S=2 Quantum Spin Chain with the Biquadratic Exchange Interaction

The S = 2 quantum spin chain with the single-ion anisotropy D and the biquadratic exchange interaction JBQ is investigated using the numerical diagonalization of finite-size clusters and the level spectroscopy analysis. It is found that the intermediate-D phase corresponding to the symmetry protected topological (SPT) phase appears in a wide region of the ground state phase diagram. We also obtain the phase diagram at the half of the saturation magnetization which includes the SPT plateau phase.