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

Abstract

We consider the Kondo-Hubbard model with ferromagnetic exchange coupling JH , 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 ∆C , ∆S, and several spin-spin correlation functions, including the hidden order parameter Z(π), as functions of JH /t and U/t, by numerical diagonalization of finite systems. Except for Z(π), all properties converge slowly to the strong-coupling limit. When JH /t ∼ 2 and U/t ∼ 7 (the effective parameters that we obtain for Y2BaNiO5), ∆S is roughly half of the value expected from a strong-coupling expansion.