Trapping of quasiparticles of a nonequilibrium superconductor

We have performed experiments where hot electrons are extracted from a norm al metal into a superconductor through a tunnel junction. We have measured the cooling performance of such NIS junctions, especially in the cases wher e another normal metal electrode, a quasiparticle trap, is attached to the superconductor at different distances from the junction in direct metal-to- metal contact or through an oxide barrier. The direct contact at a submicro n distance allows superior thermalization of the superconductor. We have an alyzed theoretically the heat transport in this system. From both experimen t and theory, it appears that NIS junctions can be used as refrigerators at low temperatures only with quasiparticle traps attached. (C) 2000 American Institute of Physics. [S0003-6951(00)00219-9].