A heat pump at a molecular scale controlled by a mechanical force

We show that a mesoscopic system such as Feynman's ratchet may operate as a heat pump, and clarify underlying physical picture. We consider a system of a particle moving along an asymmetric periodic structure. When put into contact with two distinct heat baths of equal temperature, the system transfers heat between two baths as the particle is dragged. We examine Onsager relation for the heat flow and the particle flow, and show that the reciprocity coefficient is a product of the characteristic heat and the diffusion constant of the particle. The characteristic heat is the heat transfer between the baths associated with a barrier-overcoming process. Because of the correlation between the heat flow and the particle flow, the system can work as a heat pump when the particle is dragged. This pump is particularly effective at molecular scales where the energy barrier is of the order of the thermal energy.