CRITICAL CURRENTS AND PINNING MECHANISMS IN UNTWINNED A-AXIS YBA2CU3O7-X THIN-FILMS

We have measured the angular dependence of the critical current densit y (J(c)) on magnetic field and temperature in untwinned a-axis-oriente d YBa2Cu3O7-x films. The direction of magnetic field with respect to t he crystallographic axes was found to be the dominant factor determini ng the behavior of J(c). J(c)(theta) measurements, for currents driven along the b direction, yielded a peak at 0 degrees corresponding to t he magnetic field applied parallel to the CuO2 planes. For supercurren ts flowing along the c direction, a crossover from grain-boundary pinn ing to surface and interface pinning was observed as temperatures appr oached T-c. Lawrence-Doniach and Tinkham H-c2(theta) models were used to fit our J(c)(theta) data by simply replacing H-c2(theta) with J(c)( theta). These models fit our J(c)(theta) data very well, at high and m edium-to-low temperature ranges, respectively.