Superconducting aluminum heat switch with 3 nΩ equivalent resistance.

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 and 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 8T W/K, which is equivalent to an electrical resistance of 3 nΩ according to the Wiedemann-Franz law. In the superconducting state, we measured a thermal conductance that is 2 × 10 times lower than that of the normal state at 50 mK.