Nonlinear electrodynamics of superconducting NbN and Nb thin films at microwave frequencies

Abstract

The surface resistance Rs of NbN and Nb thin films with particular reference to nonlinear effects is reported at high values of the microwave magnetic field Hrf≤500 Oe using a stripline resonator in the frequency range 0.6≤f≤12 GHz. In the stripline geometry, the microwave current is concentrated on the narrow center conductor. Thus a high microwave current density and, therefore, a high Hrf can be achieved with moderate power. For Nb thin films, Rs does not increase with Hrf for low Hrf values, as expected from weak-link theory. However, for the NbN thin films, Rs at temperature T follows a Rs(T,f)=Rs0(T,f)+S(T,f)Hrf dependence, for f<6 GHz where Rs0(T,f) is the surface resistance at zero Hrf, and the slope S(T,f) is proportional to f2,3. This nonlinear effect is consistent with Halbritter’s weak-link theory. For f>6 GHz, Rs shows a plateau in the dependence on Hrf the magnitude of which varies as f3,5, which is not predicted theoretically. When Hrf increases above a critical value Hrfc, the resonance curves for the strip-line resonator become asymmetrical and the intermodulation products saturate, indicating strong nonlinearities. The temperature dependence of Hrfc(T) for Nb, in contrast to NbN, thin films follows that of the dc Hc1. Information on the granularity of NbN, derived from the present study, provides insights into the surface impedance of the granular high-Tc copper oxide thin films.

Source-reported materials — not catalogue approval

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