Multiparticle states in the S = 1 chain system CsNiCl₃

A continuum of magnetic states has been observed by neutron scattering from the spin-1 chain compound CsNiCl₃ in its disordered gapped one-dimensional phase. Results using both triple-axis and time-of-flight spectrometers show that around the antiferromagnetic point Q_c = π, the continuum lies higher in energy than the Haldane gapped excitations. At 6K the integrated intensity of the continuum is about 12(2)% of the total spectral weight. This result is considerably larger than the 1%–3% weight predicted by the nonlinear sigma model for the three-particle continuum.