Refrigerators based on NIS tunnel junctions

by

J.P. Pekola, D.V. Anghel, T.I. Suppula, J.K. Suoknuuti, A.J. Manninen, 
and M. Manninen


We have performed experiments where hot electrons are extracted 
from a normal metal into a superconductor through a tunnel 
junction (NIS junction). We have measured the cooling performance 
of such NIS junctions, especially in the cases where 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 submicron distance 
allows superior thermalisation of the superconductor. We have analysed 
theoretically the heat transport in this system. From both experiment 
and theory, it appears that NIS junctions can be used as refrigerators 
at low temperatures only with quasiparticle traps attached. 
Configurations with several NIS junctions can be used efficiently for 
refrigerating purposes.