Observation of the transition between real and complex superconducting order parameter phases in UPt$_3$

The heavy fermion superconductor UPt$_3$ provides a rich system for studying the competition between superconductivity and other forms of electronic order because it has an unconventional pairing mechanism, coexistence of anti-ferromagnetism and superconductivity, and two distinct superconducting phases characterized by different order parameter symmetries. We have fabricated Josephson tunnel junctions on the as-grown surfaces of UPt$_3$ single crystals at a series of angles in the basal plane. By measuring their critical current, we map out the magnitude of the superconducting order parameter as a function of k-space direction and temperature. We observe a sharp node in the superconducting gap at 45$^{\circ}$ with respect to the a-axis in the high temperature phase and the onset of an out-of-phase component creating a complex order parameter in the low temperature phase.