SPIN-WAVE ANALYSIS OF EASY-AXIS QUANTUM ANTIFERROMAGNETS ON THE TRIANGULAR LATTICE

We perform the standard spin wave analysis of the triangular Heisenberg quantum antiferromagnet with nearest neighbour coupling. The exchange interaction is taken to be DELTA-J(z) = J(x) = J(y) (0 < DELTA less-than-or-equal-to 1). We give a simple explanation of the non-trivial classical degeneracy pointed out by Miyashita and Kawamura and show that it is removed by quantum fluctuations, but that the degeneracy manifests itself through the appearance of a second gapless spin-wave branch. The existence of a second gap-less mode has a drastic influence on the quasiclassical behaviour near the Ising limit: the energy gain with respect to the Ising state energy is found to be linear in DELTA, and the reduction of the sublattice magnetization on two of the three sublattices remains finite as DELTA --> 0. These findings essentially invalidate the original qualitative arguments [14] in favour of a spin-liquid ground state of the anisotropic triangular antiferromagnet.