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A.A. Zhukov, M.R. Trunin, A.T. Sokolov, N.N. Kolesnikov.
Temperature dependence of surface impedance of Tl₂Ba₂CaCuO_8-d and YBa₂Cu₃O_6.95 single crystals measured in the microwave band.
 

  Abstract:
The real part R_s and the imaginary  part X_s of the surface impedance Z_s=R_s+iX_s of   Tl₂Ba₂CaCuO_8-d  and  YBa₂Cu₃O_6.95 single crystals have been measured with high precision at frequency w/2p=9.4 GHz in the temperature range 0< T<140 K. In the Tl₂Ba₂CaCuO_8-d
crystal a linear temperature dependence R_s (T) has been found for T £ 50 K, and the magnetic field penetration depth l (4.2 K)=X_s(4.2 K)/wm₀ @ 3760 Angstrom has been measured. Along with well known features of the function Z_s(T) in high-quality YBa₂Cu₃O_6.95   single crystals, such as the linearity of l(T) and R_s(T) for T<T_c/3 and a maximum of R_s(T) at T ~ T_c/2, the linearity range of l(T) extends to T @ 50 K, and this curve has a plateau in the range 60<T<85 K. The curve of  R_s (T) in both the superconducting and normal states of YBa₂Cu₃O_6.95  is well described by a two-fluid model with the electron-phonon mechanism of quasiparticle relaxation. A formula describing the curve of
l²(0)/l²(T) throughout the studied temperature range is also given.