Influence of finite Hund rules and charge transfer on properties of Haldane systems

Abstract

We consider the Kondo-Hubbard model with ferromagnetic exchange coupling $% J_{H}$, showing that it is an approximate effective model for late transition metal-O linear systems. We study the dependence of the charge and spin gaps $\Delta_{C}$, $\Delta_{S}$, and several spin-spin correlation functions, including the hidden order parameter $Z(\pi)$, as functions of $J_{H}/t$ and $U/t$, by numerical diagonalization of finite systems. Except for $Z(\pi)$, all properties converge slowly to the strong-coupling limit. When $J_{H}/t\sim 2$ and $U/t\sim 7$ (the effective parameters that we obtain for Y_2BaNiO_5), $\Delta_{S}$ is roughly half of the value expected from a strong-coupling expansion.