Temperature dependence of surface impedance of Tl2Ba2CaCu2O8−δ and YBa2Cu3O6.95 single crystals measured in the microwave band

The real part Rs and the imaginary part Xs of the surface impedance Zs=Rs+ iXs of Tl2Ba2CaCu2O8−δ and YBa2Cu3O6.95 single crystals have been measured with high precision at frequency ω/2π=9.4 GHz in the temperature range 0<T<140 K. In the Tl2Ba2CaCu2O8−δ crystal a linear temperature dependence Rs(T) has been found for T⩽50 K, and the magnetic field penetration depth λ(4.2 K)=Xs(4.2 K)/ω μ0≈3760 Å has been measured. Along with well known features of the function Zs(T) in high-quality YBa2Cu3O6.95 single crystals, such as the linearity of λ(T) and Rs(T) for T<Tc/3 and a maximum of Rs(T) at T∼Tc/2, the linearity range of λ(T) extends to T≃50 K, and this curve has a plateau in the range 60<T<85 K. The curve of Rs(T) in both the superconducting and normal states of YBa2Cu3O6.95 is well described by a two-fluid model with the electron-phonon mechanism of quasiparticle relaxation. A formula describing the curve of λ2(0)/λ2(T) throughout the studied temperature range is also given.