Transition from columnar to point pinning in coated conductors: critical currents that are independent of magnetic field direction

We identify a sharp crossover in the vortex pinning of a high-temperature superconductor with nanocolumnar stacks of precipitates as strong vortex pinning centers. Above a particular, temperature-dependent field BX(T) the vortex response is no longer determined by the nanocolumns, and is instead determined by point-like pinning. This crossover is evident as a change in the dependence of the critical current density on the angle between the applied magnetic field and the nanocolumns. It also leads to the field-orientation-independent power law index n of the E–J curves. Below the transition, there is a strong maximum in JC when the field is aligned parallel to the columns and n depends on field direction. Above the transition, n is independent of the field direction and there is a JC minimum for H parallel to the columns. We discuss a possible mechanism for such behavior change, as well as testing and confirming a prediction that the crossover must become very broad at high temperatures and low fields. (paper)