DISPERSION OF MAGNETIC EXCITATIONS IN A SPIN-1 CHAIN WITH EASY-PLANE ANISOTROPY

We study the dispersion of magnetic excitations in a Heisenberg antiferromagnetic chain of spin S = 1 with exchange J and single-ion anisotropy D. Chains of length up to 18 sites are studied by a Lanczos method. For the Haldane phase D < D(c) almost-equal-to J we show that the gap at zero momentum and S(z) = 1 is given by the sum of the gaps in the sectors S(z) = 1 and S(z) = 0 at momentum pi. The transition region near D = J is likely to be reduced to an isolated point. Large-D perturbation theory is smoothly recovered as soon as D > 2J. We discuss the implications for experiments on Ni(C2H8N2)2NO2ClO4 (NENP) as well as CsFeBr3.