Phenomenological description for unusual surface impedance behaviors of high-Tc thin film in weak DC magnetic field

A phenomenological description is presented in this paper for unusual surface impedance (Zs) behaviour of high-Tc thin film in weak DC magnetic field (BDC). Unusual Zs surface impedance behaviour of YBCO thin film, namely, Zs decreases with BDC, is observed with microstrip technique at 77 K. In our description, the exponential dependence of the increase of superconducting electron density on BDC is assumed to explain unusual Zs behaviour. Combined with the two-fluid model (TFM) and the coupled-grain model, the field dependence of Zs is computed. Results based on this model agreed with the experimental results qualitatively.