Application of Kondo lattice theory to high-temperature superconductivity and pseudogaps in cuprate oxides

Abstract

A theory of Kondo lattices is developed for the t−J model on a square lattice. The spin susceptibility is described in a form consistent with a physical picture of Kondo lattices: Local spin fluctuations at different sites interact with each other by a bare intersite exchange interaction, which is mainly composed of two terms such as the superexchange interaction, which arises from the virtual exchange of spin-channel pair excitations of electrons across the Mott-Hubbard gap, and an exchange interaction arising from that of Gutzwiller’s quasi-particles. The bare exchange interaction is enhanced by intersite spin fluctuations, whose development is caused by itself. The enhanced exchange interaction is responsible for the formation and condensation of dγ-wave Cooper pairs between Gutzwiller’s quasiparticles. On the basis of the microscopic theory, we develop a phenomenological theory of low-temperature superconductivity and pseudogaps in the underdoped region as well as high-temperature superconductivity in the optimal-doped region. Anisotropic pseudogaps open because of dγ-wave superconducting low-energy fluctuations: Quasiparticle spectra around (±π/a,0) and (0,±π/a), with a the lattice constant, or X points at the chemical potential are swept away by strong inelastic scatterings, and quasiparticles are well defined only around (±π/2a,±π/2a) on the Fermi surface or line. As temperatures decrease in the vicinity of superconducting critical temperatures, pseudogaps become smaller and the well-defined region extends toward X points. The condensation of dγ-wave Cooper pairs eventually occurs at low enough temperatures when the pair breaking by inelastic scatterings becomes small enough.

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