In a refrigerator, heat is moved from one system to another and this movement requires an additional dissipated power. It is desirable to both isolate the cooled system, so that its temperature may differ from the bath, and to heatsink the heated system so that its temperature does not differ from the bath. In a normal-metal/insulator/superconductor (NIS) tunnel junction refrigerator, the normal-metal electrons are cooled and the dissipated power heats the superconducting electrode. This paper presents a review of the mechanisms by which heat leaves the superconductor and introduces overlayer quasiparticle traps, for more effective heatsinking. A comprehensive thermal model is presented that accounts for the described physics, including the behavior of athermal phonons generated by both quasiparticle recombination and trapped quasiparticles. We present measurements of a large area (> 400 μm2) NIS refrigerator with overlayer quasiparticle traps which cools from 300 mK to below 100 mK, and demonstrate that the model results are in good agreement with the experimental results. We use this model to predict refinements that will improve NIS refrigerator performance even further.
Citation: Physical Review B
Pub Type: Journals
nis, refrigerator, electron tunneling, normal-metal/insulator/superconcutor