Sub-100 mK Cooling Using Normal-Metal $$\backslash $$ Insulator $$\backslash $$ Superconductor Tunnel Junctions.

Normal-metal $$\backslash $$ insulator $$\backslash $$ superconductor (NIS) junctions can be used as solid-state refrigerators since the hottest electrons preferentially tunnel from the normal metal into the superconductor. In this paper, we present NIS junctions optimized to cool electrons from a bath temperature of 100 mK. We measure a temperature reduction of the electrons in the refrigerator junctions from 100 to 26 mK which agrees with device models. Independent measurements of the electron temperature using thermometer junctions measure a temperature decrease from 100 to 48 mK. Theories explaining the difference in measured temperatures are discussed. [ABSTRACT FROM AUTHOR]