Continuum spin excitations in S=1 one-dimensional antiferromagnet

We present detailed measurement of the non-hydrodynamic part of the spin excitation spectrum in the model quasi-1D S=1 antiferromagnet CsNiCl3 by inelastic magnetic neutron scattering. In the better part of the Brillouin zone the effect of the inter-chain coupling is negligible, and spin dynamics reflects that of a single Haldane chain. We find that at q ~< 0.6pi 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.