Phase Transitions of Anisotropic Heisenberg Antiferromagnets on the Triangular Lattice

Phase transitions in anisotropic Heisenberg antiferromagnets with nearest neighbour coupling constants ( J x , J y , J z ) on the triangular lattice have been investigated. In particular, for the case of Heisenberg-Ising model, | J z |>| J x |=| J y |>0, successive phase transitions due to orderings of z components and of x y components have been found. A non-trivial degeneracy is found both in the ground state and in harmonic excitations. Divergent-like behaviour of the uniform susceptibility in the x y plane at low temperatures is found, which is supposed to be caused by the non-trivial degeneracy.