Nodal gap structure of the heavy-fermion superconductor URu₂Si₂ revealed by field-angle-dependent specific-heat measurements

URu₂Si₂ is a promising candidate for a chiral d-wave superconductor whose gap is composed of a horizontal line node at equator and point nodes at the north and south poles. However, previous measurements of the specific heat and the thermal conductivity did not detect quasiparticle excitations from a horizontal line node in heavy-mass bands. Here, we have provided strong evidence for the presence of a horizontal line node from field-angle-dependent specific-heat measurements by using a high-quality single crystal of URu₂Si₂. We observed the behavior of the specific heat at low fields which does not depend on inplane field angle φ but shows a shoulder-like anomaly in the polar-angle θ dependence around θ ∼ 45°. This feature is more clearly seen at lower temperatures, suggesting the detection of low-energy quasiparticle excitations reflecting the gap structure. From theoretical analyses based on microscopic calculations, we have demonstrated that this anomaly can be explained by the existence of a horizontal line node at k_z = 0. Thus, the gap structure of URu₂Si₂ matches well with the E_g chiral k_z(k_x + ik_y) symmetry.