Development and characterization of Nb(3)Sn/Al(2)O(3) superconducting multilayers for particle accelerators

Superconducting radio-frequency (SRF) resonator cavities provide extremely high quality factors > 10(10) at 1–2 GHz and 2 K in large linear accelerators of high-energy particles. The maximum accelerating field of SRF cavities is limited by penetration of vortices into the superconductor. Present state-of-the-art Nb cavities can withstand up to 50 MV/m accelerating gradients and magnetic fields of 200–240 mT which destroy the low-dissipative Meissner state. Achieving higher accelerating gradients requires superconductors with higher thermodynamic critical fields, of which Nb(3)Sn has emerged as a leading material for the next generation accelerators. To overcome the problem of low vortex penetration field in Nb(3)Sn, it has been proposed to coat Nb cavities with thin film Nb(3)Sn multilayers with dielectric interlayers. Here, we report the growth and multi-technique characterization of stoichiometric Nb(3)Sn/Al(2)O(3) multilayers with good superconducting and RF properties. We developed an adsorption-controlled growth process by co-sputtering Nb and Sn at high temperatures with a high overpressure of Sn. The cross-sectional scanning electron transmission microscope images show no interdiffusion between Al(2)O(3) and Nb(3)Sn. Low-field RF measurements suggest that our multilayers have quality factor comparable with cavity-grade Nb at 4.2 K. These results provide a materials platform for the development and optimization of high-performance SIS multilayers which could overcome the intrinsic limits of the Nb cavity technology.