Superconducting aluminum heat switch with 3 n$\Omega$ equivalent resistance

TL;DR

This paper reports the development of a high-quality superconducting aluminum heat switch with extremely low resistance, suitable for nuclear demagnetization refrigerators, demonstrating significant thermal conductance reduction in the superconducting state.

Contribution

The paper introduces a novel aluminum heat switch design with improved contact resistance and demonstrates its low thermal conductance in the superconducting state.

Findings

Normal state resistance of 3 nΩ

Thermal conductance drops by a factor of 2 million at 50 mK

Effective joining method for Al and Cu with low contact resistance

Abstract

Superconducting heat switches with extremely low normal state resistances are needed for constructing continuous nuclear demagnetization refrigerators with high cooling power. Aluminum is a suitable superconductor for the heat switch because of its high Debye temperature and its commercial availability in high purity. We have constructed a high quality Al heat switch whose design is significantly different than that of previous heat switches. In order to join the Al to Cu with low contact resistance, we plasma etched the Al to remove its oxide layer then immediately deposited Au without breaking the vacuum of the e-beam evaporator. In the normal state of the heat switch, we measured a thermal conductance of $8 T$ W/K$^2$ which is equivalent to an electrical resistance of 3 n$\Omega$ according to the Wiedemann-Franz law. In the superconducting state we measured a thermal conductance that is $2\times10^6$ times lower than that of the normal state at 50 mK.