Supercurrent-induced Anomalous Thermal Hall Effect as a New Probe to Superconducting Gap Anisotropy

TL;DR

This paper proposes a novel method using supercurrent-induced thermal Hall effect to probe the in-plane anisotropy of the superconducting gap in twisted bilayer superconductors, especially in systems lacking external magnetic fields.

Contribution

It introduces a new, symmetry-breaking based approach to detect superconducting gap anisotropy via thermal Hall measurements without external magnetic fields.

Findings

Derived explicit formulas for thermal Hall conductivity.

Showed the effect is significant in twisted cuprates and FeSe.

Proposed the method as a generic probe for gap anisotropy.

Abstract

Two-dimensional superconductors have been realized in various atomically thin films such as the twisted bilayer graphene, some of which are anticipated to involve unconventional pairing mechanism. Due to their low dimensionality, experimental probes of the exact nature of superconductivity in these systems have been limited. We propose, by applying a \emph{vertical} supercurrent to a bilayer superconductor where the mirror symmetry is naturally broken by the twisting, there will be anomalous thermal Hall effect induced by the supercurrent that can serve as a sharp probe for the \emph{in-plane} anisotropy of the superconducting gap function. This effect occurs in the \emph{absence} of an external magnetic field and spontaneous breaking of the time-reversal symmetry in the ground state. We derive explicit formulas for the induced thermal Hall conductivity and show them to be significant in the examples of twisted cuprates and twisted FeSe where monolayer superconductivity have already been observed. Though technical challenges still exist, we propose this to be a generic probe of the gap anisotropy in a twisted bilayer superconductor.