Translational symmetry broken magnetization plateau of the S=1 antiferromagnetic Heisenberg chain with competing anisotropies

We investigate the S =1 antiferromagnetic quantum spin chain with the exchange and single-ion anisotropies in a magnetic field, using numerical exact diagonalization of finite-size clusters, level spectroscopy analysis, and the density matrix renormalization group (DMRG). We find that a translational symmetry broken magnetization plateau possibly appears at the half of the saturation magnetization, when the anisotropies compete with each other. The level spectroscopy analysis gives the phase diagram at half the saturation magnetization. The DMRG calculation presents the magnetization curves for some typical parameters and clarifies the spin structure in the plateau phase.