O(3) NonlinearσModel and the Topological Distinction between Integer- and Half-Integer-Spin Antiferromagnets in Two Dimensions

It is shown that the O(3) nonlinear σ model derived from the quantum Heisenberg antiferromagnet in two spatial dimensions, under the assumption that the N\'eel field ^Ω(r,t) is well defined at all points in (2+1)D space-time, has no Hopf term, in contradiction to recent speculation. However, the amplitudes for tunneling between configurations with different spatial topologies reveal a difference between integer and half-integer, and between even- and odd-integer, spins.