Short-range antiferromagnetic correlations in high-Tc oxide superconductors

The modified Kondo-Heisenberg model for high-Tc oxide superconductors and the Kondo-Heisenberg model are analysed by the composite-operator expansion method. We study the effects of short-range antiferromagnetic correlations on the density of states and related band dispersion near the Fermi level when the Heisenberg insulator is doped. A set of self-consistent equations for fermionic correlation functions are solved by a numerical calculation. It turns out that the next-to-nearest-neighbour copper spin-spin correlations on extended lattice clusters can be used to predict the width of the band crossing the Fermi level in the same experimental range. Results are compared with previous calculations and a detailed discussion is given on the approximations implemented. (author)