Smooth, homogeneous, high-purity Nb3Sn RF superconducting films by seed-free electrochemical synthesis

Superconducting radio-frequency (SRF) resonators are critical components for particle accelerator applications and emerging quantum computers. A promising SRF material is thin-film Nb3Sn with high-quality-factor, high-field, high-operation-temperature potentials. However, surface roughness, non-stoichiometry, and impurities in Nb3Sn deposited by conventional Sn-vapor diffusion prevent their performance from reaching theoretical capabilities. Here we design the deposition process involving seed-free electrochemical synthesis and push the limit of chemical and physical properties in Nb3Sn. Utilization of electrochemical Sn pre-deposits reduces the roughness of converted Nb3Sn by five times compared to typical vapor-diffused Nb3Sn. Quantitative mappings using chemical and atomic probes confirm improved stoichiometry and minimized impurity concentrations in electrochemical Nb3Sn. We have successfully applied this Nb3Sn to the large-scale 1.3 GHz SRF cavity and demonstrated ultra-low BCS surface resistances at multiple operation temperatures, notably lower than vapor-diffused cavities. Our smooth, homogeneous, high-purity Nb3Sn provides the route toward high efficiency and high fields for SRF use.