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We study the bulk-boundary correspondences for zigzag ribbons (ZRs) of massless Dirac fermion in two-dimensional $\alpha$-${T}_3$ lattice. By tuning the hopping parameter $\alpha\in[0,1]$, the $\alpha$-${T}_3$ lattice interpolates between pseudospin $S=1/2$ (graphene) and $S=1$ (${T}_3$ or dice lattice), for $\alpha=0$ and $1$, respectively, which is followed by continuous change of the Berry phase from $\pi$ to $0$. The range of existence for edge states in the momentum space is determined by solving tight-binding equations at the boundaries of the ZRs. We find that the transitions of in-gap bands from bulk to edge states in the momentum space do not only occur at the positions of the Dirac cones but also at additional points depending on $\alpha\notin \{0,1\}$. The $\alpha$-${T}_3$ ZRs are mapped into stub Su-Schrieffer-Heeger chains by performing unitary transforms of the bulk Hamiltonian. The non-trivial topology of the bulk bands is revealed by the Majorana representation of the eigenstates, where the $\mathbb{Z}_2$ topological invariant is manifested by the winding numbers on the complex plane and the Bloch sphere.

We investigate the bulk-boundary correspondence for massless Dirac fermion in $\alpha$-${T}_3$ lattice where the Berry phase can be continuously tuned from $\pi $ (graphene) to $0$ ($T_3$ or dice lattice) without modifying the energy dispersion. The topological origin of edge states is revealed by unitary transform of the Hamiltonian, which maps $\alpha$-${T}_3$ zigzag ribbon (ZR) into stub Su-Schrieffer-Heeger (SSH) chain. In the Majorana representation of the eigenstates, the $\mathbb{Z}_2$ invariant is manifested by azimuthal winding numbers on the Bloch sphere. Particularly, $T_3$ ZR exemplifies a topologically nontrivial system with zero Berry phase. Contrary to the conventional topological insulators, band gap closing in the corresponding stub SSH chain is not accompanied by a topological phase transition., Comment: 6 pages, 4 figures

In the fundamental understanding of magnetic interactions between atoms in solids, the crystal lattice is one of the key parameters. As the effective tool for controlling the lattice using tensile stress is limited, there are only few demonstrations of the control in magnetic properties with expanding the lattice structure. Here, we observe that the Curie temperature (Tc) of quasi two-dimensional Cr2Ge2Te6 with NiO overlayer doubles from ~60 K to ~120 K, describe a clear correlation of magnetic properties with lattice expansion, which is characterized by several probes and computational approaches, and address on the mechanisms leading to the increase in Tc via the change in exchange interactions.

A second-order topological insulator on a two-dimensional square lattice hosts zero-dimensional states inside a band gap. They are localized near $90^{\circ}$ and $270^{\circ}$ corners constituting an edge of the system. When the edge is in a staircase form consisting of these two corners, two families of edge states (i.e., one-dimensional states localized near the edge) appear as a result of the hybridization of zero-dimensional states. We identify symmetry that makes them gapless. We also show that a pair of nontrivial winding numbers associated with this symmetry guarantee a gapless spectrum of edge states, indicating that bulk--boundary correspondence holds in this topological insulator with a staircase edge., Comment: 9 pages, 14 figures

Quasiparticle - a key concept to describe interacting particles - characterizes electron-electron interaction in metals (Fermi liquid) and electron pairing in superconductors. While this concept essentially relies on the simplification of hard-to-solve many-body problem into one-particle picture and residual effects, a difficulty in disentangling many-body effects from experimental quasiparticle signature sometimes hinders unveiling intrinsic low-energy dynamics, as highlighted by the fierce controversy on the origin of Dirac-band anomaly in graphene and dispersion kink in high-temperature superconductors. Here, we propose an approach to solve this fundamental problem - the Bayesian modelling of quasiparticles. We have chosen a topological insulator $\mathrm{TlBi(S,Se)_2}$ as a model system to formulate an inverse problem of quasiparticle spectra with semiparametric Bayesian analysis, and successfully extracted one-particle and many-body characteristics, i.e. the intrinsic energy gap and unusual lifetime in Dirac-quasiparticle bands. Our approach is widely applicable to clarify the quasiparticle dynamics of quantum materials.

We study second-order topological insulators and semimetals characterized by chiral symmetry. We investigate topological phase transitions of a model for construction of the two-dimensional second-order topological insulators protected only by chiral symmetry. By the theory of the phase transitions, we propose a second-order topological semimetal and insulators with flat hinge bands in chiral-symmetric three-dimensional systems. The three-dimensional second-order topological phases can be obtained from the stacked two-dimensional second-order topological insulators with chiral symmetry. Moreover, we show that broken chiral symmetry in the three-dimensional second-order topological phase allows a second-order topological insulator with chiral hinge states. We also demonstrate the second-order topological phases by using a lattice model., Comment: 9 pages, 5 figures

Band structure is theoretically studied in partially flattened carbon nanotubes within an effective-mass scheme. Effects of inter-wall interactions are shown to be important in non-chiral nanotubes such as zigzag and armchair and can essentially be neglected in chiral nanotubes except in the close vicinity of non-chiral tubes. In fact, inter-wall interactions significantly modify states depending on relative displacement in the flattened region in non-chiral tubes and can convert semiconducting tubes into metallic and vice versa. They diminish rapidly when the chiral angle deviates from that of the zigzag or armchair tube, although the decay is slower in the vicinity of armchair tubes., Comment: 17 pages, 11 figures, accepted in Phys. Rev. B

The 1111-type iron-based superconductor LnFeAsO1-xFx (Ln stands for lanthanide) is the first material with a Tc above 50 K, other than cuprate superconductors. Electron doping into LaFeAsO by H, rather than F, revealed a double-dome-shaped Tc-x diagram, with a first dome (SC1, 0.0550 K observed in RE-1111 compounds (RE: Pr, Sm, and Gd) at ambient pressure is the merging of SC1 and SC2., Comment: 11 pages, 4 figures

Electronic structure of graphene monolayer-bilayer junction in a magnetic field is studied within an effective-mass approximation. The energy spectrum is characterized by interface Landau levels, i.e., the locally flat bands appearing near the boundary region, resulting in a series of characteristic peaks in the local density of states. Their energies are independent of boundary types such as zigzag or armchair. In the atomic scale, the local density of states shows a Kekul\'{e} pattern due to the valley mixing in the armchair boundary, while does not in the zigzag boundary., Comment: 12 pages, 9 figures

The electron transmission between monolayer and bilayer graphene is theoretically studied for zigzag and armchair boundaries within an effective-mass scheme. Due to the presence of an evanescent wave in the bilayer graphene, traveling modes are well connected to each other. The transmission through the boundary is strongly dependent on the incident angle and the dependence is opposite between the K and K' points, leading to valley polarization of transmitted wave., Comment: 14 pages, 7 figures

Optical response of finite-length metallic carbon nanotubes is calculated including effects of induced edge charges in a self-consistent manner. The results show that the main resonance corresponding to excitation of the fundamental plasmon mode with wave vector $\pi/l$ with $l$ being the tube length is quite robust and unaffected. This arises because the strong electric field associated with edge charges is screened and decays rapidly inside the nanotube. For higher-frequency resonances, the field starts to be mixed and tends to shift resonances to higher frequencies., Comment: 10 pages, 9 figures, to be published in J. Phys. Soc. Jpn

Thermoelectric power due to coherent electron transmission through a quantum dot is theoretically studied. In addition to the known features related to resonant peaks, we show that a novel significant structure appears between the peaks. This structure arises from the so-called transmission zero, which is characteristic of coherent transmission through several quantum levels. Because of sensitivity to phase-breaking effect in quantum dots, this novel structure indicates degree of coherency in the electron transmission., Comment: 6 pages, 6 figures, submitted to J. Phys. Soc. Jpn

We report a numerical study of the tunnel conductance through the Schottky barrier at the contact between a semiconducting carbon nanotube and a metal electrode. In a planar gate model the asymmetry between the p--doped and the n--doped region is shown to depend mainly on the difference between the electrode Fermi level and the band gap of carbon nanotubes. We quantitatively show how the gate/nanotube distance is important to get large on--off ratios. We explain the bend of the current versus gate voltage as the transition from a thermal--activation region to a tunneling region. A good agreement is obtained with experimental results for carbon nanotubes field--effect transistors., Comment: 5 pages, to appear in Phys. Rev. B

The possibility of superconductivity (SC) in the 2D Hubbard model (2DH) was investigated by means of the variational Monte Carlo method. The energy gain of the d-wave SC state, obtained as the difference of the minimum energy with a finite gap and that with zero gap, was examined with respect to dependences on U, electron density rho and next nearest neighbor transfer t' mainly on the 10 x 10 lattice. It was found to be maximized around U = 8 (in energy unit of t). It sharply increased for negative values of t' and had a broad peak for t' ~ -0.10. For these values of t' the energy gain was a smooth increasing function of rho almost independent of the shell structure in the region starting from ~ 0.76 up to 0.92. This clearly indicates that the result is already close to the value in the bulk limit. For t' = 0, the energy gain depended on the electronic shell state. Competition between the SC and the SDW states was also investigated. When t' = 0, the ground state is SDW in the range of rho \geq ~ 0.84$. The SC region slightly extends up to ~ 0.87 for t' ~ -0.10. Consequently the present results strongly indicate that the 2DH with t' ~ -0.1 drives SC by itself in the rho region from ~ 0.76 to ~ 0.87. The energy gain in the SC state with suitable parameters is found to be in reasonable agreement with the condensation energy in the SC state of YBa2Cu3O7. The corresponding t-J model proves to give an order-of-magnitude larger energy gain, which questions its validity., Comment: 24 pages including 13 figures, to be published in Physic C

Diffusion of electrons in a two-dimensional system with time-dependent random potentials is investigated numerically. In the absence of spin-orbit scattering, the conductivity shows universal weak localization correction. In the presence of it, however, the correction to the conductivity weakly depends on the strength of disorder, and becomes vanishingly small close to the metal-insulator transition point., Comment: 4 pages, 2 figures, Physica B in press, Proceedings for EP2DS-12

Diffusion of electrons in a two-dimensional system with time-dependent random potentials is investigated numerically. The correction to the conductivity due to inelastic scatterings by oscillating potentials is shown to be a universal function of the frequency $\omega$, which is consistent with the weak localization prediction $(e^2)/(3\pi^2 \hbar) log \omega$., Comment: 4 pages, RevTeX, 3 postscript figures, accepted in J. Phys. Soc. Jpn.(1997)Apr