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

Abstract

Scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy (XPS) have been used to investigate the differences between the surface electronic structures and chemical structure of typically prepared and N-doped Nb cutouts from superconducting radio frequency (SRF) 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 radiofrequency magnetic field, which has been observed by many groups on N-doped Nb cavities. In particular, we address the effects of N-doping on the superconducting properties at the surface of SRF cavities. XPS measurements reveal a significantly more oxidized Nb 3d states on the N-doped Nb surfaces which is confirmed by tunneling spectroscopy measurements. Analysis of the tunneling spectra in the framework of a recent model (A. Gurevich and T. Kubo Phys. Rev. B 96, 184515 ( 2017)) shows that the N-doping greatly reduces local distribution of superconducting properties on the surface, causes a significant shrinkage of the metallic suboxide and changes the contact resistance between the metallic suboxide and the bulk niobium toward an optimum value, resulting in a lower surface resistance. Combination of these factors enables one to effectively tune the density of states at the surface and to reveal the decrease of the surface resistance with the radio frequency field which follows from the BCS theory. A slightly reduced average gap and a smaller coherence length, revealed by tunneling spectroscopy in the vortex cores, have been found in the N-doped Nb samples compared to typically prepared Nb samples, indicating a stronger impurity scattering caused by nitrogen doping in a moderately disordered material.

Development and characterization of Nb3Sn/Al2O3 superconducting multilayers for particle accelerators

“…The fit was done with Γ = 0.4 meV and meV, consistent with the conventional gap value for a stoichiometric Nb 3 Sn 4 . The ratio % in our samples turns out to be about 2–3 times larger than the values observed by tunneling spectroscopy on 1–2 μm thick Nb 3 Sn films for rf applications 27 and Nb coupons 28 . The deviations of the STM data from the Dynes model at low energies may indicate the effects of local non-stoichiometry, gap anisotropy and strain 22 , scattering of quasiparticles on magnetic impurities, and a thin layer with deteriorated superconducting properties at the surface 27 – 30 .…”

Development and Characterization of Nb3Sn/Al2O3 Superconducting Multilayers for High-performance Radio-Frequency Applications

“…The ratio Γ Δ ≈ 13 ⁄ % in our samples turns out to be about 2-3 times larger than the values observed by tunneling spectroscopy on 1-2 m thick Nb3Sn films for rf applications 27 and Nb coupons 28 . The deviations of the STM data from the Dynes model at low energies < Δ may indicate the effects of local non-stoichiometry, gap anisotropy and strain 22 , scattering of quasiparticles on magnetic impurities, and a thin layer with deteriorated superconducting properties at the surface 27,28,29,30 . In turn, the subgap quasiparticles states which appear at |E| <  due to a finite  contribute to a temperature-independent residual surface resistance Ri at kBT << Δ 5,29…”

RF surface resistance tuning of superconducting niobium via thermal diffusion of native oxide

“…Performance enhancement in interstitially alloyed cavities is inherently a complex multifaceted problem. In this space, the performance is dependent on the interplay between electron mean free path [19][20][21][22][23][24], enhanced sensitivity to trapped flux [19,25,26], a normalconducting hydride precipitate blocking effect [27][28][29][30], and density of states tuning [31][32][33][34][35][36]. With interstitial alloying, these factors have often worked in concert to produce resonant cavities with unprecedented low surface resistance (high quality factor), but often with lower maximum supportable field amplitude.…”