Resistivity and critical current anisotropy of untwinned a-axis YBCO thin films

We have pulsed laser deposited untwinned a-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films on (100) LaSrGaO/sub 4/. From resistive measurements, the chain contribution to the total charge carrier density was estimated to be around 60%. Critical current density (J/sub c/) depends on the granular nature of the films and the transport across each grain boundary can be modeled as a collection of multiple parallel narrow conductive paths. Measurements in fields up to 6 T indicate that the behavior of J/sub c/ vs. B along the b- and c- directions is similar to that of standard c-axis films, with no significant weak link effects from the grain boundaries. We have also measured the temperature and magnetic field dependence of the critical current density (J/sub c/) along b- and c- directions of our films. The direction of magnetic field with respect to the crystallographic axes was found to be the dominant factor determining the J/sub c/. For supercurrents flowing along the c- direction, a cross-over from grain boundary pinning to surface and interface pinning was observed as temperatures approached T/sub c/.