Effect of a temperature dependent effective quasiparticle mass on the surface impedance of YBa2Cu3O7-x
Fachbereich Physik, Bergische Universität Wuppertal, 42097 Wuppertal, Germany
Revised: 3 November 1999
Published online: 15 April 2000
The temperature dependent surface impedance Zs(T) of single-crystalline YBa2Cu3O7-x (YBCO) was analyzed within the two-fluid model in terms of the fraction of paired charge carriers, fs(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 TD. 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 fs(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 TD=460K. Assuming an Einstein spectrum with kBTc/ñΩn=0.24, a residual resistivity pr=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.
PACS: 74.25.Nf – Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.) / 74.20.De – Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) / 74.76.Bz – High-Tc films
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2000