Superconducting gap function in antiferromagnetic heavy-fermion UPd2Al3 probed by angle-resolved magnetothermal transport measurements

Abstract

The superconducting gap structure of heavy fermion $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$, in which unconventional superconductivity coexists with antiferromagnetic (AF) order with atomic size local moments, was investigated by the thermal conductivity measurements in a magnetic field $\mathbit{H}$ rotating in various directions relative to the crystal axes. The thermal conductivity displays distinct twofold oscillation when $\mathbit{H}$ is rotated in the plane orthogonal to the basal ab plane, while no oscillation was observed when $\mathbit{H}$ is rotated within the basal plane. These results provide strong evidence that the gap function $\mathrm{\ensuremath{\Delta}}(\mathbit{k})$ has a single line node orthogonal to the $c$ axis located at the AF Brillouin zone boundary, while $\mathrm{\ensuremath{\Delta}}(\mathbit{k})$ is isotropic within the basal plane. This gap structure indicates that the pairing interaction in neighboring planes strongly dominates over the interaction in the same plane. The determined nodal structure is compatible with the resonance peak in the dynamical susceptibility observed in neutron inelastic scattering experiments. Based on these results, we conclude that the superconducting pairing function of $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ is most likely to be $d$-wave with a form $\mathrm{\ensuremath{\Delta}}(\mathbit{k})={\mathrm{\ensuremath{\Delta}}}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{cos}({k}_{z}c)$.