EAGER: CRYO: Development of a sub-Kelvin Refrigerator using Magnetic Field Activated Solid-State Thermal Switches based on Thermal Chiral Anomaly

Ohio State University, The, Columbus OH

Investigators

Abstract

This EArly-concept Grant for Exploratory Research (EAGER) project is to extend the optimization of the switches from temperatures above 4K, where it is known they work, to below 1 K, then to build a small prototype adiabatic demagnetization refrigeration (ADR) and establish the proof of concept. Quantum computers based on superconductors must be cooled to temperatures far below 1 Kelvin. Only two technologies exist to do that, dilution refrigeration and ADR. Dilution refrigeration uses a rare isotope, He3, of helium, which itself is already a scarce gas and a non-renewable resource. He3 is thus in such short supply that its scarcity may well limit the spread of quantum computers. The alternative, ADR, is a heat pump that functions like a household refrigerator but uses magnetic “work”. A household refrigerator cools its contents (the heat load) and rejecting the heat to the room (the heat sink). Its working fluid is a refrigerant, like freon, that is alternatively compressed, which heats it up, and expanded, which cools it down. Similarly, the working fluid in an ADR is a pill of a magnetic salt that is alternatively put in thermal contact with the heat load, whose heat it drains, and then with a heat sink, where it rejects the heat it has taken from the load. The work is done by alternatingly applying a magnetic field to the salt pill, which heats it up, and then removing the field, which cools the pill down. The salt pill is exposed to the field while in contact with the heat sink. The field is removed while the salt pill is in contact with the load. Existing conventional ADR’s use helium exchange gas as a thermal contact: cycling them requires pumping helium in and out of the system, which is slow and limits the cycle time. This project is to develop a pair of solid-state heat switches that connect the salt pill, one to the sink and the other to the load. The design calls for the same field to do the magnetic the work in the pill and operate both switches automatically. Heat switches are devices that let heat flow when they are “closed” but block heat when they are “open”. They are made from materials that switch between a low and a high thermal conductivity. In the switches to be developed here, heat will be carried by electrons. In almost all solids, electron conduction is decreased in a magnetic field that deflects the electron motion sideways, creating magneto-resistance. One notable exception is the “Thermal Chiral Anomaly” or “Gravitational Anomaly”. In this recently discovered effect, a temperature difference applied parallel to a magnetic field along a specific crystallographic direction in Weyl semimetals, a class of topological solids, greatly increases the thermal conductivity. The material switches from a low thermal conductivity at low field to a high one at high field. But a cyclical ADR needs two switches, one to the load and one to the sink. It turns out that the same materials, with the field oriented along the opposite direction, is not a Weyl semimetal but a “topological insulator”. It now gives the opposite effect: the switch is closed when the field is high and open when the field is low. By combining both and integrating them with a salt pill, an ADR can be made that theoretically can reach the temperatures needed for quantum computing. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.