Unexpected z -Direction Ising Antiferromagnetic Order in a Frustrated Spin-1/2 J_1 - J_2 XY Model on the Honeycomb Lattice

摘要

Using the density matrix renormalization group on wide cylinders, we study the phase diagram of the spin-1/2 XY model on the honeycomb lattice, with first-neighbor (J_1 = 1) and frustrating second-neighbor (J_2 ＞ 0) interactions. For the intermediate frustration regime 0.22 ≤ J_2 ≤ 0.36, we find a surprising antiferromagnetic Ising phase, with ordered moments pointing along the z axis, despite the absence of any S_ZS_Z interactions in the Hamiltonian. Surrounding this phase as a function of J_2 are antiferromagnetic phases with the moments pointing in the x - y plane for small J_2 and a close competition between an x - y plane magnetic collinear phase and a dimer phase for large values of J_2. We do not find any spin-liquid phases in this model.