SIGMA-MODEL STUDY OF HALDANE-GAP ANTIFERROMAGNETS

We use a quantum O(N) nonlinear sigma model to describe one-dimensiona l quantum Heisenberg antiferromagnets with integer spin S. A large N a nalytical treatment of this effective theory leads to definite predict ions for thermodynamic quantities in the low-temperature regime. We ob tain 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 mag netic susceptibility are also obtained and comparison with exact numer ical results for the S = 1 spin chain allows us to determine the domai n of validity of our approximation. By a calculation at subleading 1/N order we compute the quasiparticle lifetime. Results are given on-she ll, for the width observed at the quasiparticle pole as seen in neutro n scattering experiments, and also off-shell, for the NMR relaxation r ate T1(-1). The phenomenological interacting boson model is compared w ith our approach. We discuss the effects of anisotropy and compare our results with present experiments on NENP.