Surface-impedance measurements of superconducting NbN films

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Abstract
The temperature and frequency dependences of the surface impedance of NbN thin films have been measured from 4.2 to 13 K and from 0.5 to 10 GHz using a stripline resonator. At 4.2 K resonant-cavity measurements of surface resistance were made at 35 and 56 GHz. From surface-reactance measurements using both cavity and stripline techniques, we obtain the penetration depth extrapolated to 0 K, λ(0)=3700 Å. For the stripline, the film thickness was approximately 8000 Å, which is about two times the measured penetration depth. The weak-coupling (full) Mattis-Bardeen theory was used to fit the temperature dependence of the surface resistance. Based on the quality of the weak-coupling fit to the data, we conclude that the strong-coupling effect is not important in the local and dirty limits. We discuss the correction due to the finite film thickness of the stripline measurements of the surface resistance.