Nb$_3$Sn Thin Films Using a Cu-Sn Route for Dark Matter Detection

TL;DR

This paper develops a novel solid-state diffusion method to produce uniform Nb3Sn thin films on copper substrates at lower temperatures, enhancing superconducting cavity performance for dark matter detection.

Contribution

It introduces two new fabrication routes for Nb3Sn coatings on copper, achieving high-quality films suitable for superconducting radio-frequency cavities.

Findings

Nb3Sn coatings reached a Q factor of 77,000 at zero magnetic field.

The developed methods produce uniform Nb3Sn films at lower temperatures (~650-750°C).

Q factor drops sharply in magnetic fields, indicating room for further improvement.

Abstract

Axion dark matter searches require superconducting radio-frequency (SRF) cavities on copper (Cu) substrates with quality factors Q > 10^5 in multi-tesla magnetic fields. Copper reduces thermal noise and allows complex geometries. Nb3Sn is a strong candidate due to its superior superconducting properties. However, uniform high-Tc Nb3Sn thin films on Cu are challenging due to Sn loss and substrate strain. This work uses solid-state diffusion of Sn from high-Sn Cu-Sn alloys into Nb layers to form Nb3Sn at Cu-compatible temperatures (650-750{\deg}C), avoiding the traditional ~1100{\deg}C vapor method. Varying Cu-Sn composition yielded an optimal alloy that maintains high Sn activity. Compositional and thermal expansion analyses showed Tc is suppressed below 18 K by Cu substrate strain. Experiments on Nb and sapphire substrates isolated the strain effects. Two routes were developed: (1) Cu-Sn on Ta-coated Cu with hot Nb sputtering (Tc = 16 K), and (2) Nb on Ta/Cu with Cu-Sn evaporation and ex-situ reaction. Route 2 gave uniform Nb3Sn and was chosen for cavity coating. A hexagonal cavity combining designs from the University of Washington and Center for Axion and Precision Physics was coated using Route 2 and tested to 50 mK and 9 T. At zero field it reached Q = 77,000 (40% above bare Cu's Q = 55,000), but Q dropped sharply in field. Nb3Sn coatings on Cu cavities outperform bare Cu at zero field and provide practical routes for improved axion detectors.