Critical currents and pinning mechanisms in untwinned a-axis YBa2Cu3O7-x thin films

We have measured the angular dependence of the critical current density (Jc) on magnetic field and temperature in untwinned a-axis-oriented YBa2Cu3O7-x films. The direction of magnetic field with respect to the crystallographic axes was found to be the dominant factor determining the behavior of Jc. Jc(θ) measurements, for currents driven along the b direction, yielded a peak at 0 degree corresponding to the magnetic field applied parallel to the CuO2 planes. For supercurrents flowing along the c direction, a crossover from grain-boundary pinning to surface and interface pinning was observed as temperatures approached Tc. Lawrence-Doniach and Tinkham Hc2(θ) models were used to fit our Jc(θ) data by simply replacing Hc2(θ) with Jc(θ). These models fit our Jc(θ) data very well, at high and medium-to-low temperature ranges, respectively. copyright 1997 The American Physical Society