By means of high-resolution polarized neutron scattering from the spin-1 chain compound ${\mathrm{CsNiCl}}_3,$ we have observed at least one additional mode of spin fluctuations, not contained in spin-wave theory, in the three-dimensional antiferromagnetically ordered phase of the material. The number of observed modes, their polarization, frequency, magnetic field dependence, and intensity show that, in contrast to the doublet-spin dynamics of conventional antiferromagnets, the ${\mathrm{Ni}}^{2+}$ spins of ${\mathrm{CsNiCl}}_3$ manifest triplet fluctuations where one component is parallel to the ordered moment. The results confirm that the Ginsburg-Landau model of Affleck and Wellman gives a realistic description of the dynamics of an ordered array of coupled integer-spin chains exhibiting the Haldane gap. The longitudinal fluctuations exist because the system lies not too far from a quantum critical point.

• Received 17 December 1997

DOI:https://doi.org/10.1103/PhysRevB.59.4235