Spin Dynamics of Hole Doped {rm Y_{2-x} Ca_x Ba Ni O₅}

We propose an electronic model for the recently discovered hole doped compound ${\rm Y_{2-x} Ca_x Ba Ni O_5 }$. From a multiband Hamiltonian with oxygen and nickel orbitals, a one band model is derived. 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 ${\rm S(q,\omega)}$ in a realistic regime of couplings, spectral weight in the Haldane gap is observed in agreement with neutron scattering data. Low energy states with S=3/2 appear in the model. Several predictions are made to test these ideas.