Gap symmetry of superconductivity in U Pd 2 Al 3

The angle-dependent thermal conductivity of the heavy-fermion superconductor $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ in the vortex state was recently measured by Watanabe et al. Here we analyze this data from two perspectives: universal heat conduction and the angle dependence. We conclude that the superconducting gap function $\ensuremath{\Delta}(\mathbf{k})$ in $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ has horizontal nodes and is given by $\ensuremath{\Delta}(\mathbf{k})=\ensuremath{\Delta}\phantom{\rule{0.2em}{0ex}}\mathrm{cos}(2\ensuremath{\chi})$, with $\ensuremath{\chi}=c{k}_{z}$.