1/S EXPANSION IN A 2-DIMENSIONAL FRUSTRATED HEISENBERG-ANTIFERROMAGNET

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 Neel 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.