FIELD-THEORETICAL CALCULATIONS ON THE HALDANE GAP OF QUASI-ONE-DIMENSIONAL HEISENBERG ANTIFERROMAGNETS

One-dimensional Heisenberg antiferromagnets can be described by the O(3) nonlinear sigma-model (NL sigma M). We give a review on zero temperature and finite temperature Haldane gaps obtained from this model. Based on the coupled-chain theory, we derive the finite temperature Haldane gap for triangular antiferromagnet RbNiCl3. The Neel temperature is obtained as 11 K. In order to treat the anisotropy in crystal Ni(C2H8N2)(2)NO2(ClO4), we relax the constraint of the NL sigma M and compute the finite temperature Haldane gap using a Ginzburg-Landau mean field approach. The comparison with the experimental data is discussed.