  Polarized and unpolarized inelastic neutron scattering is used to study
Haldane-gap excitations in the mixed-spin linear-chain antiferromagnet
Nd2BaNiO5. The longitudinal mode, polarized along the direction of ordered
moments, is observed for the first time. The model of isolated Haldane chains
in a static staggered exchange field, that is known to work very well for
static properties and transverse spin excitations in R2BaNiO5 compounds, fails
to explain new results for the longitudinal spin gap.
