Keldysh theory of thermal transport in multiband Hamiltonians

We establish a comprehensive theoretical framework for systems subjected to a static uniform temperature gradient, employing the nonequilibrium Keldysh-Dyson formalism. This framework interprets the statistical force due to the temperature gradient as a mechanical force, utilizing both Luttinger's scalar and Moreno-Coleman-Tatara's vector potentials, which collectively emulate the gauge invariance stemming from the conservation of energy. Our approach has the ability to treat heat current and heat magnetization on an equal footing, thereby extending and generalizing previous formalisms. The derived result for the thermal conductivity is applied to investigate the thermal characteristics of Weyl magnons in a stacked honeycomb ferromagnet featuring a trivial insulator phase, a magnon Chern insulator phase, and three Weyl magnon phases. Against the expectation from the Berry curvature, the magnon Chern insulator phase exhibits the highest transverse thermal conductivity.