Superconducting Tunnel Junction Refrigerators for Sub-Kelvin Cooling of Electrons, Phonons, and Arbitrary, User-Supplied Payloads

Lowell, Peter Joseph

Graduate Thesis Or Dissertation

Superconducting Tunnel Junction Refrigerators for Sub-Kelvin Cooling of Electrons, Phonons, and Arbitrary, User-Supplied Payloads Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/3197xm32q

Abstract

Modern science often requires measurements at sub-Kelvin temperatures. Temperatures of 300 mK can be reached by using liquid ³He, but reaching lower temperatures requires the use of adiabatic demagnetization and dilution refrigerators which are complex, large, and costly. Normal-metal\Insulator\Superconductor (NIS) tunnel junctions provide an alternative refrigeration method that is simple to use, compact, and provides continuous cooling power that has the potential to expand the accessibility of these sub-Kelvin temperatures. When properly biased, the electron system in the normal metal of an NIS junction is cooled since the hottest electrons preferentially tunnel from the normal metal to the superconductor, transferring heat in the process. When the normal metal is extended onto a thermally isolated membrane, the cold electrons cool the phonons in the membrane through electron-phonon coupling. In previous work, NIS junctions have been used to cool detectors and bulk objects that were integrated with the membrane, but could not be considered a general-purpose refrigerator since they could not cool arbitrary objects.

The goal of this work has been to demonstrate a general-purpose NIS refrigerator to which a user can attach arbitrary bulk objects. First, we discuss NIS refrigeration and then develop a model to predict phonon cooling. We fabricated and tested NIS refrigerators capable of cooling bulk objects and used the model to explain the results. The devices were able to cool phonons from 300 mK to 154 mK with 100 pW of cooling power at 200 mK. With these devices, we were able to cool a 2 cm³ piece of copper from 290 mK to 256 mK with 700 pW of cooling power at 290 mK. This demonstration marks the emergence of NIS refrigerators as a true, general-purpose refrigerator since users can attach arbitrary objects. Measurements of Andreev reflections in the devices and next-generation refrigerators that cool electrons from 100 mK to below 50 mK are also presented.

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