Evidence for chiral d-wave superconductivity in URu₂Si₂ from the field-angle variation of its specific heat

Low-energy quasiparticle (QP) excitations in the heavy-fermion superconductor URu₂Si₂ were investigated by specific-heat C(T,H,φ,θ) measurements of a high-quality single crystal. The occurrence of QP excitations due to the Doppler-shift effect was detected regardless of the field direction in C(H) of the present clean sample, which is in sharp contrast to a previous report. Furthermore, the polar-angle-dependent C(θ) measured under a rotating magnetic field within the ac plane exhibits a shoulder-like anomaly at θ ∼ 45° and a sharp dip at θ = 90° (H||a) in the moderate-field region. These features are supported by theoretical analyses based on microscopic calculations assuming the gap symmetry of k_z(k_x + ik_y), whose gap structure is characterized by a combination of a horizontal line node at the equator and point nodes at the poles. The present results have settled the previous controversy over the gap structure of URu₂Si₂ and have authenticated its chiral d-wave superconductivity.