A heat pump at a molecular scale controlled by a mechan - ical force

– We show that a mesoscopic system such as Feynman’s ratchet may operate as a heat pump, and clarify a underlying physical picture. We consider a system of a particle moving along an asymmetric periodic structure. When put into a 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. Introduction. – In the last decade, physics and technology have moved more and more toward the manipulation of mesoscopic objects. Techniques of micro manipulation of molecules are expanding to open new possibilities to future technology. Although to control heat flow in mesoscopic scales may be among crucial issues, its study is still in a preliminary stage [1]. Heat pumps working in nano-scale devices [2, 3] or mesoscopic machines [4] are studied but the number of cases are still limited. In this Letter, we study a class of mesoscopic systems working as a heat pump and present a physical picture explaining how they work. We start from a numerical study adopting one specific model, and demonstrate that a characteristic heat in a barrier-overcoming process determines the ability of the heat pump and Onsager’s reciprocity coefficients. Then, we give a theoretical derivation of the reciprocity coefficients and show the generality of the results in a class of Feynman’s ratchet [5]. We present a simple and clear picture for the mechanism of heat pumps, which is expected to be useful in designing various mesoscopic heat pumps. Model. – We study a class of systems including Feynman’s ratchet in which a particle moves along a periodic structure. The particle interacts with the periodic structure in an asymmetric manner. The periodic structure and the particle are attached to distinct heat baths B1 and B2, whose temperatures are T1 and T2, respectively. We start from a specific but typical model shown in Fig. 1 [6], which is suitable for numerical simulation. In this