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

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 symmetries with chiral p- and d-wave superconducting phases. At the zero-temperature limit, the thermal Hall conductivity κ_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 gaps, i.e., point or line nodes, leading to characteristic power law behaviors in the temperature, as known for other quantities such as specific heat and London penetration depth. The generic behavior is analytically discussed on the basis of simple models, while the analysis of the tight-binding models is given numerically. Both arguments focus on a single-band model to clarify the role of specific nodal structures. (author)