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정호상,양우일,Myung Su Kim,이상영,Brian H. Moeckly,John. H. Claassen 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.3
We study the temperature dependences of the microwave penetration depths at 38 GHz (l(38 GHz)) and 40 GHz (l(40 GHz)) for a YBCO film with a critical temperature (T_C) of 91 K and a MgB_2 film with a T_C of 39 K, respectively, for which two TE_(021)-mode sapphire resonators are used for the measurements. The l(40 GHz) appears to be anomalously suppressed in the MgB_2 film at temperatures below 0.8T_C, with variations in the l(40 GHz) with increasing temperature appearing significantly less than that in the penetration depth measured at 10 kHz (l(10 kHz)) by using the mutual inductance method. The suppressed l(38 GHz) of the MgB_2 film is attributed to enhanced quasiparticle conductivity, which results in significant screening of the electromagnetic waves due to quasiparticles in the MgB_2 film. We discuss the quasiparticle screening effects in the MgB_2 film in comparison with those in a YBCO film.