Enhanced Surface Superconductivity of Niobium by Zirconium Doping

Superconducting radio-frequency (SRF) cavities currently rely on niobium (Nb), and could benefit from a higher-Tc surface, which would enable a higher operating temperature, lower surface resistance, and higher maximum fields. Surface zirconium (Zr) doping is one option for improvement, which has not previously been explored, likely because bulk alloy experiments showed only mild Tc enhancements of 1-2 K relative to Nb. Our ab initio results reveal a more nuanced picture: an ideal bcc Nb-Zr alloy would have Tc over twice that of niobium, but displacements of atoms away from the high-symmetry bcc positions due to the Jahn-Teller-Peierls effect almost completely eliminates this enhancement in typical disordered alloy structures. Ordered Nb-Zr alloy structures, in contrast, are able to avoid these atomic displacements and achieve higher calculated Tc up to a theoretical limit of 17.7 K. Encouraged by this, we tested two deposition methods: a physical-vapor Zr deposition method, which produced Nb-Zr surfaces with Tc values of 13.5 K, and an electrochemical deposition method, which produced surfaces with a possible 16-K Tc. An rf test of the highest-Tc surface showed a mild reduction in BCS surface resistance relative to Nb, demonstrating the potential value of this material for RF devices. Finally, our Ginzburg-Landau theory calculations show that realistic surface doping profiles should be able to reach the maximum rf fields necessary for next-generation applications, such as the ground-breaking LCLS-II accelerator. Considering the advantages of Nb-Zr compared to other candidate materials such as Nb3Sn and Nb-Ti-N, including a simple phase diagram with relatively little sensitivity to composition, and a stable, insulating ZrO2 native oxide, we conclude that Nb-Zr alloy is an excellent candidate for next-generation, high-quality-factor superconducting rf devices.