The spin correlations of the S=1 Heisenberg antiferromagnetic chains in ${\mathrm{AgVP}}_2$${\mathrm{S}}_6$ have been studied by neutron scattering at temperatures of the order of T∼${10}^{\mathrm{-}2}$ J/${\mathit{k}}_{\mathit{B}}$. On the polycrystalline samples, with a careful choice of experimental conditions, we have observed a Haldane gap of ${\mathit{E}}_{\mathit{g}}$=26 meV at the antiferromagnetic point at q=π. The excitation spectra are consistent with a mode that has a spin-wave velocity C=150 meV and a correlation length ξ≊5.5, close to C/${\mathit{E}}_{\mathit{g}}$. These results are in quantitative agreement with the predictions and numerical results on the Haldane state.

• Received 28 March 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.497