Eur. Phys. J. B 14, 605-610 (2000)
https://doi.org/10.1007/s100510051068

Effect of a temperature dependent effective quasiparticle mass on the surface impedance of YBa₂Cu₃O_7-x

Fachbereich Physik, Bergische Universität Wuppertal, 42097 Wuppertal, Germany

Received: 22 September 1999
Revised: 3 November 1999
Published online: 15 April 2000

Abstract

The temperature dependent surface impedance Z_s(T) of single-crystalline YBa₂Cu₃O_7-x (YBCO) was analyzed within the two-fluid model in terms of the fraction of paired charge carriers, f_s(T), and of the quasiparticle scattering time _T(T). The usual approach was extended by considering a temperature dependent effective quasiparticle mass m*(T), which results from a strong electron-phonon interaction. This effect must not be neglected in the description of high-temperature superconductors due to the large ratio of Tc to the Debye temperature T_D. The temperature dependence of the penetration depth, λ(T), of high-quality YBCO crystals and films could be described with an electron-phonon coupling constant Λ₀=4, and using f_s(T)=1-(T/T)^2.8 as an approximation of the BCS theory. Different trial phonon spectra were encountered in terms of their ability to reproduce the λ(T)-data. The scattering time _T(T) was described by the Bloch-Grüneisen formalism with T_D=460K. Assuming an Einstein spectrum with k_BTc/ñΩ_n=0.24, a residual resistivity p_r=1.8μΩcm and a fraction of unpaired quasiparticles ξ0.04 at T=0Kyielded a surprisingly good agreement of the model with T=0K-data measured at 87 GHz with a high-quality epitaxial YBCO film between T=4K and Tc. While an exact 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.