Continuum in the spin-excitation spectrum of a haldane chain observed by neutron scattering in CsNiCl3.

The spin-excitation continuum, expected to dominate the low-energy fluctuation spectrum in the Haldane spin chain around the Brillouin zone center, q = 0, is directly observed by inelastic magnetic neutron scattering in the S = 1 quasi-1D antiferromagnet CsNiCl3. 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 less, similar 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.