Effect of a temperature dependent effective quasiparticle mass on the surface impedance of YBa2Cu3O7-x

The temperature dependent surface impedance Z(s)(T) of single-crystalline Y Ba2Cu3O7-x (YBCO) was analyzed within the two-fluid model in terms of the f raction of paired charge carriers, f(s)(T), and of the quasiparticle scatte ring time tau(T). The usual approach was extended by considering a temperat ure dependent effective quasiparticle mass m*(T), which results from a stro ng electron-phonon interaction. This effect must not be neglected in the de scription of high-temperature superconductors due to the large ratio of T-c to the Debye temperature T-D. The temperature dependence of the penetratio n depth, lambda(T), of high-quality YBCO crystals and films could be descri bed with an electron-phonon coupling constant Lambda(o) = 4, and using f(s) (T) = 1 - (T/T-c)(2.8), an approximation of the BCS theory. Different trial phonon spectra were encountered in terms of their ability to reproduce the lambda(T)-data. The scattering time tau(T) was described by the Bloch-Grun eisen formalism with T-D = 460 K. Assuming an Einstein spectrum with k(B)T( c)/h Omega(ln) = 0.24, a residual resistivity rho(r) = 1.8 mu Ohm cm and a fraction of unpaired quasiparticles epsilon = 0.04 at T = 0 K yielded a sur prisingly good agreement of the model with Z(s)(T)-data measured at 87 GHz with a high-quality epitaxial YBCO film between T = 4 K and T-c. While an e xact reproduction of the surface impedance asks for a rigorous theoretical computation, our analysis demonstrates that strong electron-phonon coupling is relevant for discussing the unconventional transport properties of YBCO .