1/Nexpansion and long range antiferromagnetic order

Abstract
The staggered magnetization of the Heisenberg antiferromagnet in two dimensions can be systematically approximated by a 1/N expansion. Cancellation between self energy diagrams leads to a Luttinger-like theorem for the ground state. We prove (for a smooth enough self energy) that the long range order of mean field theory ($N$=$\infty$) survives corrections to all orders of 1/N. Divergences of this series provides a new route to the disordered phases of quantum antiferromagnets.Comment: 9 LaTeX pages. Figures upon request. Tech-10
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