The Angular Dependence of the Critical Current in UPt3 and the Symmetry of the Order Parameter

The superconducting critical current along the c-axis of the unconventional superconductor UPt3 has been determined as a function of magnetic field direction in the basal (a-a*) plane in the three (A, B and C) phases of the mixed state. The critical current has maxima for field directions parallel to both the a- and a*-axis in the superconducting C phase, but has maxima only parallel to the a-axis in the A and B phases. Our experiments thus show that the anisotropy of the superconducting critical current in UPt3 changes at the same field at which the symmetry of the order parameter changes. This demonstrates for the first time that the critical current is sensitive to the symmetry of the superconducting order parameter.