Steady-state thermal transport in anharmonic systems: Application to molecular junctions

We develop a general theory for thermal transport in anharmonic systems under the weak system-bath coupling approximation similar to the quantum master equation formalism. A current operator is derived, which is valid not only in the steady state, but in the transient regime as well. Here, we focus on the effects of anharmonicity on the steady-state thermal conductance of a mono and diatomic molecular junctions. We also study molecules being confined in a double-well potential. We find that when the molecules have a nonlinear on-site potential, the low-temperature thermal conductance is dramatically affected by the strength of nonlinearity, whereas for the diatomic molecule connected by an anharmonic spring the strength of anharmonicity plays almost no role in the low-temperature regime. In case of the molecules confined in a double-well potential, we find that the height of the barrier greatly affects the thermal conductance; once the molecules can feel the effect of the barrier, we observe negative differential thermal conductance at both high and low temperatures.