Haldane, large-d intermediate-d states in an S = 2 quantum spin chain with on-site and XXZ anisotropies

Using mainly numerical methods, we investigate the ground-state phasediagram of the S = 2 quantum spin chain described by mathmatical equation where δ denotes the XXZ anisotropy parameter ofthe nearest-neighbor interactions and D the on-site anisotropy parameter. We restrict ourselvesto the case of δ ≥ 0 and D ≥ 0 for simplicity. Each of the phase boundary lines is determined by the level spectroscopy or the phenomenological renormalization analysis of numerical results of exact-diagonalization calculations. The resulting phase diagram on the δ-D plane consists of four phases; the XY1 phase, the Haldane/large-D phase, the intermediate-D phase, and the Néel phase. The remarkable natures of the phase diagram are as follows: (1) there exists an intermediate-D phase which was predicted by Oshikawa in 1992; (2) the Haldane state and the large-D state belong to the same phase; (3) the shape of the phase diagram on theδ-D plane is different from that considered so far. We note that this is the first report on the observation of the intermediate-D phase.