Phenomenological theory of longitudinal spin fluctuations inCsNiCl3

The recent observation of longitudinal spin excitations at low temperatures in the spin-1 quasi-one-dimensional antiferromagnet ${\mathrm{CsNiCl}}_3$ is contrary to expectations based on conventional spin-wave theory. Affleck [Phys. Rev. Lett. 62, 474 (1989)] has proposed a field-theory model which attributes this unusual gap-mode fluctuation to quantum effects resulting from the Haldane conjecture. We demonstrate here that the field-theory results are reproduced by a phenomenological Lagrangian formulation of spin waves, in the spirit of Landau and Lifshitz, that has existed in the literature for many years.