Nonlinear sigma models and quantum spin systems - art. no. 184439

Microscopic models of quantum anti ferromagnets are investigated on the bas is of a mapping onto effective low-energy Hamiltonians. Lattice effects are carefully taken into account and their role is discussed. We show that the presence of an external staggered magnetic field modifies in a nontrivial way the usual mapping onto the nonlinear sigma model, leading to the appear ance of terms neglected in previous works. Our analysis is compared with La nezos diagonalizations of S = 1 Heisenberg chains in a staggered field, con firming the validity of the single-mode approximation for the evaluation of the dynamical structure factor. The results are relevant for the interpret ation of experiments in quasi-one-dimensional compounds. Microscopic realiz ations of SU(4) spin chains are also discussed in the framework of spin-orb ital lattice systems. The low-energy physics is shown to be described by si gma models with topological angle theta in one dimension. This mapping stro ngly suggests that the one-dimensional CP3 model (with theta = pi) undergoe s a second-order phase transition as a function of the coupling.