Physical Review Applied
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.
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
Lechner, Eric M.; Oli, Basu Dev; Makita, Junki; Ciovati, Gianluigi; Gurevich, Alex; and Iavarone, Maria, "Electron Tunneling and X-Ray Photoelectron Spectoscopy Studies of the Superconductiong Properties of Nitrogen-Doped Niobium Resonator Cavities" (2020). Physics Faculty Publications. 426.