Ground-state properties of theS=1/2 Heisenberg antiferromagnet on a triangular lattice

We study the S=1/2 Heisenberg antiferromagnet on a triangular lattice with both nearest-neighbor (J₁) and next-to-nearest-neighbor (J₂) couplings. We have performed a spin-wave analysis around the classical ground state. At large (J₂/J₁) the N\'eel state is destabilized and the system exhibits the ``order from disorder'' phenomenon with a threefold-degenerate ground state that spontaneously breaks the lattice rotational invariance. These results are in agreement with a Lanczos study of a 12-site lattice. For even larger values of J₂/J₁ the spin-wave calculation shows the existence of a second transition to an incommensurate spiral. Thus, the previously suggested existence of chiral order in this model should be reanalyzed.