Surface impedance measurements of superconducting (NbTi)N films by a ring microstrip resonator technique

In the framework of the search for new materials for sputter coated superconducting rf cavities for particle accelerators, an innovative method to measure the rf properties of superconducting films, based on an inverted microstrip ring resonator technique, is presented. Test measurements on well characterized Nb sputtered films show excellent agreement with the Bardeen, Cooper and Schrieffer theory and prove the good resolution of the method. New results obtained by this technique on the surface impedance of (NbTi)N reactively sputtered films, including the temperature dependence and the rf field amplitude dependence of the surface resistance and the temperature dependence of the magnetic field penetration depth, are then presented and discussed.