Spin dynamics of hole doped Y2-xCaxBaNiO5

We propose an electronic model for the recently discovered hole doped compound Y2-xCaxBaNiO5. From a multiband Hamiltonian with oxygen and nickel orbitals, a one band model is discussed. Holes are described using Zhang-Rice-like S = 1/2 states at the nickels propagating on a S = 1 spin chain. Using numerical techniques to calculate the dynamical spin structure factor S(q, omega) in a realistic regime of couplings, spectral weight in the Haldane gap is observed in agreement with neutron scattering data. The case of static defects relevant for Zn-doped chains is also discussed. Ferromagnetic states at high hole mobility are favored in our model, contrary to what occurs in the 1D t-J model.