Mechanism of hole attraction in the extended Hubbard model.

Abstract

The expansion in the Cu-O hybridization energy, t, usually used for the simplification of the extended Hubbard Hamiltonian, is shown to meet with difficulties for the known parameters of ${\mathrm{CuO}}_{2}$ planes of cuprate perovskites. An expansion in a power series in \ensuremath{\lambda}t, \ensuremath{\lambda}\ensuremath{\approxeq}0.1, is suggested, which is applicable for these values of parameters and in which the Hubbard repulsion and the Cu-O hybridization are considered on an equal footing. A Hamiltonian obtained with the help of the expansion for the lower part of the energy spectrum, is equivalent to the t-J Hamiltonian and the corresponding states are some generalizations of the Zhang-Rice singlets. The Hamiltonian contains terms describing a static attraction between holes and for reasonable sets of parameters the attraction is approximately equal to a half of the superexchange constant.