Tunneling study of cavity grade Nb: Possible magnetic scattering at the surface

Abstract

Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap Δ=1.55 meV, characteristic of clean, bulk Nb. However the tunneling density of states (DOS) was broadened significantly. The Nb pieces treated with the same mild baking used to improve the Q-slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.Elemental niobium (Nb) is the material of choice in two of the most important commercial superconducting devices: i) the Josephson tunnel junction (JTJ) 1 and ii) the superconducting radio frequency (SRF) cavity. The complex surface oxides of air-exposed Nb 2 have played an important role in the development of both devices over the past 30 years. For JTJs the deleterious effects of the surface oxides (reduced gaps and Josephson currents, large sub-gap quasiparticle currents, etc.) were eliminated by introducing an ultra-thin capping layer of Al, a technique first successfully established on Nb foils 3 . Prevention of oxygen exposure to the underlying Nb is the key to the construction of JTJ devices such as mixers and analog-digital converters, all of which now utilize Nb/Al bi-layer technology 4 .For SRF cavities the Nb oxide layers are relevant because they occupy a significant fraction of the region where electric and magnetic fields E and B are confined, within one magnetic penetration depth (~45 nm) from the surface. The reduction of the SRF cavity quality factor Q with increasing E field, (Q-slope problem) and its mitigation by a mild annealing procedure 5 (baking effect), are not well understood. There is evidence the baking effect is related to a decrease of Nb 2 O 5 and an increase of NbO 2 from surface probes such as X-ray photoemission spectroscopy measurements applied before and after the baking 6 . However, the particular mechanism by which oxygen affects Q-slope is still elusive.

Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy

“…This can be significant as it has been shown that <3% oxygen vacancies in Nb 2 O 5 change this compound from a nonmagnetic insulator to a conductor with localized magnetic moments [15]. The superconducting gap region probed by tunneling sometimes exhibits a smeared density of states consistent with pair breaking due to magnetic scattering [16]. There is thus a good chance that the source of rf dissipation in a pit is tied to the local chemistry (e.g.…”

Persistence of superconductivity in niobium ultrathin films grown on R-plane sapphire

“…2b Si-interface thickness), which could create a metallic silicide at the Nb/Si interface. Oxygen traces migrating through the silicon capping layer could also further contribute to the inverse proximity effect since niobium oxides with low stoichiometry are known to be metallic and even suspected to be a potential source of destruction of superconductivity by magnetic effects 29 .…”

Surface impedance of superconductors with magnetic impurities

“…Given the above properties, it was recently argued in Ref. 16 that they could indeed be attributed to the presence of magnetic impurities in the system. Although the bulk of Nb samples used for SRF cavities is typically very clean, a disordered oxide surface layer forms due to exposure to atmosphere 14 , Fig.…”

“…Scanning tunneling spectroscopy measurements of the SRF cavity samples performed in Ref. 16 revealed the surface tunneling DOS with appreciable "subgap" contribution, consider- , and (4.17) of the paper by numerically calculating the integrals over and k. In the gapped regime (e), R(ω) ∝ exp(−∆ * /T ) is exponential at lower temperatures and vanishes at T = 0. In the gapless regime (f), for moderate "subgap" DOS, the surface resistance R(ω) is exponential at lower but finite temperatures and saturates to a nonzero value at T = 0.…”