Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities

• DOI: 10.1103/physrevapplied.13.044044
• Publication Date: 2020-04-16T16:27:42Z
• Authors: Eric M. Lechner; Basu Dev Oli; Junki Makita; Gianluigi Ciovati; Alex Gurevich; Maria Iavarone
• Keywords: Suboxide

We use scanning tunneling microscopy (STM) and spectroscopy (STS), x-ray photoelectron (XPS) to investigate the effect of nitrogen doping on surface electronic chemical structures cutouts from superconducting $\mathrm{Nb}$ radio-frequency cavities. The goal this work is get insights into fundamental physics materials mechanisms behind striking decrease resistance with magnetic field, which has been observed $N$-doped Our XPS measurements reveal significantly more oxidized $3d$ states a thinner metallic suboxide layer surfaces, also confirmed by measurements. In turn, performed native surfaces as well $\mathrm{Ar}$-ion sputtered allow us separate $N$ surface-oxide that density in bulk. Analysis our spectra framework model proximity-coupled normal at [A. Gurevich T. Kubo, Phys. Rev. B 96, 184515 (2017)] consistent hypothesis $N$-doping ameliorates lateral inhomogeneities properties shrinks layer. For $\mathrm{Ar}$ we find evidence changes contact between bulk niobium toward an optimum value corresponding minimum resistance. totality experimental data suggests provides effective tuning such way it can result predicted calculations nonlinear low-frequency electromagnetic response dirty superconductors. Furthermore, STM imaging vortex cores shows slightly reduced average gap shorter coherence length samples compared typically prepared samples, indicating stronger impurity scattering caused moderately disordered material.