Document Type

Article

Publication Date

2020

DOI

10.1103/PhysRevApplied.13.044044

Publication Title

Physical Review Applied

Volume

13

Issue

4

Pages

044044

Abstract

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

Comments

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© 2020 American Physical Society

Publisher's version available at: https://doi.org/10.1103/PhysRevApplied.13.044044

Original Publication Citation

Lechner, E. M., Oli, B. D., Makita, J., Ciovati, G., Gurevich, A., & Iavarone, M. (2020). Electron tunneling and x-ray photoelectron spectoscopy studies of the superconductiong properties of nitrogen-doped niobium resonator cavities. Physical Review Applied, 13(4), 044044. doi:10.1103/PhysRevApplied.13.044044

ORCID

0000-0003-0759-8941 (Gurevich)

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