Temperature dependence of surface impedance of Tl2Ba_2CaCu_2O_8 – and YBa_2Cu_3O_(6.95) single crystals measured in the microwave band

摘要

The real part Rs and the imaginary part Xs of the surface impedance Zs = Rs + iXs of Tl2Ba2CaCu2O8 – and YBa2Cu3O6.95 single crystals have been measured with high precision at frequency /2= 9.4 GHz in the temperature range 0 < T < 140 K. In the Tl2Ba2CaCu2O8 – crystal a linear temperature dependence Rs(T) has been found for T50 K, and the magnetic field penetration depth (4.2 K) = Xs(4.2 K)/μ03760 ? 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 (T) and Rs(T) for T < Tc/3 and a maximum of Rs(T) at T ~ Tc/2, the linearity range of (T) extends to T50 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 2(0)/2(T) throughout the studied temperature range is also given.