Microscopic calculation of the spin-stiffness constant for the spin-(1/2 square-lattice Heisenberg antiferromagnet

We discuss a systematic, microscopic calculation of the spin-stiffness constant ρₛ for the spin-(1/2 square-lattice Heisenberg antiferromagnet. An infinitesimal twist is imposed upon the system by gradually rotating the direction of antiferromagnetic ordering. The difference in the ground-state energy of this system with respect to the uniformly ordered ground state can be related to the spin-stiffness constant ρₛ. Series expansions and extrapolation for the energy of the twisted system lead to the estimate (=4ρₛ/J)=0.72±{}0.0. The ratio of the series for ρₛ and perpendicular susceptibility χ_⊥ leads to an estimate for the spin-wave (=cₛ/ {}2 J)=1.18±{}0.02. The experiments on La₂{CuO}₄$ are quantitatively consistent with a nearest-neighbor Heisenberg model when one takes into account these quantum renormalizations.