Two-Peak Temperature Dependence of the Microwave Surface Impedance in Single-Crystalline YBCO Films

摘要

Temperature dependencies of microwave surface impedance were measured forperfect c-oriented YBCO thin films deposited on CeO2-buffered sapphiresubstrates. The measurements revealed a distinct two-peak structure of Rs(T)and Xs(T) dependencies with peaks at 28-30 K and 50 K. The peaks become smearedat higher frequencies as well as in applied dc magnetic field about 1 kOe,while the peak positions remain almost unchanged. For less perfect, e.g., PLDfilms, Rs(T) and Xs(T) dependencies are monotonous (power law). The two-peakZs(T)dependencies for YBCO films differ from those for high quality YBCO singlecrystals, where only one much broader frequency-dependent peak of Rs(T) wasdetected earlier. The two-peak Zs(T) behavior is believed to be an intrinsicelectron property of extremely perfect quasi-single-crystalline YBCO films. Atheoretical model is suggested to explain the observed anomalous Zs(T)behavior. The model is based on the Boltzman kinetic equation forquasiparticles in layered HTS cuprates. It takes into account the supposeds+d-wave symmetry of electron pairing and strong energy dependent relaxationtime of quasiparticles, determined mainly by their elastic scattering onextended defects parallel to the c-axis (e.g., c-oriented dislocations and twinboundaries).