The Transition Between Real and Complex Superconducting Order Parameter Phases in UPt3

Mind the Gap 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 UPt3 has long been known to exhibit unconventional pairing. Now, Strand et al. (p. 1368) have measured the momentum space dependence of UPt3'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 UPt3 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 UPt3 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.