Broken discrete and continuous symmetries in two-dimensional spiral antiferromagnets.

We study the occurrence of symmetry breaking, at zero and finite temperatures, in the J1-J3 antiferromagnetic Heisenberg model on the square lattice using Schwinger boson mean field theory. For spin-1/2 the ground state always breaks the SU(2) symmetry with a continuous quasi-critical transition at J3/J1 ∼ 0.38, from Néel to spiral long range order… (More)