FLOW EQUATIONS AND EXTENDED BOGOLIUBOV TRANSFORMATION FOR THE HEISENBERG-ANTIFERROMAGNET NEAR THE CLASSICAL LIMIT

The Heisenberg spin-S quantum antiferromagnet is studied near the larg e-spin limit, applying a new continuous unitary transformation which e xtends the usual Bogoliubov transformation to higher order in the 1/S- expansion of the Hamiltonian. This allows to diagonalize the bosonic H amiltonian resulting from the Holstein-Primakoff representation beyond the conventional spin-wave approximation. The zero-temperature flow e quations derived from the extension of the Bogoliubov transformation t o order O(1/S-2) for the ground-state energy, the spin-wave velocity, and the staggered magnetization are solved exactly and yield results w hich are in agreement with those obtained by a perturbative treatment of the magnon interactions.