arXiv subjects

Ryo Okugawa

Publications and source records attributed to Ryo Okugawa.

25 records · Page 2Linked to original sources

arXiv2018

Topological semimetals studied by ab initio calculations

In topological semimetals such as Weyl, Dirac, and nodal-line semimetals, the band gap closes at points or along lines in k space which are not necessarily located at high-symmetry positions in the Brillouin zone. Therefore, it is not straightforward to find these topological semimetals by ab initio calculations because the band structure is usually calculated only along high-symmetry lines. In this paper, we review recent studies on topological semimetals by ab initio calculations. We explain theoretical frameworks which can be used for the search for topological semimetal materials, and some numerical methods used in the ab initio calculations.

cond-mat.mtrl-sci↗

arXiv2017

Universal phase transition and band structures for spinless nodal-line and Weyl semimetals

We study a general phase transition between spinless topological nodal-line semimetal and Weyl semimetal phases. We classify topological nodal lines into two types based on their positions and shapes, and their phase transitions depends on their types. We show that the topological nodal-line semimetal becomes the Weyl semimetal by breaking time-reversal symmetry when the nodal lines enclose time-reversal invariant momenta (type-A nodal lines). We also discuss an effect of crystallographic symmetries determining the band structure of the topological nodal-line semimetals. Thanks to protection by the symmetries, the topological nodal-line semimetals can transit into the spinless Weyl semimetals or maintain the nodal lines in many crystals after inversion symmetry is broken.

cond-mat.mes-hall↗

arXiv2017

Topological Dirac Nodal Lines and Surface Charges in fcc Alkaline Earth Metals

In nodal-line semimetals, the gaps close along loops in ${\bf k}$ space, which are not at high-symmetry points. Typical mechanisms for the emergence of nodal lines involve mirror symmetry and the $π$ Berry phase. Here, we show via ab initio calculations that fcc calcium (Ca), strontium (Sr) and ytterbium (Yb) have topological nodal lines with the $π$ Berry phase near the Fermi level, when spin-orbit interaction is neglected. In particular, Ca becomes a nodal-line semimetal at high pressure. Owing to nodal lines, the Zak phase becomes either $π$ or 0 depending on the wavavector ${\bf k}$, and the $π$ Zak phase leads to surface polarization charge. Carriers eventually screen it, leaving behind large surface dipoles. In materials with nodal lines, both the large surface polarization charge and the emergent drumhead surface states enhance Rashba splitting when heavy adatoms are present, as we have shown to occur in Bi/Sr(111) and in Bi/Ag(111).

cond-mat.mtrl-sci↗

arXiv2016

Emergence of topological semimetals in gap closing in semiconductors without inversion symmetry

A band gap for electronic states in crystals governs various properties of solids, such as transport, optical and magnetic properties. Its estimation and control have been an important issue in solid state physics. The band gap can be controlled externally by various parameters, such as pressure, atomic compositions and external field. Sometimes, the gap even collapses by tuning some parameter. In the field of topological insulators, such closing of the gap at a time-reversal invariant momentum indicates a band inversion, i.e. it leads to a topological phase transition from a normal insulator to a topological insulator. Here we show that the gap losing in inversion-asymmetric crystals is universal, in the sense that the gap closing always leads either to a Weyl semimetal or a nodal-line semimetal, from an exhaustive study on possible space groups. We here consider three-dimensional spinful systems with time-reversal symmetry. The space group of the system and the wavevector at the gap closing uniquely determine which possibility occurs and where the gap-closing points or lines lie in the wavevector space after closing of the gap. In particular, we show that an insulator-to-insulator transition never happens, which is in sharp contrast with inversion-symmetric systems.

cond-mat.mes-hall↗

arXiv2015

In-plane electric polarization of bilayer graphene nanoribbons by interlayer bias voltage

We theoretically show that an interlayer bias voltage in the AB-stacked bilayer graphene nanoribbons with armchair edges induces an electric polarization along the ribbon. Both tight-binding and ab initio calculations consistently indicate that when the bias voltage is weak, the polarization shows opposite signs depending on the ribbon width modulo three. This nontrivial dependence is explained using a two-band effective model. A strong limit of the bias voltage in the tight-binding model shows either one-third or zero polarization, which agrees with topological argument.

cond-mat.mes-hall↗

arXiv2015

Weyl Node and Spin Texture in Trigonal Tellurium and Selenium

We study Weyl nodes in materials with broken inversion symmetry. We find based on first-principles calculations that trigonal Te and Se have multiple Weyl nodes near the Fermi level. The conduction bands have a spin splitting similar to the Rashba splitting around the H points, but unlike the Rashba splitting the spin directions are radial, forming a hedgehog spin texture around the H points, with a nonzero Pontryagin index for each spin-split conduction band. The Weyl semimetal phase, which has never been observed in real materials without inversion symmetry, is realized under pressure. The evolution of the spin texture by varying the pressure can be explained by the evolution of the Weyl nodes in k space.

cond-mat.mtrl-sci↗

arXiv2014

Dispersion of Fermi arcs in Weyl semimetals and their evolutions to Dirac cones

We study dispersions of Fermi arcs in the Weyl semimetal phase by constructing a simple effective model. We calculate how the surface Fermi-arc dispersions for the top- and bottom surfaces merge into the bulk Dirac cones in the Weyl semimetal at both ends of the arcs, and show that they have opposite velocities. This result is also confirmed by a calculation using a tight-binding model. Furthermore, by changing a parameter in the system while preserving time-reversal symmetry, we show that two Fermi arcs evolve into a surface Dirac cone when the system transits from the Weyl semimetal to the topological insulator phase. We also demonstrate that choices of surface terminations affect the pairing of Weyl nodes, from which the Fermi arcs are formed.

cond-mat.mes-hall↗