Generalized first-principles method to study near-field heat transfer mediated by Coulomb interaction

We present a first-principles approach to calculate the Coulomb-interaction-mediated heat current in the near field. With the nonequilibrium Green's functions method, the heat flux is calculated from a Landauer formula combining with a Caroli formula of transmission coefficient. The central physical quantities are the screened Coulomb potential and the spectrum function of polarizability. Within the random phase approximation, we calculate the polarizability using the linear response density functional theory and obtain the screened Coulomb potential from a retarded Dyson equation. We adopt single-layer graphene as an example to calculate the Coulomb heat flux between two parallel sheets with different gap sizes. Our results show that the heat flux saturates at the extreme near field with a convergent ratio of $5\times10^4$ to the black-body limit. With increasing distances, the heat flux asymptotically shows a 1/$d^2$ dependence. From the energy current spectrum, we infer that the near-field enhancement of Coulomb heat flux stem from electron transitions around the Fermi level. Strain effects on the heat flux are also discussed at different distances. We found that the heat flux is positively correlated to the strain for most of the distances while a negative correlation is shown at the near-to-medium field. Our method is general and can be easily applied to various kinds of materials which provides a benchmarking reference for both theory and experiment.