Determination of the gap structure in UPt3 by thermal conductivity.

The thermal conductivity {kappa} of the heavy-fermion superconductor UPt{sub 3} was measured down to {ital T}{sub {ital c}}/10, along the {ital b} and {ital c} axis of a single crystal. The anisotropy ratio {kappa}{sub {ital c}}/{kappa}{sub {ital b}} as {ital T}{r_arrow}0 is shown to be a powerful new probe of the gap structure in this compound. Our observation of a nonvanishing value for this ratio establishes a property of the gap in the {ital B} phase: the presence of nodes along the {ital c} axis. Furthermore, recent calculations by Fledderjohann and Hirschfeld strongly suggest these cannot be point nodes with a linear {ital k} dependence. {copyright} {ital 1996 The American Physical Society.}