Speaker: Juergen Halbritter, Research Center Karlsruhe
Abstract: For superconducting Nb the electric surface impedance ZE is confined to a narrow (≲10nm) Nb2O5-Nb interfaces and the magnetic surface impedance ZH to λB≲100 nm as magnetic penetration depth into Nb. The corresponding surface resistances RE and RH are small below 2K but their increase with oxidation is for Nb cavities the limiting obstacle at low E⊥≲10V/m and at high rf magnetic fields B||≈0.1T and high rf electric fields E⊥≳5MV/m. Nb oxidation has been studied by angle resolved X-ray photon electron spectroscopy (ARXPS), by surface resistance RB(T,B,ω), by flux pinning, by penetration depth λB(T,B,ω), and by voltage tunnel spectroscopy (VTS). Our studies showed that oxidation is strongly inhomogeneous: by Nb2O5-y consisting of crystalline blocks (CB) built of {NbO6} octahedra to sizes ~ 1nm with a barrier height ΦB = EC -EF ≃ 1 eV as difference between conduction band EC and Fermi energy EF, by CBs separated by crystallographic shear planes (CS) with ΦS ≃ 0.1 eV housing localized states nL(z) ≃ 1019/cm3, and by nuclei in Nb easing the strain relieve due to the CB volume increase by more than 300% causing injection of oxidized weak links and NbOx precipitates up to depth between 0.1 - 50μm degrading locally the superconducting energy gap δΔ/Δ≃100x/Δ by Ox. The latter deteriorations of superconductivity have been minimized successfully in recent years by Nb with minimal density of nuclei. Hence, here we focus on the Nb-oxides and on Nb2O5-y/NbOz/Nb interface reaching up to 0.1μm deep for high purity Nb surfaces. Wet, fast grown Nb2O5-y is thicker then 4 nm and highly perturbed with a high density of CS and of nL enforcing weak link and NbOx injection into Nb to relieve the strain. Slower oxidation is quantified by the Cabrera Mott process, where nuclei poor Nb single crystals show the slowest Nb2O5-y growth rate with smaller CS and nL densities on one side and, on the other side, the smallest amount of O injection into Nb. Differently grown Nb2O5-y yield interfacial tunnel exchange (ITE) differing by oxide thickness and CS and nL density, yielding, e.g., greatly varying RE ∝ nCSnL dominated by nL and CS densities.
The details of Nb2O5-y structure and growth will be elucidated explaining via ITE Rres < 5nΩ(f/GHz)2, noise, electronic two level systems (ETLS) confined to the Nb surface and oxidation dependent Q-drops quantitatively, especially the much reduced Q-drop after UHV annealing. Differences between Nb, Al, and NbN oxidation and their consequences for ZE and ZH will be elucidated.