1/N EXPANSION AND LONG-RANGE ANTIFERROMAGNETIC ORDER

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=infinity) survives corrections to all orders of 1/N. Divergences of this series provide a new route to the disordered phases of quantum antiferromagnets.