DYNAMIC PROPERTIES OF A HALDANE-GAP ANTIFERROMAGNET

We study the dynamic spin correlation function of a spin-I antiferromagnetic chain with easy-plane single-ion anisotropy. We use exact diagonalization by the Lanczos method for chains of length up to N = 16 spins. We show that a single-mode approximation is an excellent description of the dynamical properties. A variational calculation allows us to clarify the nature of the excitations. The existence of a two-particle continuum near-zero wavevector is dearly seen, both in finite-size effects and in the dynamical structure factor. The recent neutron scattering experiments on the quasi-one-dimensional antiferromagnet NENP are fully explained by our results.