Nonequilibrium Green's function method for phonon-phonon interactions and ballistic-diffusive thermal transport

Phonon-phonon interactions are systematically studied by nonequilibrium Green's function (NEGF) formalism in momentum space at finite temperatures. Within the quasiparticle approximation, phonon frequency shift and lifetime are obtained from the retarded self-energy. The lowest-order NEGF provides the same phonon lifetime as Fermi's golden rule. Thermal conductance is predicted by the Landauer formula with a phenomenological transmission function. The main advantage of our method is that it covers both ballistic and diffusive limits, and thermal conductance of different system sizes can be easily obtained once the mode-dependent phonon mean-free path is calculated by NEGF. As an illustration, the method is applied to two one-dimensional atom chain models [the Fermi-Pasta-Ulam (FPU)-beta model and the phi(4) model] with an additional harmonic on-site potential. The obtained thermal conductance is compared with that from a quasiclassical molecular-dynamics method. The harmonic on-site potential is shown to remove the divergence of thermal conductivity in the FPU-beta model.