Neutron-scattering study of the dynamic spin correlations inCsNiCl3above Néel ordering

An experimental study of the spectral distribution of magnetic correlations in the quasi-one-dimensional antiferromagnet ${\mathrm{CsNiCl}}_3$ above the Néel temperature has been performed. Using the cold-source three-axis spectrometer with the energy resolution 0.02 THz, we made a number of q scans around the antiferromagnetic Bragg point (1/3, 1) / 3 ,1) at different energies. This allowed us to distinguish between the quasielastic and dynamic contributions to S(q,ω) in ${\mathrm{CsNiCl}}_3$ at different temperatures. Attention was also paid to the separate examination of the temperature evolution of the in-chain and interchain correlations. It is shown that the interchain and the low-energy correlations (including three-dimensional critical scattering) gradually disappear above ${\mathit{T}}_{\mathit{N}}$=4.84 K, giving rise to well-defined gapped excitations, as predicted by Haldane. These excitations, notably broadening, persist up to 21 K, in agreement with the previous studies.