Neutron scattering study of magnetic excitation in a one-dimensional Heisenberg antiferromagnet CsVCl3

We report a high energy-resolution neutron scattering study of magnetic excitations in a one-dimensional Heisenberg antiferromagnet CsVCl3 with S = 3/2. Because of the weak antiferromagnetic coupling J'/J ∼ 10-4 in the hexagonal basal plane, the spin direction rotates by 120 deg with respect to the nearest neighbor spins, and the spinwave dispersion in the basal plane consists of three branches. In the present experiment we observed an almost dispersionless mode and an acoustic branch in neutron profiles which are in good agreement with the previous studies. Accurate absolute cross-section measurements revealed, however, that the intensity of the acoustic branch could be accounted for neither by the linear spin-wave theory (the observed intensity is weaker by a factor 33) nor by the magneto-vibrational scattering (stronger by two orders of magnitude). Quantum effects, which cause large corrections in quasi-one dimensional antiferromagnets, are also discussed on the basis of the 1/S expansion, but also fall short of providing adequate explanation. (author)