Unconventional Superconducting Class in a Heavy Fermion System UPt₃

Abstract

The origin of the double transition phenomenon recently found in a typical heavy Fermion material UPt 3 is investigated. It is shown group-theoretically solely on the basis of the known symmetries that there exists a non-trivial coupling term between the preexisting spin density wave order parameter and the superconducting pairing function belonging to a degenerate representation with internal degree of freedom. This coupling ultimately leads to the T c splitting. Superconducting properties associated with this unconventional pairing function, including the phase diagram, are discussed both in the absence and presence of an applied field. Several experiments are proposed to test the present theory, in particular, the field dependence of the double transition is crucial to identify the symmetry of the pairing function. The existing data are most consistent with those in the two-dimensional representations E 1 g or E 1 u of the hexagonal symmetry. The possibility of similar phenomenon in other heavy Fermion systems is also examined.