Ground-State Properties and Phase Diagram of the Quantum XXZ Antiferromagnet on a Triangular Lattice

抄録

Long-range order, especially chiral order, in the two-dimensional spin 1/2 XXZ antiferromagnet H=JΣ(S_i^xS_j^x+S_i^yS_j^y+ΔS_i^zS_j^z) on a triangular lattice is studied, partially by showing exact ground states and partially using the spin-wave theory. Exact ground states in the region Δ≤−0.5 are found. These states are not disordered by quantum fluctuation. Most of the ground states at Δ=−0.5 have a so-called 120° structure. Ground-state properties in the region −0.5≤Δ≤1 are studied using the spin-wave theory. It is shown that quantum fluctuations are enhanced in the ground states as Δ is increased. The phase diagram of the relevant system at low temperatures is also discussed using the spin-wave theory.