The existence of these natural conduction loops may explain the period of recently observed quantum-interference behaviour in these films.