Role of single-ion excitations in the mixed-spin quasi-one-dimensional quantum antiferromagnet Nd2BaNiO5

Inelastic neutron scattering experiments on Nd2BaNiO5 single crystals and p owder samples are used to study the dynamic coupling of one-dimensional Hal dane-gap excitations in the S=1 Ni2+-chains to local crystal-field transiti ons, associated with the rare earth ions. Substantial interference between the two types of excitations is observed even in the one-dimensional parama gnetic phase. Despite that, the results provide solid justification for the previously proposed "static staggered field" model for R2BaNiO5 nickelates . The observed behavior is qualitatively explained by a simple chain-random -phase-approximation model.