We present a numerical diagonalization study of two one-dimensional
S=21 antiferromagnetic Heisenberg chains, having nearest-neighbor and Haldane-Shastry (
r21) interactions, respectively. We have obtained the
T=0 dynamical correlation function, ${S}^{\ensuremath{\alpha}\ensuremath{\alpha}}(q, \ensuremath{\omega})$, for chains of length $N=8\ensuremath{-}28$. We have studied ${S}^{\mathrm{zz}}(q, \ensuremath{\omega})$ for the Heisenberg chain in zero field, and from finite-size scaling we have obtained a limiting behavior that for large $\ensuremath{\omega}$ deviates from the conjecture proposed earlier by M\"uller et al. For both chains we describe the behavior of ${S}^{\mathrm{zz}}(q, \ensuremath{\omega})$ and ${S}^{\mathrm{xx}}(q, \ensuremath{\omega})$ for selected values of the applied field, and compare with previous work by M\"uller et al. and Talstra and Haldane. Suggestions for future finite-field neutron scattering experiments are made.
Neutron scattering has shown that the lowest spin excitations in the spin-1 antiferromagnet
CsNiCl3 in its one-dimensional phase occur at finite frequency. After allowance for the known weak interchain coupling the gap is found to be in good agreement with recent numerical calculations. The results support the Haldane conjecture that in integral-spin chains with isotropic interactions a gap separates the ground state from the excitations.