Spin Dynamics of Hole DopedY2−xCaxBaNiO5

We propose an electronic model for the recently discovered hole doped compound ${\mathrm{Y}}_{2-x}{\mathrm{Ca}}_x{\mathrm{BaNiO}}_5$. From a multiband Hamiltonian with oxygen and nickel orbitals, a one band model is discussed. Holes are described using Zhang-Rice-like $S=\frac{1}{2}$ states at the nickels propagating on a $S=1\mathrm{}$ 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.