SPIN DYNAMICS OF HOLE-DOPED Y2BANIO5

Starting from a multiband Hamiltonian containing the relevant Ni and O orbitals, we derive an effective Hamiltonian H-eff for the low-energy physics of doped Y2BaNiO5. For hole doping, H-eff describes O fermion s interacting with S = 1 Ni spins in a chain, and cannot be further re duced to a simple one-band model. Using numerical techniques, we obtai n a dynamical spin structure factor with weight inside the Haldane gap . The nature of these low-energy excitations is identified and the eme rging physical picture is consistent with most of the experimental inf ormation on Y2 - xCaxBaNiO5.