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

TL;DR

This paper develops a theoretical framework for understanding thermal transport in anharmonic molecular systems, revealing how non-linearity and potential barriers influence conductance, including phenomena like negative differential thermal conductance.

Contribution

It introduces a general quantum master equation approach for steady and transient thermal transport in anharmonic systems, with specific insights into molecular junctions and double-well potentials.

Findings

Low-temperature conductance is sensitive to non-linearity in on-site potentials.

Anharmonic springs in diatomic molecules have minimal impact on low-temperature conductance.

Barrier height in double-well potentials significantly affects thermal conductance and can induce negative differential conductance.

Abstract

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 non-linear on-site potential the low-temperature thermal conductance is dramatically affected by the strength of non-linearity, 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.