The spin-excitation continuum, expected to dominate the low-energy fluctuation spectrum in the Haldane spin chain around the Brillouin zone center, $q=0\mathrm{}$, is directly observed by inelastic magnetic neutron scattering in the $S=1\mathrm{}$ quasi-1D antiferromagnet ${\mathrm{CsNiCl}}_3$. We find that the single mode approximation fails, and that a finite energy width appears in the dynamic correlation function $S(q,\omega )$ for $q\lesssim 0.5\pi$. The width increases with decreasing $q$, while $S(q,\omega )$ acquires an asymmetric shape qualitatively similar to that predicted for the two-magnon continuum in the nonlinear $\sigma$-model.

• Received 4 October 2000

DOI:https://doi.org/10.1103/PhysRevLett.87.017202