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

We consider the Kondo-Hubbard model with ferromagnetic exchange coupling JH , showing that it is an approximate effective model for late transition-met al-O linear systems. We study the dependence of the charge and spin gaps: D elta(C), Delta(S), and several spin-spin correlation functions, including t he hidden order parameter Z(pi), as functions of J(H)/t and U/t, by numeric al diagonalization of finite systems. Except for Z(pi), all properties conv erge slowly to the strong-coupling limit. When J(H)/t similar to 2 and U/t similar to 7 (the effective parameters that we obtain, for Y2BaNiO5), Delta (S) is roughly half Of the value expected from a strong-coupling expansion. [S0163-1829(99)05215-7].