SPIN STIFFNESSES OF THE QUANTUM HEISENBERG-ANTIFERROMAGNET ON A TRIANGULAR LATTICE

The two spin stiffnesses (rho(parallel to), rho(perpendicular to)) of the quantum Heisenberg; antiferromagnet on the triangular lattice are investigated by a first-order spin-wave theory. At the thermodynamic l imit, spin-wave calculations predict a large reduction of the spin sti ffnesses by quantum fluctuations: relative to their classical values, the reduction is 68% for rho(parallel to), 12% for rho(perpendicular t o), and 40% for the average spin stiffness rho(m). In this approach qu antum fluctuations, not large enough to destroy the rigidity of Neel o rder, are nevertheless changing the sign of the anisotropy of the spin stiffnesses tenser. A method using exact diagonalizations on finite l attices is used to countercheck the importance of quantum fluctuations on small sizes. These last results confirm qualitatively the conclusi ons of the first-order spin-wave calculation.