SIGMA-MODEL STUDY OF HALDANE-GAP ANTIFERROMAGNETS

We use a quantum O(N) nonlinear sigma model to describe one-dimensional quantum Heisenberg antiferromagnets with integer spin S. A large N analytical treatment of this effective theory leads to definite predictions for thermodynamic quantities in the low-temperature regime. We obtain the Haldane gap as a function of the temperature and note a trend similar to that found in the compound NENP. The specific heat and magnetic susceptibility are also obtained and comparison with exact numerical results for the S = 1 spin chain allows us to determine the domain of validity of our approximation. By a calculation at subleading 1/N order we compute the quasiparticle lifetime. Results are given on-shell, for the width observed at the quasiparticle pole as seen in neutron scattering experiments, and also off-shell, for the NMR relaxation rate T1(-1). The phenomenological interacting boson model is compared with our approach. We discuss the effects of anisotropy and compare our results with present experiments on NENP.