Omnidirectional Measurements of Angle-Resolved Heat Capacity for Complete Detection of Superconducting Gap Structure in the Heavy-Fermion Antiferromagnet UPd2Al3

Quasiparticle excitations in UPd2Al3 were studied by means of heat-capacity (C) measurements under rotating magnetic fields using a high-quality single crystal. The field dependence shows C(H)[infinity]H1/2-like behavior at low temperatures for both two hexagonal crystal axes, i.e., H//[0001] (c axis) and H//[1120] (a axis), suggesting the presence of nodal quasiparticle excitations from heavy bands. At low temperatures, the polar-angle ([theta]) dependence of C exhibits a maximum along H//[0001] with a twofold symmetric oscillation below 0.5 T, and an unusual shoulder or hump anomaly has been found around 30[degrees]-60[degrees] from the c axis in C([theta]) at intermediate fields (1[< or =][mu]0H[< or =]2T). 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 here as a novel spectroscopic method for nodal gap determination, which can be applied to other exotic superconductors.