Germany

Tunnelling through nanocrystalline Nb₂O_5-y is not only crucial for cryo electronics but also for Cabrera Mott oxidation of Nb and, therewith, for the dielectric surface impedance Z^E. In the talk it will be elaborated how oxidation deteriorates the superconducting Nb depending on defects, i.e., why thin films show severe Z^B(B) deterioration being minimal in single crystals, and how different oxides change Z^E(E), i.e., the high field Q-drop. The physics and material science behind is summarized in the abstract below.

Niobium oxides consists of edge connected {NbO₆} octahedra blocks (CB, size by 3 –5 {NbO₆} about nm², stoichiometry Nb⁶⁺) with Φ_B = E_c – E_F≃ 1 eV as difference between conduction band E_C and Fermi energy E_F. The CB’s are bordered by side connected {NbO₆} octahedra with Φ_S≃ 0.1 eV, named, crystallographic shear planes (CS). By nonlocal charge transfer CS’s yield the correct stoichiometry Nb⁵⁺ in average. As common defect in Nb₂O_5-y O-vacancies populated by an electron pair pin E_F 1 eV below E_C of CB yielding as density of localized states n_L ≃10²⁰ -10¹⁹/cm³, mostly defect complexes Nb⁴⁺V₀Nb⁴⁺ at CS, easing the tunnel and hopping charge transport. Hence, Cabrera Mott electrons tunneling to the adsorbed air O₂ by the Nb–O binding energy of 5eV, charge O up and the potential (≲ 5V) across the Nb₂O_5-y layer drives O^2- towards the metallic NbO_x (x ≲1) interface speeded up by CS O vacancies. There new CB may be nucleated if the volume expansion by a factor 3 is possible, e.g., at nuclei releasing the strain. If the release is retarded, as in the case of the hard NbN or of defect free Nb single crystals one obtains very thin Nb₂O_5-y(≲ nm) oxide coatings, which are dominated by Φ_B and n_L ≪ 10¹⁹/cm³. Below the metallic NbO_x(x ≲1)-interface injected O exists in solid solution and, defects adjacent to Nb surfaces, nucleate strain release yielding crack corrosion, i.e., segregation by Nb-d-bonds into Nb₂O_5-y filled weak links and into NbO_x(x≃0.02). Those segregates are injected up to depths of 0.1 – 50µm, depending on oxidizing potential and nuclei density. Despite this variance, the oxidation still follows the Cabrera Mott process with the metallic interface oxide NbO_x(x ≈1) as anode where the rate depends on density of nuclei in Nb and density of CS in Nb₂O_5-y.