Xiaohang NMN Zhang, Peter J. Lowell, Brandon L. Wilson, Galen C. O'Neil, Joel N. Ullom
In this paper, we demonstrate a general-purpose macroscopic refrigerator based on the transport of hot electrons through superconducting tunnel junctions. Our refrigerator is intended to provide access to temperatures below those accessible using pumped 3He. The refrigerator is cooled by 96 Normal-metal/Insulator/Superconductor (NIS) junctions divided among three separate silicon substrates. The use of thin-film devices on different substrates shows the potential to achieve higher cooling powers by connecting NIS devices in parallel. Improving on previous work, we demonstrate a larger temperature reduction, a more robust mechanical suspension, and a new electromechanical heat switch that will make it easier to integrate our refrigerator into other cryostats. The electromechanical heat switch has a measured thermal conductance in the on state of 1.2 ± 0.3 μW/K at 0.3 K and no thermal conductance in the off state. We observe a temperature reduction from 291 mK to 228 mK in a copper stage with 28 cm2 of area for attaching user-supplied devices.