  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 it electron levels, nuclear vibrations and their structures.
We consider cooling mechasims 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 nduced
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.
