CROSSOVER AND ORDER-DISORDER TRANSITION OF 2-DIMENSIONAL QUANTUM ANTIFERROMAGNETS AT LOW-TEMPERATURES

We study the low-temperature behavior of quantum antiferromagnets in two space dimensions. Our model is the quantum nonlinear sigma-model (NL-sigma-M), which is the long-wavelength limit of the two-dimensional quantum Heisenberg model. Introducing fermion fields in NL-sigma-M, we propose a model of doped antiferromagnets. We show that renormalization-group theory predicts an order-disorder transition at finite hole concentration delta(c). Very small doping changes the temperature dependence of the correlation length-xi from exponential to power law, xi is-proportional-to 1/T, for hole concentration delta < delta(c). We also discuss a crossover between classical and quantum regions with varying coupling constant g.