Quasiparticle excitations in UPd2Al3 were investigated by means of heat-capacity (C) measurements under rotating magnetic fields using a high-quality single crystal. The field dependence shows C(H)∝H1/2-like behavior at low temperatures for both two hexagonal crystal axes, i.e., H∥ (c axis) and H∥[11¯20] (a axis), indicating the presence of nodal quasiparticle excitations from heavy bands. At low temperatures, the polar-angle (θ) dependence of C exhibits a maximum along H∥ with a twofold symmetric oscillation below 0.5 T, and an unusual shoulder or hump anomaly has been found around 30°–60° from the c axis in C(θ) at fields of 1≲μ0H≲2 T. These behaviors in UPd2Al3 purely come from the superconducting nodal quasiparticle excitations, and can be successfully reproduced by theoretical calculations assuming the gap symmetry with a horizontal linear line node. We demonstrate the whole angle-resolved heat-capacity measurements done in the present work as a novel spectroscopic method for nodal gap determination, which can be applied to other exotic superconductors. The present paper has been selected as an Editor’s suggestion of Physical review letters.
PHYSICAL REVIEW LETTERS Vol. 117, 037001 (2016).