**A.A. Zhukov,
M.R. Trunin, A.T. Sokolov, N.N. Kolesnikov.**

Temperature dependence of surface
impedance of Tl_{2}Ba_{2}CaCuO_{8-d}
and YBa_{2}Cu_{3}O_{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
Tl_{2}Ba_{2}CaCuO_{8-d }
and YBa_{2}Cu_{3}O_{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 Tl_{2}Ba_{2}CaCuO_{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)/wm_{0}
@
3760 Angstrom has been measured. Along with well known features of the
function *Z*_{s}(T) in high-quality YBa_{2}Cu_{3}O_{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 YBa_{2}Cu_{3}O_{6.95
}is well described by a two-fluid model with the electron-phonon mechanism
of quasiparticle relaxation. A formula describing the curve of

l^{2}(0)/l^{2}(*T*)
throughout the studied temperature range is also given.