Role of single-ion excitations in the mixed-spin quasi-one-dimensional quantum antiferromagnet ${\mathrm{Nd}}_2{\mathrm{BaNiO}}_5$

Inelastic neutron scattering experiments on ${\mathrm{Nd}}_2{\mathrm{BaNiO}}_5$ single crystals and powder samples are used to study the dynamic coupling of one-dimensional Haldane-gap excitations in the $S=1\mathrm{}$ ${\mathrm{Ni}}^{2+}$-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 one-dimensional paramagnetic phase. Despite that, the results provide solid justification for the previously proposed “static staggered field” model for $R_2{\mathrm{BaNiO}}_5$ nickelates. The observed behavior is qualitatively explained by a simple chain-random-phase-approximation model.