GROUND-STATE PROPERTIES OF THE S = 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 Néel 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. © 1990 The American Physical Society.