Frustration-induced phase transitions in the spin-S orthogonal-dimer chain

We investigate quantum phase transitions in the frustrated orthogonal-dimer chain with an arbitrary spin $S \geq 1/2$. When the ratio of the competing exchange couplings is varied, first-order phase transitions occur 2S times among distinct spin-gap phases. The introduction of single-ion anisotropy further enriches the phase diagram. The phase transitions described by the present model possess most of the essential properties inherent in frustrated quantum spin systems.Comment: 4 pages, 9 figure

Quantum Phase Transitions in the Shastry-Sutherland Model for SrCu2(BO3)2\rm\bf SrCu_2 (BO_3)_2

We investigate the quantum phase transitions in the frustrated antiferromagnetic Heisenberg model for $\rm SrCu_2(BO_3)_2$ by using the series expansion method. It is found that a novel spin-gap phase, which is adiabatically connected to the plaquette-singlet phase, exists between the dimer and the magnetically ordered phases known so far. When the ratio of the competing exchange couplings $\alpha(=J'/J)$ is varied, this spin-gap phase exhibits the first- (second-) order quantum phase transition to the dimer (the magnetically ordered) phase at the critical point $\alpha_{c1}=0.677(2)$ ($\alpha_{c2}=0.86(1)$). Our results shed light on some controversial arguments about the nature of the quantum phase transitions in this model.Comment: 4 pages, accepted for publication in Phys. Rev. Let

Reentrant topological transitions in a quantum wire/superconductor system with quasiperiodic lattice modulation

We study the condition for a topological superconductor (TS) phase with end Majorana fermions to appear when a quasiperiodic lattice modulation is applied to a one-dimensional quantum wire with strong spin-orbit interaction situated under a magnetic field and in proximity to a superconductor. By density-matrix renormalization group analysis, we find that multiple topological phases with Majorana end modes are realized in finite ranges of the filling factor, showing a sequence of reentrant transitions as the chemical potential is tuned. The locations of these phases reflect the structure of bands in the non-interacting case, which exhibits a distinct self-similar structure. The stability of the TS in the presence of an on-site interaction or a harmonic trap potential is also discussed.Comment: 5 pages, 4 figures, v4: minor corrections; published in Phys. Rev. B Rapid Communicatio

Topological edge Mott insulating state in two dimensions at finite temperatures -bulk and edge analysis-

We study a bilayer Kane-Mele-Hubbard model with lattice distortion and inter-layer spin exchange interaction under cylinder geometry. Our analysis based on real-space dynamical mean field theory with continuous-time quantum Monte Carlo demonstrates the emergence of a topological edge Mott insulating (TEMI) state which hosts gapless edge modes only in collective spin excitations. This is confirmed by the numerical calculations at finite temperatures for the spin-Hall conductivity and the single-particle excitation spectrum; the spin Hall conductivity is almost quantized, $\sigma^{xy}_\mathrm{spin}\sim2(e/2\pi)$, predicting gapless edge modes carrying the spin current, while the helical edge modes in the single-particle spectrum are gapped out with respecting symmetry. It is clarified how the TEMI state evolves from the ordinary spin Hall insulating state with increasing the Hubbard interaction at a given temperature and then undergoes a phase transition to a trivial Mott insulating state. With a bosonization approach at zero temperature, we further address which collective modes host gapless edge modes in the TEMI state.Comment: 9 pages, 6 figure

Reentrant topological transitions with Majorana end states in 1D superconductors by lattice modulation

The possibility to observe and manipulate Majorana fermions as end states of one-dimensional topological superconductors has been actively discussed recently. In a quantum wire with strong spin-orbit coupling placed in proximity to a bulk superconductor, a topological superconductor has been expected to be realized when the band energy is split by the application of a magnetic field. When a periodic lattice modulation is applied multiple topological superconductor phases appear in the phase diagram. Some of them occur for higher filling factors compared to the case without the modulation. We study the effects of phase jumps and argue that the topologically nontrivial state of the whole system is retained even if they are present. We also study the effect of the spatial modulation in the hopping parameter.Comment: 10 pages, 9 figures, submitted to Phys. Rev.