A two-dimensional condensed-matter lattice model is presented which exhibits a nonzero quantization of the Hall conductance a" in the absence of an external magnetic field. Massless fermions without spectral doubling occur at critical values of the model parameters, and exhibit the so-called "parity anomaly" of (2+1)-dimensional field theories.

The continuum field theory describing the low-energy dynamics of the large-spin onedimensional Heisenberg Bntiferromagnet is found to be the O(3) nonlinear sigma model. When weak easy-axis anisotropy is present, soliton solutions of the equations of motion are obtained and semiclassically quantized. Integer and half-integer spin systems are distinguished.

No abstract

No abstract

No abstract