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 Luttingerlike theorem for the ground state. We prove (for a smooth enough self energy) that the long range order of mean field theory (N=∞) survives corrections to all orders of 1/N . Divergences of this series provides a new route to the disordered phases of quantum antiferromagnets. PACS number(s): 75.10.Jm, 75.50.Ee, 67.40.Db Email: [email protected] Email: [email protected]