Molecular wires acting as quantum heat ratchets

TL;DR

This paper investigates how molecular junctions can produce directed heat and electric currents through time-periodic temperature modulations, demonstrating a heat ratchet effect that can be controlled and potentially utilized in nanoscale thermal management.

Contribution

It introduces a novel mechanism for heat transfer in molecular junctions driven by unbiased, time-periodic temperature modulations, revealing controllable heat and electric currents without net thermal bias.

Findings

Sizeable directed heat currents can be generated and detected.

The ratchet effect can produce heat flow against thermal bias.

The phase control enables steering of heat flow direction.

Abstract

We explore heat transfer in molecular junctions between two leads in the absence of a finite net thermal bias. The application of an unbiased, time-periodic temperature modulation of the leads entails a dynamical breaking of reflection symmetry, such that a directed heat current may emerge (ratchet effect). In particular, we consider two cases of adiabatically slow driving, namely (i) periodic temperature modulation of only one lead and (ii) temperature modulation of both leads with an ac driving that contains a second harmonic, thus generating harmonic mixing. Both scenarios yield sizeable directed heat currents which should be detectable with present techniques. Adding a static thermal bias, allows one to compute the heat current-thermal load characteristics which includes the ratchet effect of negative thermal bias with positive-valued heat flow against the thermal bias, up to the thermal stop-load. The ratchet heat flow in turn generates also an electric current. An applied electric stop-voltage, yielding effective zero electric current flow, then mimics a solely heat-ratchet-induced thermopower (``ratchet Seebeck effect''), although no net thermal bias is acting. Moreover, we find that the relative phase between the two harmonics in scenario (ii) enables steering the net heat current into a direction of choice.