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

TL;DR

This study provides strong evidence for chiral d-wave superconductivity in URu2Si2 by analyzing specific heat variations with magnetic field angle, confirming a complex gap structure with line and point nodes.

Contribution

The paper demonstrates the presence of chiral d-wave superconductivity in URu2Si2 through detailed specific heat measurements and theoretical analysis, resolving previous controversies.

Findings

Detection of quasiparticle excitations via Doppler-shift effect.

Observation of a shoulder-like anomaly at 45° and a sharp dip at 90° in specific heat.

Confirmation of a gap structure with horizontal line and point nodes.

Abstract

Low-energy quasiparticle (QP) excitations in the heavy-fermion superconductor URu$_2$Si$_2$ were investigated by specific-heat $C(T, H, \phi, \theta)$ 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 $C(\theta)$ measured under a rotating magnetic field within the ac plane exhibits a shoulder-like anomaly at $\theta \sim 45$ deg and a sharp dip at $\theta = 90$ deg ($H \parallel a$) 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 URu$_2$Si$_2$ and have authenticated its chiral $d$-wave superconductivity.