Haldane, Large-D, and 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 phase diagram of the S 2 quantum spin chain described by H = Sigma(j)((SjSj+1x)-S-x + (SjSj+1y)-S-y + Delta(SjSj+1z)-S-z) + D Sigma(j)(S-j(z))(2), where Delta denotes the XXZ anisotropy parameter of the nearest-neighbor interactions and D the on-site anisotropy parameter. We restrict ourselves to the case of Delta >= 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 Delta-D plane consists of four phases; the XY1 phase, the Haldane/ large-D phase, the intermediateD phase, and the Neel 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 the Delta-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.