Spin-wave stiffness of the Heisenberg antiferromagnet at zero temperature

Abstract
The spin-wave stiffness ρs of the Heisenberg antiferromagnet at zero temperature is calculated in two ways, by spin-wave theory and by series expansion about the Ising limit, extending previous results. The two methods give values of ρs for the isotropic model which are in good agreement, and which satisfy the hydrodynamic relation.