NIS chip refrigeration

摘要

A normal-metal/insulator/superconductor (NIS)tunnel junction can be applied to cool electrons by biasing thejunction suitably with external voltage. Two NIS junctions inseries can form an efficient microrefrigerator because of thesymmetry with bias voltage. Our SINIS microrefrigerator has beencapable of reaching electronic temperatures of about 100 mKstarting from 300 mK. To achieve appreciable refrigeration of theunderlying lattice, the microrefrigerator must be thermallydecoupled from the bulk substrate. We have demonstratedexperimentally the reduction of lattice temperature by 23 mK at180 mK by extending the normal electrode on a thin dielectricmembrane with four suspended bridges. Methods to improve theefficiency of refrigeration, e.g., by using a superconductor with alarge energy gap and/or by increasing the number of tunneljunctions, are discussed.