arXiv subjects

Ryuji Takahashi

Publications and source records attributed to Ryuji Takahashi.

9 recordsLinked to original sources

arXiv2018

Landau-Zener tunneling problem for Bloch states

We study the Landau-Zener tunneling problem for particles bound in periodic lattice insulators. To this end, we construct the path integral based on the Bloch and Wannier functions in the presence with an external force, and the transition amplitude is calculated for the Su-Schrieffer-Heeger model. We find that the tunneling probability in bulk periodic systems becomes drastically larger than that by the Landau-Zener formula. This enhancement is prominent for small values of the external field or small hopping integral compared with the gap, and comes from the difference between the Dirac and the periodic dispersions. In addition, when the lattice effect is strong, another analytical formula of the tunneling probability is given with a different behavior from the Landau-Zener formula. Finally, we discuss the observation scheme for the lattice effect.

cond-mat.mes-hall↗

arXiv2018

Current control of systems with a Peierls distortion by magnetic field

We study the tunneling phenomenon of a ladder system with a Peierls distortion in a magnetic flux, and the response of electrons the insulator is investigated when the tunneling current flows on one-dimensional gapped chains along the external electric field. Without the magnetic field, the ladder system is insulated by the charge density wave order. Then, by the increase of the magnetic field, it becomes metallic with the disappearance of the distortion of the lattice. Finally, the gap appears, and it becomes a insulator. At the metallic state, the topological transition also occurs. To show this phenomenon, we consider the distortion by the phonon in the ladder model, and calculate the distortion gap and the transition probability by using both Landau-Zener formula and the instanton method. The transition to the metallic states will be applied to the current control by the magnetic field.

cond-mat.mes-hall↗

arXiv2018

Semiclassical Lattice effects on interband tunneling of a two-state system

Previously, we have shown that the transition probability of the Landau-Zener problem in periodic lattice systems becomes large by taking into account the nonlinearity of the energy spectra, compared with the probability by the conventional Landau-Zener formula. The enhancement comes from the nonlinearity peculiar to the periodic lattice system, and this effect from the lattice on transition action cannot be neglected in the transition process. In the present paper, we first give a brief review of the previous work, and construct the transfer matrix of the Landau-Zener problem by the semiclassical description for lattice systems. Next, we study a ladder lattice system and show that the transition action obtains a phase due to the nonlinearity. Then, we consider the double-passage problem of the ladder system within the semiclassical description. We find the oscillation of the probability by the transition phase by the lattice effect. This phase comes from the semiclassical analysis unlike the Stokes phase, and we show that the oscillation is mainly contributed by the transition phase by the lattice effect, when the hybridization of the ladder is strong.

cond-mat.mes-hall↗

arXiv2016

Berry curvature in magnon-phonon hybrid systems

We theoretically study the Berry curvature of the magnon induced by the hybridization with the acoustic phonons via the spin-orbit and dipolar interactions. We first discuss the magnon-phonon hybridization via the dipolar interaction, and show that the dispersions have gapless points in momentum space, some of which form a loop. Next, when both spin-orbit and dipolar interactions are considered, we show anisotropic texture of the Berry curvature and its divergence with and without gap-closing. Realistic evaluation of the consequent anomalous velocity is given for yttrium iron garnet.

cond-mat.mes-hall↗

arXiv2014

Interfacial Fermi Loops from Interfacial Symmetries

We propose a concept of interfacial symmetries such as interfacial particle-hole symmetry and interfacial time-reversal symmetry, which appear in interfaces between two regions related to each other by particle-hole or time-reversal transformations. These symmetries result in novel dispersion of interface states. In particular for the interfacial particle-hole symmetry the gap closes along a loop ("Fermi loop") at the interface. We numerically demonstrate this for the Fu-Kane-Mele tight-binding model. We show that the Fermi loop originates from a sign change of a Pfaffian of a product between the Hamiltonian and a constant matrix.

cond-mat.mes-hall↗

arXiv2013

Completely flat bands and fully localized states on surfaces of anisotropic diamond-lattice models

We discuss flat-band surface states on the (111) surface in the tight-binding model with nearest-neighbor hopping on the diamond lattice, in analogy to the flat-band edge states in graphene with a zigzag edge. The bulk band is gapless, and the gap closes along a loop in the Brillouin zone. The verge of the flat-band surface states is identical with this gap-closing loop projected onto the surface plane. When anisotropies in the hopping integrals increase, the bulk gap-closing points move and the distribution of the flat-band states expands in the Brillouin zone. Then when the anisotropy is sufficiently large, the surface flat bands cover the whole Brillouin zone. Because of the completely flat bands, we can construct surface-state wavefunctions which are localized in all the three directions.

cond-mat.str-el↗

arXiv2011

Gapless interface states between topological insulators with opposite Dirac velocities

The Dirac cone on a surface of a topological insulator shows linear dispersion analogous to optics and its velocity depends on materials. We consider a junction of two topological insulators with different velocities, and calculate the reflectance and transmittance. We find that they reflect the backscattering-free nature of the helical surface states. When the two velocities have opposite signs, both transmission and reflection are prohibited for normal incidence, when a mirror symmetry normal to the junction is preserved. In this case we show that there necessarily exist gapless states at the interface between the two topological insulators. Their existence is protected by mirror symmetry, and they have characteristic dispersions depending on the symmetry of the system.

cond-mat.mes-hall↗

arXiv2010

Thermoelectric transport of perfectly conducting channels in two- and three-dimensional topological insulators

Topological insulators have gapless edge/surface states with novel transport properties. Among these, there are two classes of perfectly conducting channels which are free from backscattering: the edge states of two-dimensional topological insulators and the one-dimensional states localized on dislocations of certain three-dimensional topological insulators. We show how these novel states affect thermoelectric properties of the systems and discuss possibilities to improve the thermoelectric figure of merit using these materials with perfectly conducting channels.

cond-mat.mes-hall↗

arXiv2010

Thermoelectricity by Perfectly Conducting Channels in Quantum Spin Hall Systems

Thermoelectric transport of two-dimensional quantum spin Hall systems are theoretically studied in narrow ribbon geometry. We find that at high temperature electrons in the bulk states dominate. By lowering temperature, the "perfectly conducting" edge channels becomes dominant, and a bulk-to-edge crossover occurs. Correspondingly, by lowering temperature, the figure of merit first decreases and then will increase again due to edge-state-dominated thermoelectric transport.

cond-mat.mes-hall↗