S(K) FOR HALDANE-GAP ANTIFERROMAGNETS - LARGE-SCALE NUMERICAL RESULTSVERSUS FIELD-THEORY AND EXPERIMENT

The structure function S(k) for the s = 1, Haldane-gap antiferromagnet ic chain is measured accurately using the recent density-matrix renorm alization-group method, with a chain length of 100. Excellent agreemen t with the nonlinear-sigma-model prediction is obtained, both at k alm ost-equal-to pi, where a single-magnon process dominates, and at k alm ost-equal-to 0, where a two-magnon process dominates. We repeat our ca lculation with crystal-field anisotropy chosen to model Ni(C2H8N2)2NO2 (ClO4), obtaining good agreement with both field-theory predictions an d recent experiments. Correlation lengths, gaps, and velocities are de termined for both polarizations.