Dominance of long-lived excitations in the antiferromagnetic spin-1 chain NENP

We have measured the dynamic spin correlation function for the quasi-one-dimensional S=1 antiferromagnet NENP by inelastic magnetic neutron scattering. Long-lived excitations (ωτ>10) exist for 0.3π<=q~<=π, and within error exhuast the total moment sum rule. The dispersion relation retains the fundamental periodicity of the spin chain, explicitly demonstrating the absence of broken translational symmetry in the ground state. The instantaneous spin correlation function follows a square-root Lorentzian for q~~=π but deviates substantially from this form for q~<0.8π.