A pathway to improve Nb$_3$Sn for SRF accelerator cavities via Zr inclusion

Nb$_3$Sn has the potential to exceed the performance of niobium-based superconducting radio-frequency (SRF) cavities, both in particle accelerators and in high-$Q_0$ cavities operating in multi-tesla magnetic fields for axion dark matter searches. We studied how the incorporation of Zr into the Nb$_3$Sn matrix by co-sputtering affects the microstructure and superconducting properties of Nb$_3$Sn films. As the atomic fraction of Zr increased, the surface morphology improved and the bulk and surface void densities decreased. At the optimal Zr content, the superconducting properties improved and a thinner surface niobium oxide formed. These results identify co-sputtered Nb$_3$Sn:Zr as a promising material to advance the performance limits of SRF cavities.