Theory Results on Novel Surface Preparations for Superconducting Radio-Frequency Cavities

In the last decade, rapid advances in the materials science of niobium surfaces have pushed the performance of superconducting radio-frequency (SRF) cavities far beyond what were thought to be the limits of this humble elemental superconductor. At the same time, an improving fundamental understanding of SRF has made clear that further advances are possible. We present theoretical results on new “surfaces by design” under active development by our collaborators: using density-functional theory (DFT) and time-dependent Ginzburg-Landau (TDGL) theory, we investigate novel doping treatments which can alter the electronic structure of the surface, inhibit the development of weakly-superconducting phases, or promote the growth of new phases with enhanced superconducting properties.<br/><br/>Our two-pronged theoretical approach explores firstly the dependence of fundamental properties like Fermi-level density of states, electron-phonon coupling strength, and elastic constants on the composition and structure of niobium-based materials of interest, and secondly the effect of processing conditions such as temperature and chemical potential of different species on the kinetics and thermodynamics that determine the ultimate structure of the surface. Specifically, we employ Wannier function techniques to precisely characterize the electronic states at the Fermi level and smoothly integrate over reciprocal space to determine scattering amplitudes for electron-phonon and electron-defect scattering processes. These fundamental material properties determine phase stability, superconducting properties and, as shown in TDGL simulations, SRF performance.<br/><br/>In addition to inspiring new sample preparation recipes, we show encouraging agreement between our calculations and experimental measurements, in particular explaining the significantly improved properties of our sample coupons relative to similar samples documented in the literature. As applications for SRF cavities continue to expand, from a growing demand for electron beams to the construction of large particle accelerators to exotic uses in particle detection and quantum computing, a richer understanding of niobium-based superconductors and how to optimize their properties for SRF stands to pay great dividends.