Exact dynamics in the Haldane-Shastry spin chain model in magnetic field

Dynamical spin correlation function S-zz(Q, w) with Q smaller than pi (m) o ver bar ((m) over bar being magnetization) is derived exactly for the spin chain with the 1/r(2) interaction in magnetic field. It is found that only two spinons and one antispinon contribute to S-zz(Q, w) in the thermodynami c limit. In deriving the result we use the solution of the Sutherland model with internal symmetry in the strong coupling limit. Our theory keeps the full symmetry of the model in contrast to the previous one which is based o n mapping onto the single-component Sutherland model. (C) 2000 Elsevier Sci ence B.V. All rights reserved.