Spontaneous thermal Hall effect in three-dimensional chiral superconductors with gap nodes

TL;DR

This paper investigates the spontaneous thermal Hall effect in three-dimensional chiral superconductors with nodal gaps, revealing how it depends on gap structure and temperature, and providing both analytical and numerical insights.

Contribution

It offers the first detailed analysis of the spontaneous thermal Hall effect in nodal chiral superconductors with specific crystal symmetries, linking it to gap structure and surface states.

Findings

Thermal Hall conductivity reflects gap structure and surface Andreev states.

Temperature dependence shows power-law behavior based on gap nodes.

Analytical models and numerical simulations elucidate the effect's origin.

Abstract

Generic chiral superconductors with three-dimensional electronic structure have nodal gaps and are not strictly topological. Nevertheless, they exhibit a spontaneous thermal Hall effect (THE), i.e. a transverse temperature gradient in response to a heat current even in the absence of an external magnetic field. While in some cases this THE can be quantized analogous to the Quantum Hall effect, this is not the case for nodal superconductors in general. In this study we determine the spontaneous THE for tight binding models with tetragonal and hexagonal crystal symmetry with chiral $p$- and d-wave superconducting phase. At the zero-temperature limit, the thermal Hall conductivity $ \kappa_{xy} $ provides information on the structure of the gap function on the Fermi surface and the Andreev bound states on the surface. The temperature dependence at very low temperatures is determined by the types of gap nodes, point or line nodes, leading to characteristic power law behaviors in the temperature, as known for other quantities such as specific heat or London penetration depth. The generic behavior is discussed on simple models analytically, while the analysis of the tight-binding models is given numerically.