Identification of odd-parity superconductivity in UPt₃ from paramagnetic effects on the upper critical field

We present a theoretical explanation of the unusual temperature-dependent anisotropy of Hc2 reported by Shivaram et al. for single crystals of UPt₃, which we argue provides strong support for unconventional odd-parity pairing. We show that the observed low-temperature crossover in the anisotropy ratio, Hc2^⊥/Hc2?, can be explained by paramagnetic limiting for H parallel to the sixfold axis within BCS pairing theory if the order parameter has odd parity and there is strong spin-orbit coupling of the pair spin to the crystal axes.