Two Step Restoration of SU(2) Symmetry in a Frustrated Ring-Exchange Magnet

Abstract

We demonstrate the existence of a spin-nematic, moment-free phase in a quantum four-spin ring exchange model on the square lattice. This unusual quantum state is created by the interplay of frustration and quantum fluctuations which lead to a partial restoration of SU(2) symmetry when going from a four-sublattice orthogonal biaxial Neel order to this exotic uniaxial magnet. A further increase of frustration drives a transition to a fully gapped SU(2) symmetric valence bond crystal.