High Resolution Thermometry Using the Magnetic Penetration Depth of Superconducting Films

Abstract

A Penetration Depth Thermometer (PDT) has been under development for potential use in microgravity critical point experiments. The active element is a superconducting film whose thickness is comparable to the magnetic penetration depth (5–20 nm). Field penetration is strongly temperature dependent just below the transition (~0.9 – 1.0 T_c). Fields are generated and sensed by using excitation and pickup coils located near the film. Using a dc SQUID sensor for readout, temperature resolution on the order of 0.1 nK/✓Hz can be obtained. Earlier work using aluminum films (T_c=1.69 K) demonstrated a maximum sensitivity of 1 nK/✓Hz. Improvements in coil design have since yielded a factor of 3 improvement. PDT’s have been made with aluminum films deposited on a planar sapphire substrate with transition temperatures greater than the lambda point in superfluid helium. For performance tests, a cryostat has been constructed where the PDT will be in an experimental cell filled with superfluid. This will provide thermal stability better than 0.1 nK/✓Hz . Results of the current design are presented.