Magnetic field dependent microwave spectroscopy on superconducting Pb stripline resonators

Planar superconducting resonators have multiple applications, e.g. in quantum information processing or spectroscopy on unconventional solids. Here, Pb can be used as a basis of microwave resonators with a critical temperature of 7.2 K and a critical magnetic field of 80 mT. It shows interesting behavior with residual normal conducting areas after entering the superconducting phase with applied external magnetic field. We present a study of superconducting Pb microwave stripline resonators in variable external magnetic field up to 140 mT and a temperature range of 1.6 K to 6.5 K. We are able to determine the complex conductivity of Pb in variable magnetic field and we find striking similarities to measurements in variable temperature, like a coherence peak. The quality factor of the resonances shows strong hysteresis effects after exceeding the critical magnetic field, indicating residual normal conducting areas in the resonator persisting in the superconducting state. We are able to determine the critical magnetic field for a set of given temperatures both for the Pb resonator as well as a Sn sample placed on top of the underlying Pb resonator. These results demonstrate that Pb resonators can be applied for different spectroscopy studies in zero as well as finite magnetic field.