The Transition Between Real and Complex Superconducting Order Parameter Phases in UPt 3

The symmetry of the pairing gap is one of the most important properties of a superconductor. Whereas conventional superconductors such as lead have a gap that is uniform in momentum space, the enigmatic cuprates have point nodes and a characteristic d -wave symmetry. The heavy fermion compound UPt 3 has long been known to exhibit unconventional pairing. Now, Strand et al. (p. 1368 ) have measured the momentum space dependence of UPt 3 's superconducting gap as a function of temperature. A real gap with line nodes was observed to develop first; then, at a lower critical temperature, a complex component appeared, with the total gap expected to become fully uniform at absolute zero. The temperature evolution of two superconducting phases has been observed in a heavy fermion superconductor. Order parameter symmetry is one of the basic characteristics of a superconductor. The heavy fermion compound UPt 3 provides a rich system for studying the competition between superconductivity and other forms of electronic order and exhibits two distinct superconducting phases that are characterized by different symmetries. We fabricated a series of Josephson tunnel junctions on the as-grown surfaces of UPt 3 single crystals spanning the a - b plane. By measuring their critical current, we mapped out the magnitude of the superconducting order parameter as a function of the momentum-space direction and temperature. In the high-temperature phase, we observed a sharp node in the superconducting gap at 45° with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.