DETERMINATION OF THE GAP STRUCTURE IN UPT3 BY THERMAL-CONDUCTIVITY

The thermal conductivity kappa of the heavy-fermion superconductor UPt 3 was measured down to T-c/10, along the b and c axis of a single crys tal. The anisotropy ratio kappa(c)/kappa(b) as T-->0 is shown to be a powerful new probe of the gap structure in this compound. Our observat ion of a nonvanishing value for this ratio establishes a property of t he gap in the B phase: the presence of nodes along the c axis. Further more, recent calculations by Fleddejohann and Hirschfeld strongly sugg est these cannot be point nodes with a linear k dependence.