The spin dynamics in $S=3/2,$ one-dimensional (1D) Heisenberg antiferromagnets, ${\mathrm{CsVCl}}_3$ and ${\mathrm{CsVBr}}_3,$ was investigated using inelastic neutron scattering. The magnetic excitations were measured at temperatures above the three-dimensional ordering temperature in order to discuss the 1D properties in the present systems. We report on the spin dynamics at low temperatures and its temperature (T) dependence. From the observed dispersion relation, using the renormalization constant calculated from the quantum Monte Carlo method, we obtained the exchange constant (J) in good agreement with those taken from bulk-susceptibility measurements. The observed structure factor was well described by that calculated from the parameters describing the dispersion relation. The energy width (Γ) was independent of the 1D momentum transfer (q) at large energy transfers, however, its q dependence exhibits the minimum at the magnetic zone center. The parameters describing the energy scale in the spin dynamics at low T were found to be scaled by J. We also investigated the T dependence of the spin dynamics: $\Gamma \mathrm{}\left(T\right)\mathrm{}$ can be scaled by J, $\Gamma \mathrm{}\left(T\right)\mathrm{}$ at $T>J$ is proportional to T as predicted by classical theory but at $T>J,$ Γ decreases with decreasing T and becomes finite. On the other hand, we found that $\kappa \mathrm{}\left(T\right)\mathrm{}$ is proportional to T at any T. The scaling by J in the dynamics at low T as well as $\Gamma \mathrm{}\left(T\right)\mathrm{}$ suggests that the observed spin dynamics is of 1D origin. We conclude that the spin dynamics at high temperatures $T>J,$ is well described by classical theory, however, at low temperatures $T<J,$ the finite width indicates some quantum fluctuations. Therefore, we observed the crossover from the quantum state at low T to the classical state at high T in $S=3/2\mathrm{}$ systems.

• Received 14 December 1998

DOI:https://doi.org/10.1103/PhysRevB.59.14406