Anti-renormalization of paramagnetic fluctuations in CsMnBr3

The critical behavior of the spin-wave dispersion in the triangular antiferromagnet CsMnBr3 was investigated in the paramagnetic phase using inelastic neutron scattering. Unexpectedly, we find that the energy gap of the excitations increases with increasing temperature. This effect cannot be explained by perturbed spin wave theory taking only intra-chain exchange interactions into account. We speculate that a description of the spin dynamics in terms of an extension of an approach for an S = 1/2 model for a Heisenberg chain to S = 5/2 may be more appropriate to explain our results.