Point contact tunnelling as a probe for the superconducting properties of Niobium RF cavities

Niobium, with its very high H$_{C1}$, has been used in superconducting RF cavities for accelerator systems for 40 years with continuous improvement. The quality of cavities (Q) is governed by the surface impedance \textbf{R}$_{BCS}$, which depends on the quasiparticle gap, \textbf{$\Delta $}, and the superfluid density, \textbf{n}$_{S}$. Both of these parameters are seriously affected by surface imperfections (metallic phases, dissolved oxygen, magnetic impurities). Surface treatments of Nb cavities improved the Q factor, but are not understood from a fundamental point of view. Point contact tunneling spectroscopy is an ideal, surface sensitive probe as the quasiparticle current measures \textbf{$\Delta $}${\rm u}$ We present some measurements on SRF cavity-grade Nb using point contact tunneling demonstrating that the nature of the Nb oxide has a significant effect on $\Delta $. Atomic Layer Deposition was used to grow an alumina oxygen diffusion barrier on Nb providing control of the Nb oxidation layer composition with subsequent annealing. Point contact tunneling on these samples help to unravel the complicated effect of Nb surface oxidation on\textbf{${\rm g}\Delta $}.