Large-N expansion for frustrated quantum antiferromagnets

A large-N expansion technique based on symplectic [Sp(N)] symmetry for frustrated magnetic systems is proposed and applied to the square-lattice quantum antiferromagnet with first-, second-, and third-neighbor antiferromangetic coupling. In addition to disordered states similar to those in unfrustrated systems, phases with incommensurate coplanar spin correlations and unconfined bosonic spinons are found. The occurrence of ‘‘order from disorder’’ is discussed. Neither chirally ordered nor spin-nematic states are found.