Examining the possibility of chiral superconductivity in Sr$_2$RuO$_4$ and other compounds via applied supercurrent

TL;DR

This paper proposes using in-plane supercurrent as a novel method to detect chiral superconductivity in Sr$_2$RuO$_4$, analyzing phase diagrams, specific heat anomalies, and edge state responses to distinguish chiral states from other pairings.

Contribution

It introduces an alternative symmetry-breaking perturbation—supercurrent—to probe chiral superconductivity and predicts observable signatures in thermodynamic and tunneling measurements.

Findings

Supercurrent splits the transition of chiral order parameter components.

Visible specific heat anomalies can arise from supercurrent-induced splitting.

Supercurrent direction affects edge states and tunneling spectra differently.

Abstract

One approach to probe the still controversial superconductivity in Sr$_2$RuO$_4$ is to apply external perturbations that break the underlying tetragonal crystalline symmetry. Chiral $p_x+ip_y$ and $d_{xz}+id_{yz}$ states respond to such perturbations in ways that may help to distinguish them from other superconducting pairings. However, past experimental efforts along this line, using uniaxial strains and magnetic fields parallel to the RuO$_2$ plane, have not been able to reach an unambiguous conclusion. In this study, we propose to further examine the possibility of chiral superconducting order in Sr$_2$RuO$_4$ using an alternative tetragonal-symmetry-breaking perturbation -- in-plane supercurrent. We study the superconducting phase diagram as a function of both temperature and the applied supercurrent. Supercurrent generically splits the transition of the two chiral order parameter components, and we show that the splitting can give rise to visible specific heat anomalies. Furthermore, supercurrent parallel and anti-parallel to the unidirectional propagation of the chiral edge modes impact the edge states in different manner. This difference manifests in the tunneling spectrum, thereby providing an additional means to probe the chirality even when the related spontaneous edge current is vanishingly small. Finally, we discuss the distinction of supercurrent responses in non-chiral time-reversal-symmetry-breaking superconducting states. Our proposal can be applied to other candidate chiral superconductors.