OBSERVATION OF THE HALDANE GAP IN RBNICL3

The Haldane gap is observed by neutron scattering in RbNiCl3. The three-dimensional-phase magnetic spectrum of RbNiCl3 is found to be very similar to that of CsNiCl3, even though the two materials differ significantly in the exchange parameters. Application of Affleck's field theory to RbNiCl3 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.