ORIGIN OF HIGH-TEMPERATURE SUPERCONDUCTIVITY IN COPPER OXIDES - CLUES FROM THE NORMAL-STATE RESISTIVITY

The in-plane resistivity data, ρ{variant}∥(T), as a function of temperature for various Cu-oxide superconductors have provided strong evidence for Fermi-liquid normal state and also important clues for understanding the mechanism responsible for high-temperature superconductivity. A dominant quadratic temperature dependence of ρ{variant}∥(T) aboveTc is observed in the electron-doped NdCeCuO system and several relatively low-Tc hole-doped cuprates. On the other hand, the high-Tc Cu-oxides are always characterized by a linear temperature dependence of ρ{variant}∥(T). Within the framework of the Fermi-liquid model, the correlation of Tc with the temperature variation of Tc can be understood as caused by effects arising from a two-dimensional Fermi surface (FS). The flatness of FS and the amount of nesting determine the temperature dependence of ρ{variant}∥(T) and the magnitude of Tc. Both the T2 and the T dependences are due to electron-electron interaction and the linear temperature variation of ρ{variant}∥(T) is mostly a manifestation of partial FS nesting. Even with a modest electron-phonon interaction (λ≲1), a Tc of the order of 100 K can be achieved with the aid of a sharply varying density of states N(E) near the Fermi energy EF. Such a van Hove like singularity in N(E) has been proposed in the past as a Tc-enhancement mechanism for A15 superconductors and more recently for the Cu oxides. It is shown that Tc can be limited by various N(E) broadening effects on the scale of kBTc. However, these effects are not sufficient to invalidate the high-Tc mechanism in Cu oxides. In addition, experimental findings such as the near-zero isotope effect, the Cu-substitution effect on Tc, and the lack of correlation between λ and energy gap to Tc ratio 2Δ(0)/kBTc are consistent with this high-Tc mechanism. © 1990.