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 he supplied locally to the high field region and doe to the nonlinearities causing harmonics and rf breakdown, The separation of rf residual losses R-res(T,f) from the intrinsic losses R-BCS(Tf) yields the quasi-exponential increases of the electric surface resistance with the electric field E-perpendicular to perpendicular to the surface deltaR(E)(E-perpendicular to) proportional to exp (-c/E-perpendicular to) and the power law increases of the magnetic surface impedances with the magnetic field H-parallel to parallel to the surface deltaR(H)(H-parallel to) proportional to) (H-parallel to)(2n) (R = 1, 2. .). By the Nb/Nb2O5-y interfaces at external and internal surfaces R-res(H) (T,f) and R-res(E) (f, E-perpendicular to) can be explained quantitatively, Especially the drop of Q(o)(E-perpendicular to) proportional to 1/R-res(E) (E-perpendicular to) and its reduction by EP- and BCP-smoothening and qv better interfaces by UHV anneal are well accounted for by interface tunnel exchange.(1)