Determination of the gap structure inUPt3by thermal conductivity

The thermal conductivity κ of the heavy-fermion superconductor ${\mathrm{UPt}}_3$ was measured down to ${\mathit{T}}_{\mathit{c}}$/10, along the b and c axis of a single crystal. The anisotropy ratio ${\mathrm{\kappa }}_{\mathit{c}}$/${\mathrm{\kappa }}_{\mathit{b}}$ as T→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 B phase: the presence of nodes along the c axis. Furthermore, recent calculations by Fledderjohann and Hirschfeld strongly suggest these cannot be point nodes with a linear k dependence. © 1996 The American Physical Society.