SPIN-1/2 QUANTUM ANTIFERROMAGNETS ON THE TRIANGULAR LATTICE

The spin-1/2 anisotropic Heisenberg antiferromagnet is studied at T = 0 on the triangular lattice via numerical diagonalization for system sizes up to N = 36 sites. Extrapolation to the thermodynamic limit suggests that the isotropic system possesses no, or very small, square-root 3 x square-root 3 magnetic order; no helical or chiral order; and spin-spin correlations consistent with that of a critical phase. For XY-like anisotropy there is long-ranged square-root 3 x square-root 3 magnetic order. In contrast to bipartite lattices, the standard first- and second-order spin-wave theories are not quantitatively accurate. Excitation energy gaps suggest that the lowest-lying excitations for the isotropic point are not spin-flip excitations in the thermodynamic limit. The results for the isotropic point appear to agree with recent series expansion, large-N expansion, and the original resonanting valence bond picture of Anderson, although they cannot be considered as conclusive evidence supporting any of these theories.