LONGITUDINAL MODES IN QUASI-ONE-DIMENSIONAL ANTIFERROMAGNETS

Neutron-scattering data on CsNiCl3, a quasi-one-dimensional spin-one antiferromagnet, exhibit an anomalous mode. It was later proposed, based on a Landau-Ginsburg model, that this should be viewed as a longitudinal fluctuation of the sublattice magnetization. This theory is elaborated in more detail here and compared with experimental data on CsNiCl3 and RbNiCl3. In particular, we give explicitly a renormalization-group argument for the existence of such modes in Neel-ordered antiferromagnets which are nearly disordered by quantum fluctuations, due to quasi-one-dimensionality or other effects. We then discuss the non-Neel case of a stacked triangular lattice such as CsNiCl3 where longitudinal and transverse modes mix. In this case the quantum disorder transition is driven first order by fluctuations and the longitudinal mode always has a finite width. Effects of a magnetic field on the magnon spectrum are calculated both in conventional spin-wave theory and in the Landau-Ginsburg model and are compared with experimental data on CsNiCl3. This model is compared with an alternative Lagrangian-based one that was proposed recently.