Identification of odd-parity superconductivity in UPt_{3} from paramagnetic effects on the upper critical field

Abstract

We present a theoretical explanation of the unusual temperature-dependent anisotropy of ${\mathit{H}}_{\mathit{c}2}$ reported by Shivaram et al. for single crystals of ${\mathrm{UPt}}_{3}$, which we argue provides strong support for unconventional odd-parity pairing. We show that the observed low-temperature crossover in the anisotropy ratio, ${\mathit{H}}_{\mathit{c}2}^{\mathrm{\ensuremath{\perp}}}$/${\mathit{H}}_{\mathit{c}2}^{\mathrm{?}}$, 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.