Longitudinal spin fluctuations of coupled integer-spin chains:: Haldane triplet dynamics in the ordered phase of CsNiCl3

By means of high-resolution polarized neutron scattering from the spin-1 chain compound CsNiCl3, 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 Ni2+ spins of CsNiCl3 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. [S0163-1829(98)00437-8].