Large-Nexpansion 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.