Observation of the Haldane gap in ${\mathrm{RbNiCl}}_3$

The Haldane gap is observed by neutron scattering in ${\mathrm{RbNiCl}}_3$. The three-dimensional-phase magnetic spectrum of ${\mathrm{RbNiCl}}_3$ is found to be very similar to that of ${\mathrm{CsNiCl}}_3$, even though the two materials differ significantly in the exchange parameters. Application of Affleck’s field theory to ${\mathrm{RbNiCl}}_3$ shows that the theory is capable of explaining the dispersion of the gap mode and its relative intensity near the three-dimensional ordering wave vector (1/3,1/3,1). However, the theory does not provide a satisfactory explanation for the nearly degenerate magnon frequencies at (0,0,1). We argue that the degeneracy at (0,0,1) may not be related to the existence of the Haldane gap in integer-spin systems. Rather, it could be a general feature of certain antiferromagnetic materials.