A Schwinger boson theory of the 1/3 magnetization plateau on the triangular lattice

The classical ground state of the spin 1/2 SU(2) Heisenberg model on the triangular lattice describe a unique noncollinear, coplanar ordering pattern. However, with the addition of an external magnetic field to the model, a degenerate classical ground state manifold arises. It has been known for a while that if the lattice is homogeneous and isotropic, a quantum fluctuation-driven ordering mechanism selects a coplanar configuration for the ground state and breaks the classical degeneracy. This order-by-disorder mechanism also leads to a gapped state for a select magnetic field range where the net magnetization is quantized at an 1/3 fraction of the classical spin moments. In this paper, we consider the Schwinger boson representation of SU(2) spins to demonstrate the order by disorder mechanism and the emergence of the 1/3 magnetization plateau with a variational analysis.