Unusual Angular and Temperature Dependence of the Upper Critical Field in UPt3

Abstract

The report measurements of the upper critical field, $H_{c2\mathrm{}}$, inclined at various angles with respect to the $c$ axis in the heavy-fermion superconductor U${\mathrm{Pt}}_3$. The angular anisotropy observed near $T_c=0.53\mathrm{}$ K cannot be explained quantitatively by presently available theoretical expressions which consider either isotropic or anisotropic pairing. In addition, we find that the anisotropy apparently disappears at $T\sim 200$ mK, only to reemerge at lower temperatures with an opposite sense. We have also studied $H_{c2\mathrm{}}$ in the basal plane of this hexagonal crystal and find no angular dependence within the limits of our measurements.