Phases of Two-Dimensional Heisenberg Spin Systems from Strong-Coupling Expansions

Abstract
A quantum-mechanical Hamiltonian formulation of lattice spin systems is used to search for phase transitions in the O(2), O(3), and O(4) Heisenberg models in two dimensions. Strong-coupling expansions which have been calculated through eighth order indicate a phase transition at nonzero coupling for the O(2) model, but the non-Abelian models are predicted to exist only in the strong-coupling phase.