arXiv subjects

Ryo Okugawa

Publications and source records attributed to Ryo Okugawa.

At least 19 recordsLinked to original sources

arXiv2026

Floquet-Weyl states at one-photon resonances in three-dimensional topological insulators

Quantum materials exhibit exotic phases and electronic responses under irradiation by circularly polarized light, which breaks time-reversal symmetry and generates Floquet replica bands. Recently, Floquet topological states arising from direct resonances have attracted much attention, e.g., the emergence of Floquet-Weyl points at a one-photon resonance, rather than topological features within the modulated original bands via high-frequency expansion. In this study, we investigate the effects of a one-photon resonance in a representative three-dimensional topological insulator, Bi$_2$Se$_3$, applying Floquet theory under circularly polarized light. We find that four pairs of Floquet-Weyl points emerge in the intermediate-frequency regime, mediated by hybridization between the original and one-photon-resonant Floquet bands, preserving the threefold rotational symmetry of the crystalline structure. Our numerical calculations demonstrate that tuning the chemical potential via hole doping yields a large anomalous Hall conductivity, directly associated with these Floquet-Weyl points. This work provides a highly accessible route toward the experimental realization of one-photon-resonant Floquet-Weyl semimetals.

cond-mat.str-el↗

arXiv2026

Quasiperiodicity-induced non-Hermitian skin effect from the breakdown of scale-free localization

Non-reciprocal systems exhibit extreme sensitivity to boundary conditions, typically manifesting as the non-Hermitian skin effect (NHSE) under open boundaries. By bridging the boundaries with a tunable impurity bond, one can access intermediate regimes where scale-free localization (SFL) can emerge. Here, we investigate the competition between such boundary coupling and quasiperiodic disorder in a one-dimensional non-reciprocal lattice. Our analyses reveal a quasiperiodicity-induced breakdown of the SFL regime, which evolves into either the NHSE or an extended regime, depending on boundary conditions. These results uncover the crucial roles of boundary effects and quasiperiodicity in non-Hermitian systems.

cond-mat.mes-hall↗

arXiv2026

Quantum-geometric origin of superfluid weight in quasicrystals with critical states

A distinctive feature of many quasiperiodic systems is the presence of critical states that are neither extended nor exponentially localized. We investigate the geometric effect on the superfluid weight in quasiperiodic systems with critical states at zero temperature. We employ both real-space and momentum-space approaches to superfluid weight in quasicrystals, which allows us to separate the conventional and quantum geometric contributions. We find that the superfluid weight is dominated by the geometric contribution in quasiperiodic systems with critical states. This finding reveals a fundamental interplay between superconductivity and critical states in quasicrystals.

cond-mat.supr-con↗

arXiv2026

Chern insulators and topological flat bands in cavity-embedded kagome systems

We investigate topological band structures of a kagome system coupled to a circularly polarized cavity mode, using a model based on a muffin-tin potential and quantum light-matter interaction. We show that Chern insulating phases emerge in the cavity-embedded kagome system due to the light-matter interaction that breaks time-reversal symmetry. We also find that a nearly flat band can be topologically nontrivial with a nonzero Chern number. By varying the light-matter interaction, we also reveal that topological phase transitions occur between different Chern insulating phases in the ultrastrong coupling regime. The phase transitions change the sign of the Chern number, switching the direction of the edge current. We demonstrate the existence of topological edge modes in the cavity-embedded kagome Chern insulators by constructing a low-energy effective tight-binding model.

cond-mat.mes-hall↗

arXiv2025

Quasiperiodicity-induced bulk localization with self similarity in non-Hermitian systems

We analyze the localization behavior in a non-Hermitian system subject to a quasiperiodic onsite potential. We characterize localization transitions using multiple quantitative indicators, including inverse participation ratio (IPR), eigenstate fractal dimension (EFD), extended eigenstate ratio (EER), and spectral survival ratio. Despite the breaking of self-dual symmetry due to non-Hermiticity, our results reveal the existence of a critical potential strength, with its value increasing linearly with the nearest-neighbor antisymmetric hopping term. On the other hand, the inclusion of longer-range hopping not only enriches the topological properties but also gives rise to novel localization phenomena. In particular, it induces the emergence of mobility edges, as evidenced by both IPR and EFD, along with distinct features in the spectrum fractal dimension, which we extract using the box-counting method applied to the complex energy spectrum. Additionally, we uncover self-similar structures in various quantities, such as EER and complex eigenvalue ratio, as the potential strength varies. These findings highlight important aspects of localization and fractal phenomena in non-Hermitian quasiperiodic systems.

cond-mat.mes-hall↗

arXiv2025

Exceptional second-order topological insulators

Point-gap topological phases of non-Hermitian systems exhibit exotic boundary states that have no counterparts in Hermitian systems. Here, we develop classification of second-order point-gap topological phases protected by reflection symmetry. Based on this classification, we propose exceptional second-order topological insulators, exhibiting second-order boundary states stabilized by point-gap topology. As an illustrative example, we uncover a two-dimensional exceptional second-order topological insulator with point-gapless corner states. Furthermore, we identify a three-dimensional exceptional second-order topological insulator that features hinge states with isolated exceptional points, representing second-order topological phases intrinsic to non-Hermitian systems. Our work enlarges the family of point-gap topological phases in non-Hermitian systems.

cond-mat.mes-hall↗

arXiv2025

Gapless superconductivity and its real-space topology in quasicrystals

We study superconductivity in Ammann-Beenker quasicrystals under magnetic field. By assuming an intrinsic $s$-wave pairing interaction and solving for mean-field equations self-consistently, we find gapless superconductivity in the quasicrystals at and near half filling. We show that gapless superconductivity originates in broken translational symmetry and confined states unique to the quasicrystals. When Rashba spin-orbit coupling is present, the quasicrystalline gapless superconductor can be topologically nontrivial and characterized by a nonzero pseudospectrum invariant given by a spectral localizer. The gapless topological superconducting phase exhibits edge states with near-zero energy. These findings suggest that quasicrystals can be a unique platform for realizing gapless superconductivity with nontrivial topology.

cond-mat.supr-con↗

arXiv2024

Weyl superconductivity and quasiperiodic Majorana arcs in quasicrystals

Weyl superconductivity is a topological phase in three-dimensional crystals in which the Weyl equation describes quasiparticle excitation near band-touching points in momentum space called Weyl nodes. For quasicrystals which lack translational symmetry, a theory of Weyl superconductivity has not been established, in spite of recent extensive studies on quasicrystalline topological phases. Here, we demonstrate the occurrence of quasicrystalline Weyl superconductivity by extending the definition of Weyl superconductivity to periodically stacked, two-dimensional superconducting quasicrystals. We identify quasicrystalline Weyl nodes -- topologically protected point nodes in one-dimensional momentum space corresponding to the stacking direction -- in terms of a topological invariant given by a change in the Bott index in quasicrystalline layers. We find that these Weyl nodes exist in pairs and that Majorana zero-energy modes protected by the nonzero Bott index between a pair of quasicrystalline Weyl nodes appear on surfaces. These Majorana zero modes form an infinite number of arcs in momentum space, densely and quasiperiodically distributed as a function of momentum in the direction of surfaces within each quasicrystalline layer. In Ammann-Beenker (Penrose) quasicrystals, the quasiperiodicity of Majorana arcs is governed by the silver (golden) ratio associated with the quasicrystalline structure.

cond-mat.supr-con↗

arXiv2024

Non-Hermitian skin effect enforced by nonsymmorphic symmetries

Crystal symmetries play an essential role in band structures of non-Hermitian Hamiltonian. In this paper, we propose a non-Hermitian skin effect (NHSE) enforced by nonsymmorphic symmetries. We show that the NHSE inevitably occurs if a two-dimensional non-Hermitian system satisfies conditions derived from the nonsymmorphic symmetry of the doubled Hermitian Hamiltonian. This NHSE occurs in symmetry classes with and without time-reversal symmetry. The NHSE enforced by nonsymmorphic symmetries always occurs simultaneously with the closing of the point gap at zero energy. We also show that such a NHSE can occur in specific three-dimensional space groups with nonsymmorphic symmetries.

cond-mat.mes-hall↗

arXiv2022

Rotoinversion-symmetric bulk-hinge correspondence and its applications to higher-order Weyl semimetals

We give a bulk-hinge correspondence for higher-order topological phases protected by rotoinversion $C_{4}\mathcal{I}$ symmetry in magnetic systems. Our approach allows us to show the emergence of the chiral hinge modes only from the information of the $C_{4}\mathcal{I}$ eigenvalues at the high-symmetry points in the Brillouin zone. In addition, based on the bulk-hinge correspondence, we propose a class of higher-order Weyl semimetals (HOWSMs) being Weyl semimetals with hinge modes and Fermi-arc surface states. The HOWSM is characterized by topological invariants for three-dimensional higher-order topological insulators, and the topological invariants are determined by the $C_{4}\mathcal{I}$ symmetry eigenvalues at the high-symmetry points. This HOWSM has chiral hinge modes as a direct consequence of the three-dimensional higher-order topology in the bulk.

cond-mat.mes-hall↗

arXiv2021

Discriminant indicators with generalized inversion symmetry

We propose indicators of the discriminant for systems with generalized inversion symmetry which are computed from data only at high-symmetry points in the Brillouin zone. Our approach captures the exceptional points and their symmetry-protected variants without ambiguity arising from the reference energy, which is advantage over the previously known indicators for non-Hermitian systems. As demonstrations, we systematically analyze $3\times 3$-Hamiltonians where the proper choice of the reference energy is not obvious.

cond-mat.mes-hall↗

arXiv2021

Mirror-symmetry-protected dynamical quantum phase transitions in topological crystalline insulators

Dynamical quantum phase transitions (DQPTs) are topologically characterized in quantum quench dynamics in topological systems. In this paper, we study Loschmidt amplitudes and DQPTs in quantum quenches in mirror-symmetric topological phases. Based on the topological classification of mirror-symmetric insulators, we show that mirror symmetry creates symmetry-protected DQPTs. If mirror symmetry is present, topologically robust DQPTs can occur in quantum quenches, even in high-dimensional time-reversal invariant systems. Then, we also show that symmetry-protected DQPTs occur in quenches in two-dimensional chiral-symmetric systems with mirror symmetry. Mirror-symmetry-protected DQPTs can be easily captured by a reduced rate function. Moreover, we introduce dynamical topological order parameters for mirror-symmetry-protected DQPTs. Finally, we demonstrate DQPTs using lattice models for a time-reversal invariant topological crystalline insulator and a higher-order topological insulator.

cond-mat.mes-hall↗

arXiv2021

Non-Hermitian band topology with generalized inversion symmetry

Non-Hermitian skin effects and exceptional points are topological phenomena characterized by integer winding numbers. In this study, we give methods to theoretically detect skin effects and exceptional points by generalizing inversion symmetry. The generalization of inversion symmetry is unique to non-Hermitian systems. We show that parities of the winding numbers can be determined from energy eigenvalues on the inversion-invariant momenta when generalized inversion symmetry is present. The simple expressions for the winding numbers allow us to easily analyze skin effects and exceptional points in non-Hermitian bands. We also demonstrate the methods for (second-order) skin effects and exceptional points by using lattice models.

cond-mat.mes-hall↗

arXiv2020

Second-order topological non-Hermitian skin effects

Higher-order topology realizes topologically robust corner modes as a manifestation of nontriviality. We theoretically propose non-Hermitian skin effects which stem from second-order topology of chiral-symmetric Hermitian systems. It is found that the skin modes are localized at the corners. We demonstrate two types of second-order topological skin effects by two-dimensional intrinsic and extrinsic second-order topology. The intrinsic second-order topological skin effect is characterized topologically by bulk inversion symmetry as well as chiral symmetry. Meanwhile, the extrinsic second-order topological skin effect occurs from the topological correspondence between the edges and corners. We show that the non-Hermitian skin modes emerge by using a relationship between second-order and conventional first-order topology.

cond-mat.mes-hall↗

arXiv2019

Second-order topological phases protected by chiral symmetry

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.

cond-mat.mes-hall↗

arXiv2019

Topological exceptional surfaces in non-Hermitian systems with parity-time and parity-particle-hole symmetries

We study a topological band degeneracy in non-Hermitian systems with parity-time ($PT$) and parity-particle-hole ($CP$) symmetries. In $d$-dimensional non-Hermitian systems, it is shown that $(d-1)$-dimensional exceptional surfaces can appear from band touching thanks to $PT$ or $CP$ symmetry. We investigate the topological stability and zero-gap quasiparticles for the exceptional surfaces due to the band degeneracy. We also demonstrate the band degeneracy by using lattice models of a topological semimetal and a superconductor.

cond-mat.mes-hall↗

arXiv2018

Generic phase diagram for Weyl superconductivity in mirror-symmetric superconductors

We study topological phase transitions in three-dimensional odd-parity or noncentrosymmetric superconductors with mirror symmetry when time-reversal symmetry is broken. We construct a generic phase diagram for Weyl superconductivity in the mirror-symmetric superconductors. It is shown that Weyl superconductivity generally emerges between the trivial and the topological crystalline superconductor phases. We demonstrate how a trajectory of the Weyl nodes determines the change in mirror Chern numbers in the topological phase transition. We also discuss a relationship between particle-hole symmetry and the trajectory of the Weyl nodes which realizes the topological crystalline superconductor phase.

cond-mat.supr-con↗

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↗