1/S expansion for thermodynamic quantities in a two-dimensional Heisenberg antiferromagnet at zero temperature

We calculate the spin-wave dispersion, the perpendicular susceptibility, the spin-stiffness constant, and the sublattice magnetization of a two-dimensional Heisenberg antiferromagnet at T=0, to order 1/(2S${)}^2$, treating carefully the umklapp processes. Our numerical estimates for the thermodynamic quantities are in good agreement with series-expansion estimates, and satisfy the hydrodynamic relation very accurately.