Anisotropy of Heat Conduction in the Heavy Fermion Superconductor UPt3

We report on the first measurement of the anisotropy of heat conduction in a heavy fermion superconductor, performed on a single crystal of U${\mathrm{Pt}}_3$ with a current parallel and perpendicular to the hexagonal axis. Beyond the temperature-independent anisotropy of the normal state, a clear additional anisotropy develops in the superconducting state. This direct measure of gap anisotropy places precise constraints on the possible states for the two zero-field phases. An axial gap is excluded for both, and a comparison with existing calculations favors a $d$-wave gap for the low-temperature phase.