Anomalous Suppression of the Microwave Penetration Depth due to Enhanced Microwave Conductivity in 39-K MgB2 Films

We study the temperature dependences of the microwave in-plane penetration depths at 38 GHz (lambda(ab) (38 GHz)) and 40 GHz (lambda(ab) (40 GHz)) for a YBCO film with a critical temperature (T-C) of 91 K and a MgB2 film with a T-C of 39 K, respectively, for which two TE021-mode sapphire resonators are used for the measurements. The lambda(ab) (40 GHz) appears to be anomalously suppressed in the MgB2 film at temperatures below 0.8T(C), with variations in the lambda(ab) (40 GHZ) with increasing temperature appearing to be significantly less than those in the in-plane penetration depth measured at 10 kHz (lambda(ab) (10 kHz)) by using the mutual inductance method. The suppressed lambda(ab) (38 GHz) of the MgB2 film is attributed to enhanced quasiparticle conductivity, which results in a significant screening of the electromagnetic waves due to the quasiparticles in the MgB2 film. We discuss the quasiparticle screening effects in the MgB2 film in comparison with those in a YBCO film.