  We investigate spin-vortex excitations in the two-dimensional antiferromagnet
on the basis of the nonlinear sigma model. The model of two-dimensional
Heisenberg quantum antiferromagnet is mapped onto the (2+1)D nonlinear sigma
model. The 2D nonlinear sigma model has an instanton (or skyrmion) solution
which describes an excitation of spin-vortex type. Quantum fluctuations of
instantons are reduced to the study of the Coulomb gas, and the gas of
instantons of the 2D nonlinear sigma model is in the plasma phase. We
generalize this picture of instanton gas to the (2+1)D nonlinear sigma model.
We show, using some approximation, that there is a Kosterlitz-Thouless
transition from the plasma phase to the molecular phase as the temperature is
lowered.
