Electric surface resistance RE(T, f, E⊥) of Nb/Nb2O5-y-interfaces and Q-drop of superconducting Nb cavities

The RF losses, especially actual level and increase with RF fields limit most stringently the application of superconducting RF cavities. This is due to the needed cooling power to be supplied locally to the high field region and due to the nonlinearities causing harmonics and RF breakdown. The separation of RF residual losses R_res(T,f) from the intrinsic losses R_BSC(T,f) yields the quasi-exponential increases of the electric surface resistance with the electric field E^l perpendicular to the surface δR^E (E^⊥)∝exp (-c/E^⊥) and the power law increases of the magnetic surface impedances with the magnetic field H^|| parallel to the surface δR^H(H ^||)∝(H^||)²ⁿ (n=1, 2..). By the Nb/Nb₂O_5-y interfaces at external and internal surfaces R^H_res(T,f) and R^E_res(f,E^⊥) can be explained quantitatively. Especially, the drop of Q₀(E^⊥)∝1/R^E_res(E ^⊥) and its reduction by EP- and BCP-smoothening and by better interfaces by UHV anneal are well accounted for by interface tunnel exchange