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The microwave response of high quality niobium films in a perpendicularstatic magnetic field has been investigated. The complex frequency shift wasmeasured up to the upper critical fields. The data have been analyzed by theeffective conductivity model for the type-II superconductors in the mixedstate. This model is found to yield consistent results for the coherencelengths in high-kappa superconducting samples, and can be used with HTSC evenat temperatures much below T_c. It is shown that for samples with high valuesof depinning frequency, one should measure both components of the complexfrequency shift in order to determine the flow resistivity. The thick Nb film(160 nm) has low resistivity at 10 K, comparable to the best single crystals,and low kappa value. In contrast, the thinnest (10 nm) film has kappa ~ 9.5 andexhibits a high depinning frequency (~20 GHz). The upper critical fielddetermined from microwave measurements is related to the radius ofnonoverlaping vortices, and appears to be larger than the one determined by thetransition to the normal state.Comment: 8 pages, 7 figures; submitted to PRB; measured rho_n; changes due to  the referees' comments (abstract, conclusions, extended introduction

Microwave response of thin niobium films under perpendicular static magnetic fields