Edge Modes in the Intermediate-D and Large-D Phases of the S=2 Quantum Spin Chain with XXZ and On-Site Anisotropies

We investigate the edge modes at T=0 in the intermediate-D (ID) phase and the large-D (LD) phase of the S=2 quantum spin chain with the XXZ anisotropy and the generalized on-site anisotropies by use of the DMRG. There exists a gapless edge mode in the ID phase, while no gapless edge mode in the LD phase. These results are consistent with the physical pictures of these phases. We also show the ground-state phase diagrams obtained by use of the exact diagonalization and the level spectroscopy analysis.Comment: Submitted to "Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013)

Anomalous behavior of the spin gap of a spin-1/2 two-leg antiferromagnetic ladder with Ising-like rung interactions

Using mainly numerical methods, we investigate the width of the spin gap of a spin-1/2 two-leg ladder described by \cH= J_\rl \sum_{j=1}^{N/2} [ \vS_{j,a} \cdot \vS_{j+1,a} + \vS_{j,b} \cdot \vS_{j+1,b} ] + J_\rr \sum_{j=1}^{N/2} [\lambda (S^x_{j,a} S^x_{j,b} + S^y_{j,a} S^y_{j,b}) + S^z_{j,a} S^z_{j,b}] , where $S^\alpha_{j,a(b)}$ denotes the $\alpha$-component of the spin-1/2 operator at the $j$-th site of the $a (b)$ chain. We mainly focus on the J_\rr \gg J_\rl > 0 and $|\lambda| \ll 1$ case. The width of the spin gap as a function of $\lambda$ anomalously increases near $\lambda = 0$; for instance, for $-0.1 < \lambda < 0.1$ when $J_{\rm l}/J_{\rm r} = 0.1$. The gap formation mechanism is thought to be different for the $\lambda 0$ cases. Since, in usual cases, the width of the gap becomes zero or small at the point where the gap formation mechanism changes, the above gap-increasing phenomenon in the present case is anomalous. We explain the origin of this anomalous phenomenon by use of the degenerate perturbation theory. We also draw the ground-state phase diagram.Comment: 4 pages, 11 figures; Proc. "The International Conference on Quantum Criticality and Novel Phases" (2012), to be published in Phys. Stat. Solidi

Spin-Chirality Separation and S_3-Symmetry Breakings in the Magnetization Plateau of the Quantum Spin Tube

We study the magnetization plateau state of the three-leg spin-1/2 tube in the strong rung coupling region, where S_3-symmetry breakings and low-energy chirality degree of freedom play crucial roles. On the basis of the effective chirality model and density matrix renormalization group, we clarify that, as the leg coupling increases, the chirality liquid with gapless non-magnetic excitations, the spin imbalance phase and the vector-spin-chirality ordered phase emerge without closing the plateau spin gap. The relevance of these results to experiments is also discussed.Comment: 6 pages, 6 figures, detailed results of the spin imbalance state are adde