Toward the optimization of electronic refrigerators

Abstract

Normal metal–insulator–superconductor (NIS) junction can act as refrigerator of electron gas in a normal metal electrode under certain conditions. If majority of high energy electrons in a normal metal is moved with a bias voltage to energy levels above the superconductor's energy gap, their extraction from a normal metal is enabled by tunnelling. Their substitution with lower energy electrons results in lowering of the normal metal electron gas energy. Due to excess quasiparticle density in the superconductive electrode in vicinity of the junction, which is a result of tunnelling of quasiparticles from the normal metal and low capability of removal of these particles from the junction area the cooling capability of junctions deteriorates at lower environment temperatures. This is mainly the results of back tunnelling of excess quasiparticles to a normal metal and of dissipated energy, released by Coooper pair formation in proximity of the junction. Quantitative description of quasiparticle behavior in the superconductive electrode of the NIS junction is given. Energy dependent diffusion of quasiparticles in superconductor, their consumption due to Cooper pair creation and inflow of additional quasiparticles from a normal metal are considered. The model results in position dependence of quasiparticles' density in the junction proximity.