arXiv subjects

Katsunori Wakabayashi

Publications and source records attributed to Katsunori Wakabayashi.

At least 37 records · Page 2Linked to original sources

arXiv2024

Wilson Loop and Topological Properties in 3D Woodpile Photonic Crystal

We numerically study the first and the second order topological states of electromagnetic (EM) wave in the three-dimensional (3D) woodpile photonic crystal (PhC). The recent studies on 3D PhCs have mainly focused on the observation of the topological states. Here, we not only focus on finding the topological states but also propose a numerical calculation method for topological invariants, which is based on the Wilson loop. For the 3D woodpile PhC, the topological states emerge due to the finite difference in the winding number or partial Chern number. The selection rule for the emergence of topological hinge states is also pointed out based on the topological invariants. Our numerical calculation results are essential and put a step toward the experimental realization of topological waveguide in 3D PhCs.

physics.optics↗

arXiv2024

Identifying the phases of Kane-Mele Hubbard Hamiltonian in momentum space: A many-body configuration interaction study

We investigate the magnetic and conduction properties of Kane-Mele Hubbard model in quasi one-dimensional honeycomb ribbon systems at half-filling by varying the strength of both spin-orbit interaction and on-site Coulomb correlation term. We use the numerical many-body configuration interaction (CI) method to investigate the dispersions of charge and spin gaps along with the momentum resolved spin-density profile over the full Brillouin zone. While the spin sector retains its topological nature at all values of spin-orbit coupling and Hubbard term, we report a new signature of the topological phase transition in the charge sector. This phase transition from a topological band insulating phase to a antiferromagnetically ordered Mott insulating phase is characterized by a shift of the many-body charge gap minima from Brillouin zone boundary to Dirac point. Our results provide a better understanding of the shifting of the gap-closing point in the momentum space which was reported in an earlier mean-field study of the same model and suggests an alternative numerical route to detect topological phase transition in strongly-correlated systems in terms of their momentum space behaviors.

cond-mat.str-el↗

arXiv2024

Superconductivity in Ca-intercalated bilayer silicene

Within first-principles calculations, we explore superconductivity in Ca-intercalated bilayer silicene compound, Si2CaSi2. This arises from the coupling of interlayer flower-like Γ-centered Fermi surface formed by the hybridization of Ca-3d and Si-3pz orbitals with low-energy out-of-plane vibrations enabled by silicene's buckling. The consequent large electron-phonon coupling, as evident from the Eliashberg spectral function leads to superconductivity below 5.4 K in this two-dimensional covalent system. Our results reveal the key control parameters to achieve superconductivity in experimentally synthesizable silicon-based thin materials that can find diverse applications.

cond-mat.supr-con↗

arXiv2024

Topological Edge and Corner States in Biphenylene Network

The electronic states and topological properties of the biphenylene network (BPN) are analyzed using a tight-binding model based on the $π$-electron network. It is shown that tuning the hopping parameters induces topological phase transitions, leading to the emergence of edge states owing to the nontrivial topological Zak phase of the bulk BPN. Elementary band analysis clearly gives the number of edge states, which are associated with the location of Wannier centers. In addition, we have presented the conditions for the emergence of corner states owing to the higher-order topological nature of BPN.

cond-mat.mtrl-sci↗

arXiv2023

Topological Edge and Corner States in Biphenylene Photonic Crystal

The biphenylene network (BPN) has a unique two-dimensional atomic structure, where hexagonal unit cells are arranged on a square lattice. Inspired by such a BPN structure, we design a counterpart in the fashion of photonic crystals (PhCs), which we refer to as the BPN PhC. We study the photonic band structure using the finite element method and characterize the topological properties of the BPN PhC through the use of the Wilson loop. Our findings reveal the emergence of topological edge states in the BPN PhC, specifically in the zigzag edge and the chiral edge, as a consequence of the nontrivial Zak phase in the corresponding directions. In addition, we find the localization of electromagnetic waves at the corners formed by the chiral edges, which can be considered as second-order topological states, i.e., topological corner states.

physics.optics↗

arXiv2023

Approximating Maximally Localized Wannier Functions with Position Scaling-Eigenfunction

Position scaling-eigenfunctions are generated by transforming compactly supported orthonormal scaling functions and utilized for faster alternatives to maximally localized Wannier functions (MLWFs). The position scaling-eigenfunctions are first applied to numerical procedures solving Schrödinger and Maxwell's equations, and the solutions well agree with preceding results. Subsequently, by projecting the position scaling-eigenfunctions onto the space spanned by the Bloch functions, approximated MLWFs are obtained. They show good agreements with preceding results using MLWFs. In addition, analytical explanations of the agreements and an estimate of the error associated with the approximation are provided.

cond-mat.mes-hall↗

arXiv2023

Microwave hinge states in a simple-cubic-lattice photonic crystal insulator

We numerically and experimentally demonstrated a higher-order topological state in a three-dimensional (3D) photonic crystal (PhC) with a complete photonic bandgap. Two types of cubic lattices were designed with different topological invariants, which were theoretically and numerically confirmed by the finite difference of their Zak phases. Topological boundary states in the two-dimensional interfaces and hinge states in the one-dimensional corners were formed according to the higher-order of bulk-boundary correspondence. Microwave measurements of the fabricated 3D PhC containing two boundaries and one corner showed a localized intensity, which confirmed the boundary and hinge states.

cond-mat.mes-hall↗

arXiv2023

Symmetry manipulation of nonlinear optical effect for metallic TMDC

Nonlinear optical (NLO) effect plays a crucial role to engineer optical angular frequency and symmetry of electronic system. Metallic transition-metal dichalcogenide (TMDC) is one of two-dimensional (2D) materials, which has no inversion symmetry for odd-number-layer. In particular, odd-number-layered NbSe$_2$ has spin splitting owing to Ising-type spin-orbit coupling. In this paper, we numerically calculate the NLO charge and spin conductivities of NbSe$_2$ based on an effective tight-binding model for several different optical effects, i.e., symmetry manipulation by bi-circular light (BCL) and bulk photovoltaic effect (shift and injection current). Under irradiation of BCL which can control the symmetry of electronic system, the current can be generated even in even-number-layered NbSe$_2$. Also, we find that shift current can be generated for odd-number-layered NbSe$_2$, which is robust against electronic scattering, i.e., topological current. The direction of generated shift current can be switched by altering polarization of light. Our results will serve to design opt-spintronics devices based on 2D materials to manipulate the charge and spin current and their directions by controlling the polarization of incident light which recasts the symmetry of electronic system.

cond-mat.mes-hall↗

arXiv2022

An epitaxial graphene platform for zero-energy edge state nanoelectronics

Graphene's original promise to succeed silicon faltered due to pervasive edge disorder in lithographically patterned deposited graphene and the lack of a new electronics paradigm. Here we demonstrate that the annealed edges in conventionally patterned graphene epitaxially grown on a silicon carbide substrate (epigraphene) are stabilized by the substrate and support a protected edge state. The edge state has a mean free path that is greater than 50 microns, 5000 times greater that the bulk states and involves a theoretically unexpected Majorana-like zero-energy non-degenerate quasiparticle that does not produce a Hall voltage. In seamless integrated structures, the edge state forms a zero-energy one-dimensional ballistic network with essentially dissipationless nodes at ribbon-ribbon junctions. Seamless device structures offer a variety of switching possibilities including quantum coherent devices at low temperatures. This makes epigraphene a technologically viable graphene nanoelectronics platform that has the potential to succeed silicon nanoelectronics.

cond-mat.mes-hall↗

arXiv2022

Enhanced valley polarization of graphene on hBN under circularly polarized light irradiation

Graphene on hBN (G/hBN) has a long period moiré superstructure owing to the lattice mismatch between two materials. Long periodic potential caused by the moiré superstructure induces modulation of electronic properties of the system. In this paper, we numerically calculate optical conductivity of G/hBN under circularly polarized light irradiation. The lack of spatial inversion symmetry in G/hBN induces the valley polarization. In further, the valley polarization becomes most pronounced in the infrared and terahertz regions if the twist angle between two materials is close to zero for non-doping case, however, insensitive with twist angle for hole-doped case. These results will serve to design the valleytronics devices using G/hBN.

cond-mat.mes-hall↗

arXiv2022

Nonlinear optical Hall effect of few-layered NbSe2

NbSe$_2$ is one of metallic two-dimensional (2D) transition-metal dichalcogenide (TMDC) materials. Because of broken crystal inversion symmetry, large spin splitting is induced by Ising-type spin-orbit coupling in odd-number-layered NbSe$_2$, but absent for even-number-layered NbSe$_2$ with the inversion symmetry. In this paper, we numerically calculate nonlinear optical charge and spin Hall conductivities of few-layered NbSe$_2$ based on an effective tight-binding model which includes $d_{z^2}$, $d_{x^2-y^2}$ and $d_{xy}$ orbitals of Nb atom. We show that the nonlinear optical Hall conductivity for second harmonic generation (SHG) process has nonvanishing value in odd-number-layered NbSe$_2$. Also, we provide nonlinear optical selection rule in few-layered NbSe$_2$ and their polarization dependences. In further, for even-number-layered case, the nonlinear optical Hall currents can be generated by applying electric fields which break inversion symmetry. We also discuss that the nonlinear optical Hall effect is expected to occur in TMDC materials in general. Thus, our results will serve to design potential opt-spintronics devices based on 2D materials to generate the spin Hall current by SHG.

cond-mat.mes-hall↗

arXiv2022

Additive rule of real and reciprocal space topologies at disclinations

Topological materials are renowned for their ability to harbor states localized at their peripheries, such as surfaces, edges, and corners. Accompanying these states, fractional charges appear on peripheral unit cells. Recently, topologically bound states and fractional charges at disclinations of crystalline defects have been theoretically predicted. This so-called bulk-disclination correspondence has been experimentally confirmed in artificial crystalline structures, such as microwave-circuit arrays and photonic crystals. Here, we demonstrate an additive rule between the real-space topological invariant $\mathbf{s}$ (related to the Burgers vector $\mathbf{B}$) and the reciprocal-space topological invariant $\mathbf{p}$ (vectored Zak's phase of bulk wave functions). The bound states and fractional charges concur at a disclination center only if $\mathbf{s}+\mathbf{p}/2π$ is topologically nontrivial; otherwise, no bound state forms even if fractional charges are trapped. Besides the dissociation of fractional charges from bound states, the additive rule also dictates the existence of half-bound states extending over only half of a sample and ultra-stable bound states protected by both real-space and reciprocal-space topologies. Our results add another dimension to the ongoing study of topological matter and may germinate interesting applications.

cond-mat.supr-con↗

arXiv2021

Valley-dependent Corner States in Honeycomb Photonic Crystal without Inversion Symmetry

We study topological states of honeycomb photonic crystals in absence of inversion symmetry using plane wave expansion and finite element methods. The breaking of inversion symmetry in honeycomb lattice leads to contrasting topological valley indices, i.e., the valley-dependent Chern numbers in momentum space. We find that the topological corner states appear for 60$^\circ$ degree corners, but absent for other corners, which can be understood as the sign flip of valley Chern number at the corner. Our results provide an experimentally feasible platform for exploring valley-dependent higher-order topology in photonic systems.

physics.optics↗

arXiv2021

Optical induced Spin Current in Monolayer NbSe$_2$

Monolayer NbSe2 is a metallic two-dimensional (2D) transition-metal dichalcogenide material. Owing to the lattice structure and the strong atomic spin-orbit coupling (SOC) field, monolayer NbSe2 possesses Ising-type SOC which acts as effective Zeeman field, leading to the unconventional topological spin properties. In this paper, we numerically calculate spin-dependent optical conductivity of monolayer NbSe2 using Kubo formula based on an effective tight-binding model which includes $d_{z^2}$, $d_{x^2-y^2}$ and $d_{xy}$ orbitals of Nb atom. Numerical calculation indicates that the up- and down-spin have opposite sign of Hall current, so the pure spin Hall current can be generated in monolayer NbSe2 under light irradiation, owing to the topological nature of monolayer NbSe2, i.e., finite spin Berry curvature. The spin Hall angle is also evaluated. The optical induced spin Hall current can be enhanced by the electron doping and persists even at room temperature. Our results will serve to design opt-spintronics devices such as spin current harvesting by light irradiation on the basis of 2D materials.

cond-mat.mes-hall↗

arXiv2020

Cooperative nanoparticle self-assembly and photothermal heating in a flexible plasmonic metamaterial

We theoretically investigate equilibrium behaviors and photothermal effects of a flexible plasmonic metamaterial composed of aramid nanofibers and gold nanoparticles. The fiber matrix is considered as an external field to reconfigure a nanoparticle assembly. We find that the heating process tunes particle-particle and fiber-particle interactions, which alter adsorption of nanoparticles on fiber surfaces or clustering in pore spaces. Thus, it is possible to control the nanoparticle self-assembly by laser illumination. Gold nanoparticles strongly absorb radiations and efficiently dissipate absorbed energy into heat. By solving the heat transfer equation associated with an effective medium approximation, we calculate the spatial temperature rise. Remarkably, our theoretical results quantitatively agree with prior experiments. This indicates that we can ignore plasmonic coupling effects induced by particle clustering. Effects of the laser spot size and intensity on the photothermal heating are also discussed.

cond-mat.mes-hall↗

arXiv2020

Theory of pressure-induced rejuvenation and strain-hardening in metallic glasses

We theoretically investigate high-pressure effects on the atomic dynamics of metallic glasses. The theory predicts compression-induced rejuvenation and the resulting strain hardening that have been recently observed in metallic glasses. Structural relaxation under pressure is mainly governed by local cage dynamics. The external pressure restricts the dynamical constraints and slows down the atomic mobility. In addition, the compression induces a rejuvenated metastable state (local minimum) at a higher energy in the free energy landscape. Thus, compressed metallic glasses can rejuvenate and the corresponding relaxation is reversible. This behavior leads to strain hardening in mechanical deformation experiments. Theoretical predictions agree well with experiments.

cond-mat.soft↗

arXiv2020

Coupling between structural relaxation and diffusion in glass-forming liquids under pressure variation

We theoretically investigate structural relaxation and activated diffusion of glass-forming liquids at different pressures using both the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory and molecular dynamics (MD) simulation. An external pressure restricts local motions of a single molecule within its cage and triggers the slowing down of cooperative mobility. While the ECNLE theory and simulation generally predict a monotonic increase of the glass transition temperature and dynamic fragility with pressure, the simulation indicates a decrease of fragility as pressure above 1000 bar. The structural relaxation time is found to be linearly coupled with the inverse diffusion constant. Remarkably, this coupling is independent of compression. Theoretical calculations agree quantitatively well with simulations and are also consistent with prior works.

cond-mat.soft↗

arXiv2020

Higher-order topology in monolayer graphene

We show that monolayer graphene intrinsically hosts higher-order topological corner states, in which electrons are localized topologically at atomic sizes. The emergence of the topological corner states in graphene is due to a nontrivial product of the Zak phases for two independent directions, which can be handily calculated graphically by using the bulk wavefunctions. We give an explicit expression that indicates the existence of topological corner states for various geometric edges and corner angles. We also demonstrate the nontrivial localization nature of the topological corner states in graphene by putting an imaginary onsite potential mask.

cond-mat.mes-hall↗