Nonlinear sigma model treatment of quantum antiferromagnets in a magnetic field

We present a theoretical analysis of the properties of low-dimensional quan tum antiferromagnets in applied magnetic fields. In a nonlinear a model des cription, we use a spin stiffness analysis, a 1/N expansion, and a renormal ization group approach to describe the broken-symmetry regimes of finite ma gnetization, and, in cases of most interest, a low-field regime where symme try is restored by quantum fluctuations. We compute the magnetization, crit ical fields, spin correlation functions, and decay exponents accessible by nuclear magnetic resonance experiments. The model is relevant to many syste ms exhibiting Haldane physics, and provides good agreement with data for th e two-chain spin ladder compound CuHpCl.