Abstract

Superconducting properties of La 2CuO 4 are studied within the model of weak coupling for the Emery hamiltonian. The weak coupling model for the 2D CuO 2 lattice is obtained by means of the canonical transformation from the strong coupling model. The elementary excitations within the Emery model are the Zhang-Rice magnetic polarons whose motion depends essentially on the spin state of the Cu-sublattice. The effective spectrum of magnetic polarons for the RVB ground state is shown to satisfy the nesting condition near the Fermi surface in some narrow range of doping level. Several mechanisms are found to contribute to effective interaction, i.e. the usual Hubbard repulsion and sign changing momentum-dependent correlation contributions. Superconducting d-pairing emerges provided repulsion interaction dominates, and s-pairing can be realized if the sign-changing interaction gives the main contribution. In both cases the superconductivity arises owing to nesting which is responsible for the strong anisotropy of the pairing interaction. Nesting conditions are not satisfied for the Neel state and hence superconductivity does not arise in this case.

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