1⁄S Expansion in a Two-Dimensional Frustrated Heisenberg Antiferromagnet

抄録

We calculate the spin-wave dispersion and the sublattice magnetization up to second order in the 1⁄S expansion for the antiferromagnetic Heisenberg model with nearest-neighbor (J₁) and second-neighbor (J₂) couplings on a square lattice. The corrections to the linear spin-wave theory for the spin-wave dispersion and the sublattice magnetization grow large when the frustration increases. The expansion seems to converge asymptotically well for J₂⁄J₁<0.35, leading to quantitative estimates for both quantities. The corrections are positive for J₂⁄J₁<0.4, making the Néel ordered state more stable than what the linear spin-wave theory predicts. When J₂⁄J₁ exceeds 0.4, the second-order corrections grow very large with negative sign.