The role of single-ion excitations in the mixed-spin quasi-1-dimensional quantum antiferromagnet Nd_2BaNiO_5

Inelastic neutron scattering experiments on Nd2BaNiO5 single-crystals and powder samples are used to study the dynamic coupling of 1-dimensional Haldane-gap excitations in the S=1 Ni2+ -chains to local crystal-field transitions, associated with the rare earth ions. Substantial interference between the two types of excitations is observed even in the 1-dimensional paramagnetic phase. Despite that, the results provide solid justification for the previously proposed ``static staggered field'' model for R2BaNiO5 nickelates. The observed behaviour is qualitatively explained by a simple chain-Random-Phase-Approximation (chain-RPA) model.