  Using the nonequilibrium theory of superconductivity with the tunnel
Hamiltonian, we consider a mesoscopic NISINISIN heterostructure, i.e., a
structure consisting of five intermittent normal-metal (N) and superconducting
(S) regions separated by insulating tunnel barriers (I). Applying the bias
voltage between the outer normal electrodes one can drive the central N island
very far from equilibrium. Depending on the resistance ratio of outer and inner
tunnel junctions, one can realize either effective electron cooling in the
central N island or create highly nonequilibrium energy distributions of
electrons in both S and N islands. These distributions exhibit multiple peaks
at a distance of integer multiples of the superconducting chemical potential.
In the latter case the superconducting gap in the S islands is strongly
suppressed as compared to its equilibrium value.
