Searching for gap zeros in Sr2RuO4 via field-angle-dependent specific-heat measurement

TL;DR

This study uses field-angle-dependent specific heat measurements at very low temperatures to investigate the gap structure of Sr2RuO4, revealing evidence for horizontal line nodes and challenging previous assumptions about its superconducting order parameter.

Contribution

It provides new experimental evidence for horizontal line nodes in Sr2RuO4, prompting a reevaluation of its superconducting gap symmetry and order parameter.

Findings

Fourfold specific-heat oscillation remains unchanged down to 0.06 K.

Data consistent with horizontal line nodes on the Fermi surface.

Horizontal line nodes and anisotropic Fermi velocity explain observed behaviors.

Abstract

The gap structure of Sr$_2$RuO$_4$, which is a longstanding candidate for a chiral p-wave superconductor, has been investigated from the perspective of the dependence of its specific heat on magnetic field angles at temperatures as low as 0.06 K ($\sim 0.04T_{\rm c}$). Except near $H_{\rm c2}$, its fourfold specific-heat oscillation under an in-plane rotating magnetic field is unlikely to change its sign down to the lowest temperature of 0.06 K. This feature is qualitatively different from nodal quasiparticle excitations of a quasi-two-dimensional superconductor possessing vertical lines of gap minima. The overall specific-heat behavior of Sr$_2$RuO$_4$ can be explained by Doppler-shifted quasiparticles around horizontal line nodes on the Fermi surface, whose in-plane Fermi velocity is highly anisotropic, along with the occurrence of the Pauli-paramagnetic effect. These findings, in particular, the presence of horizontal line nodes in the gap, call for a reconsideration of the order parameter of Sr$_2$RuO$_4$.