Macroscopic Subkelvin Refrigerator Employing Superconducting Tunnel Junctions

Abstract

In this paper, we demonstrate a general-purpose macroscopic refrigerator based on the transport of electrons through superconducting tunnel junctions. Our refrigerator is intended to provide access to temperatures below those achievable using pumped He3. The refrigerator is cooled by 96 normal-metal–insulator–superconductor (N−I−S) 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 N−I−S devices in parallel. Improving on previous work by Lowell et al. [Appl. Phys. Lett. 102, 082601 (2013)], 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 300 mK and no measurable thermal conductance in the off state. We observe a temperature reduction from 291 to 233 mK and infer cooling to 228 mK on longer time scales. The cooled payload is a metal stage whose mass exceeds 150 g and with 28 cm2 of area for attaching user-supplied devices. Using the product of the cooled mass and the temperature reduction as a performance metric, this work is a more than tenfold advance over previous efforts.

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