Observations and modeling of large area normal-metal/insulator/superconductor refrigerator cooling from 300 mK to below 100 mK

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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 compare the model to measurements of a large area (>400 um²) NIS refrigerator with overlayer quasiparticle traps, and demonstrate that the model is in good agreement experiment. The refrigerator IV curve at a bath temperature of 300 mK is consistent with an electron temperature of 82 mK. However, evidence from independent thermometer junctions suggests that the refrigerator junction is creating an athermal electron distribution whose total excitation energy corresponds to a higher temperature than is indicated by the refrigerator IV curve.