Kinematic Interaction for the Heisenberg Antiferromagnet at Low Temperature

The general theory of antiferromagnetic spin waves is examined from a new viewpoint and the nature of the antiferromagnetic ground state is clarified. It is shown that the Anderson canonical transformation to free spin waves is nonunitary with the magnon state vectors having large nonphysical projections, and a projection technique which restores unitarity at low temperatures is developed. The nonphysical projection of the state vectors is shown to give a large kinematic interaction even at zero temperature.