EQUAL-TIME CORRELATIONS IN HALDANE-GAP ANTIFERROMAGNETS

The S = 1 antiferromagnetic Heisenberg chain both with and without sin gle-ion anisotropy is studied. Using the recently proposed density-mat rix renormalization-group technique of White, we calculate the energy gaps as well as several different correlation functions. The two gaps, DELTA(parallel-to) and DELTA(perpendicular-to), along with associated correlation lengths and velocities are determined. The numerical resu lts are shown to be in good agreement with theoretical predictions der ived from the nonlinear sigma model and a free-boson model. We also st udy the S = 1/2 excitations that occur at the ends of open chains; in particular we study the behavior associated with open boundary conditi ons, using a model of S = 1/2 spins coupled to the free bosons.