On-chip refrigeration by evaporation of hot electrons at sub-Kelvin temperatures

Evaporation of hot electrons from a normal-metal into a superconductor can be used for efficient Peltier type cooling in micrometer size tunnel juncti ons. We have cooled the electrons to one third,(1) and, as the main result of the present paper, a separate silicon nitride membrane to about one half of its starting temperature; all results have been obtained at temperature s well below 1 K, where the lattice is weakly coupled to electrons thermall y. The micromachined membrane can serve as a thermal bath for tiny samples, like bolometric radiation detectors in astronomy.