Cooling mechanisms in molecular conduction junctions

While heating of a current carrying Ohmic conductors is an obvious consequence of the diffusive nature of the conduction in such systems, current-induced cooling has been recently reported in some molecular conduction junctions. In this paper, we demonstrate by simple models the possibility of cooling molecular junctions under applied bias, and discuss several mechanisms for such an effect. Our model is characterized by single electron tunneling between electrodes represented by free electron reservoirs through a system characterized by its electron levels, nuclear vibrations and their structures. We consider cooling mechanisms resulting from (a) cooling of one electrode surface by tunneling-induced depletion of high-energy electrons; (b) cooling by coherent sub resonance electronic transport analogous to atomic laser-induced cooling and (c) the incoherent analog of process (b)-cooling by driven activated transport. The non-equilibrium Green function formulation of junction transport is used in the first two cases, while a master equation approach is applied in the analysis of the third.