Origin of high-temperature superconductivity in copper oxides - clues from the normal-state resistivity

Abstract

The in-plane resistivity data, ϱ ∥( 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 ∥( T) above T c is observed in the electron-doped NdCeCuO system and several relatively low- T c hole-doped cuprates. On the other hand, the high- T c Cu-oxides are always characterized by a linear temperature dependence of ϱ ∥( T). Within the framework of the Fermi-liquid model, the correlation of T c with the temperature variation of T c 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 ∥( T) and the magnitude of T c. Both the T 2 and the T dependences are due to electron-electron interaction and the linear temperature variation of ϱ ∥( T) is mostly a manifestation of partial FS nesting. Even with a modest electron-phonon interaction (λ≲1), a T c 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 E F. Such a van Hove like singularity in N( E) has been proposed in the past as a T c-enhancement mechanism for A15 superconductors and more recently for the Cu oxides. It is shown that T c can be limited by various N( E) broadening effects on the scale of k B T c. However, these effects are not sufficient to invalidate the high- T c mechanism in Cu oxides. In addition, experimental findings such as the near-zero isotope effect, the Cu-substitution effect on T c, and the lack of correlation between λ and energy gap to T c ratio 2Δ(0)/ k B T c are consistent with this high- T c mechanism.