Superconducting nanowire resonators with circular cross-section.

We report on a significant advance in implementing vibrating nanowire resonators as mechanical probes in helium superfluids as bolometers, thermometers and detectors of turbulence. Nanomechanical resonators produced lithographically typically have a flat, rectangular cross-section. Sharp edges result in velocity enhancement for potential flow reducing the operational velocity before exceeding the critical velocity. A circular cross-section can operate at higher velocities and allows for a robust description of the flow dynamics. The wires are made by drawing multifilament superconducting cables through a series of diamond dies to the desired diameter. The filaments of the cable are isolated by etching the copper matrix and then transported to a desired location to form an electro-mechanical resonator. We have characterised superconducting nano-mechanical vibrating wire resonators with diameters in the range from 200 nm to 1 µm and lengths of up to 2 mm. The nanowires have been tested in superfluid helium-4, superfluid helium-3 and helium-3 gas. We have measured critical temperatures and critical currents for varying diameters of the wires. Resonant frequencies are typically in the range of 1-20 kHz and the contact resistance between a nanowire and an aluminium bond wire has been measured to be no more than 30 mOhm. In our current work these wires will be used as probes within a superfluid helium-3 dark matter detector.