Continuum spin excitations in S=1 one-dimensional antiferromagnet

We present detailed measurement of the nonhydrodynamic part of the spin excitation spectrum in the model quasi-one-dimensional S=1 antiferromagnet CsNiCl 3 by inelastic magnetic neutron scattering. In the better part of the Brillouin zone the effect of interchain coupling is negligible, and the spin dynamics reflects that of a single Haldane chain. We find that at q≲0.6π this quantum spin system ceases to support coherent propagating excitation, which gradually turns into a continuum band of states, whose width grows with decreasing q. This finding is consistently verified under different resolution conditions, obtained with two different high-luminosity experimental setups.