Effect of a temperature dependent effective quasiparticle mass on the surface impedance of YBa_2Cu_3O_(7 - x)

摘要

The temperature dependent surface impedance Z_s(T) of single-crystalline YBa_2Cu_3O_(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 τ(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 mush not be neglected in the description of high-temperature superconductors due to the large ratio of T_c 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 Λ_0 = 4, and using f_s(T) = 1 - (T/T_c)~(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 τ(T) was described by the Bloch-Gruneisen formalism with T_D = 460 K. Assuming an Einstein spectrum with κ_B/T_c/hΩ_(ln) = 0.24, a residual resistivity ρ_r = 1.8 μΩcm and a fraction of unpaired quasiparticles ε = 0.04 at T = 0 K yielded a surprisingly 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 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.