A Tunable Kinetic Inductance Superconducting Wire Resonator for Use in Axion Dark Matter Searches

The search for axion dark matter in the 10 – 100 μeV mass range by means of a tunable haloscope is one of several promising approaches currently being pursued. A plasmonic metamaterial haloscope utilizing a wire array with a tunable plasma frequency can scan a large mass range with no change in physical dimension of the device [A. J. Millar, et al., Phys. Rev. D 107, 055013 (2023)]. A promising approach to tuning the plasma frequency of the wire metamaterial is through the dc-current-dependent kinetic inductance of narrow superconducting wires. The current converts superfluid into normal fluid, increasing the kinetic inductance of the wire, while also increasing the losses. To test the kinetic inductance tuning mechanism, we have prepared a dc-current-tunable superconducting microwave resonator composed of a 1 μm wide Nb center conductor wire in a stripline geometry. The resonator utilizes a unique integrated low-pass filter for the dc current-bias lines. We measure the resonant frequency and quality factor as a function of dc-current and parallel magnetic field, mimicking the conditions in the plasmonic haloscope. We extract the de-pairing critical current and kinetic inductance fraction of the resonator as a function of temperature and dc magnetic field, and determine the degree to which microwave losses are enhanced by current-induced depairing. Methods to increase the plasmonic tuning range while minimizing losses will be discussed.