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

TL;DR

This study investigates the surface superconducting properties of niobium used in RF cavities, revealing magnetic scattering as a key factor affecting superconductivity and cavity performance.

Contribution

It demonstrates that magnetic scattering at the surface of niobium contributes to gapless superconductivity, providing new insights into the Q-slope issue in SRF cavities.

Findings

Broadened tunneling DOS in air-exposed Nb

Sharpened DOS after mild baking treatment

Magnetic scattering explains gapless superconductivity

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 delta=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.