arXiv subjects

Toshikaze Kariyado

Publications and source records attributed to Toshikaze Kariyado.

At least 19 recordsLinked to original sources

arXiv2026

Strain Correlated Linearly Polarized Photoluminescence in WS2/WSe2 Moiré Superlattices

Reliable optical control of valley degrees of freedom in moiré excitons requires that the emitted polarization faithfully reflect the underlying valley state. Here, we show that linearly polarized photoluminescence from WSe2/WS2 moiré excitons is largely insensitive to the excitation polarization and therefore does not arise from valley coherence. Automated polarization-resolved photoluminescence and Raman mapping at cryogenic temperature reveals that the degree of linear polarization correlates strongly with local Raman shifts and moiré-exciton observables, identifying strain as the dominant experimental correlate. Linear-regression analysis further shows that strain-related descriptors provide the best prediction of the observed polarization. Guided by theory, we attribute this behavior to strain-amplified breaking of C3 symmetry in the moiré potential: weak uniaxial strain produces only partial cancellation of locally elliptical emission, yielding a finite far-field degree of linear polarization. These results establish strain as a key control parameter for reliable optical readout in TMD moiré superlattices.

cond-mat.mtrl-sci↗

arXiv2026

Layer-Selective Proximity Symmetry Breaking Enables Anomalous and Nonlinear Hall Responses in 1H-TMD Metals

Nonlinear Hall responses are a direct electrical probe of quantum geometry, but they are symmetry-forbidden in many pristine two-dimensional metals. We show that layer-selective magnetic proximity unlocks intrinsic linear and nonlinear Hall effects in metallic $1H-NbX_2$ ($X=\mathrm{S,Se,Te}$), where native $D_{3h}$ symmetry forces both the anomalous Hall conductivity and the Berry-curvature dipole (BCD) to vanish. Fully relativistic density-functional theory combined with Wannier interpolation reveals that an out-of-plane proximity exchange that preserves $C_3$ generates a sizable sheet anomalous Hall conductivity, $σ^{\mathrm{sheet}}_{xy} \sim 10^{-2}(e^2/h)$, while keeping the BCD exactly zero. Breaking $C_3$ by adding an in-plane exchange component (or an orthogonal two-sided exchange texture) produces a strongly tunable BCD and hence a nonlinear Hall conductivity that is odd and approximately linear in the in-plane exchange scale, reaching $|D_y|$ of order $10^{-2}$ angstrom and maximized in NbTe$_2$. These magnitudes imply a readily measurable second-harmonic Hall voltage in micron-scale Hall bars under mA ac drive. We further propose a dual-interface device in which the signs of the first- and second-harmonic Hall voltages provide two-bit readout using the same contacts.

cond-mat.mes-hall↗

arXiv2026

Moiré in $Γ$-valley square lattice: Copper- and iron-based superconductor simulation in a single device

Novel superconducting phases have been found in various moiré heterostructures based on hexagonal lattices. However, the archetypal high-temperature superconductors (cuprates, iron-based and nickelate families) all share a square lattice foundation. These materials host a rich landscape of correlated phenomena, such as charge and spin stripes, pseudogap behavior, and unconventional metallicity, which continue to challenge our fundamental understanding of strongly correlated electrons. In this work, we investigate the possibility of simulating the effective models governing these high-$T_c$ superconductors using twisted homobilayers of $Γ$-valley square-lattice systems. We develop a universal theoretical framework and carry out a detailed analysis of a promising candidate material ZnF$_2$. We find that the first moiré band realizes a single-orbital square-lattice Hubbard model, widely believed to capture cuprate physics, while the second and third moiré bands map to a $p_x,p_y$ two-orbital square-lattice Hubbard model, which shares common physics to the minimal $d_{xz}, d_{yz}$ models proposed for iron pnictides. Our study combines continuum Hamiltonian modeling, first-principle calculations, and Hartree-Fock mean field theory. The latter focuses on the quarter-filling regime of the two-orbital model and in particular leads to, among others, a stable antiferro-orbital, ferromagnetic insulating phase. These results highlight $Γ$-valley square-lattice moiré systems as a new and important generation of van der Waals heterostructures to realize interesting strongly correlated phases of matter.

cond-mat.str-el↗

arXiv2025

Domain Wall Sliding-induced Polarization Switching in Multilayer Graphene

Electric polarization and metallicity are long believed not to coexist until the emergence of exceptionally rare material examples including the bulk polar metals and more recently two-dimensional (2D) van der Waals (vdW) materials such as 1T' WTe2. The electric polarization for the latter represents a new and distinguishable paradigm in materials science and physics because its electric polarization states embedded in the conduction electron sea are able to couple with (and controlled by) the external electric field. However, the microscopic polarization switching process and mechanism in these 2D vdW metallic materials have not been experimentally observed and remain elusive. Here, we report the first direct experimental imaging of the microscopic mechanism behind electric-field-coupled polarization switching in a metallic system. Our gate-tunable nanoscale optical imaging identifies the robust coexistence of electric polarization and appreciable carrier densities in adjacent polar domains hosting opposite electric polarizations in a 2D elemental metallic material, tetralayer graphene. We directly visualize and verify that the sliding domain wall (DW) solitons confined at the middlemost interface are responsible for the polarization switching in tetralayer graphene upon the application of electric fields and mechanical forces. Our work provides the first direct visualization of domain wall sliding-induced polarization switching in 2D elemental carbon at room temperature, significantly expanding and advancing the research of "ferroelectric metal" initially dubbed by P. W. Anderson and coauthors in 1965.

cond-mat.mes-hall↗

arXiv2025

Single-band square lattice Hubbard model from twisted bilayer C568

We propose twisted homobilayer of a carbon allotrope, C$_{568}$, to be a promising platform to realize controllable square lattice single-band extended Hubbard model. This setup has the advantage of a widely tunable $t'/t$ ratio without adding external fields, and the intermediate temperature $t\ll T\ll U$ regime can be easily achieved. We first analyze the continuum model obtained from symmetry analysis and first-principle calculations, and calculate the band structures. Subsequently, we derive the corresponding tight-binding models and fit the hopping parameters as well as the Coulomb interactions. When displacement field is applied, anisotropic nearest neighbor hoppings can further be achieved. If successfully fabricated, the device could be an important stepping stone towards understanding high-temperature superconductivity.

cond-mat.str-el↗

arXiv2025

Strongly hybridized phonons in one-dimensional van der Waals crystals

The phenomena of pronounced electron-electron and electron-phonon interactions in one-dimensional (1D) systems are ubiquitous, which are well described by frameworks of Luttinger liquid, Peierls instability and concomitant charge density wave. However, the experimental observation of strongly hybridized phonons in 1D was not demonstrated. Herein we report the first observation of strongly hybridized phonons in 1D condensed matters by using double-walled carbon nanotubes (DWNTs), representative 1D van der Waals crystals, with combining the spectroscopic and microscopic tools as well as the ab initio density functional theory (DFT) calculations. We observe uncharted phonon modes in one commensurate and three incommensurate DWNT crystals, three of which concurrently exhibit strongly-reconstructed electronic band structures. Our DFT calculations for the experimentally observed commensurate DWNT (7,7) @ (12,12) reveal that this new phonon mode originates from a (nearly) degenerate coupling between two transverse acoustic modes (ZA modes) of constituent inner and outer nanotubes having approximately trigonal and pentagonal rotational symmetry along the nanotube circumferences. Such coupling strongly hybridizes the two phonon modes in different shells and leads to the formation of a unique lattice motion featuring evenly distributed vibrational amplitudes over inner and outer nanotubes, distinct from any known phonon modes in 1D systems. All four DWNTs that exhibit the pronounced new phonon modes show small chiral angle twists, closely matched diameter ratios of 3/5 and decreased frequencies of new phonon modes with increased diameters, all supporting the uncovered coupling mechanism. Our discovery of strongly hybridized phonons in DWNTs open new opportunities for engineering phonons and exploring novel phonon-related phenomena in 1D condensed matters.

cond-mat.mes-hall↗

arXiv2024

Comment on "Absence of Topological Protection of the Interface States in $\mathbb{Z}_2$ Photonic Crystals"

In the Letter, Xu et al. reported that edge modes disappear in the expanded structure of Wu-Hu model characterized by $\mathbb{Z}_2$ topological index, while appear in the trivial shrunken structure, when the edge cuts through the hexagonal unit cell. They then concluded that these edge modes are defect modes lacking topological protection. Unfortunately, their approach is not justified, rendering the conclusion unsolid.

physics.optics↗

arXiv2024

Realization of topological phase in a chiral honeycomb lattice model

We investigate topological properties of a chiral honeycomb lattice model with next-nearest-neighbor hoppings characterized by the reflection symmetry breaking. Topological nontriviality is detected by analyzing effective Dirac Hamiltonian, and confirmed by numerical and analytical study of the emergence of topological edge states at the boundaries between topologically distinct regions. We have also discovered that a novel asymmetric edge current attributable to chirality can be excited without any involved phase shifts in input sources to pick up one of the pseudospin components.

cond-mat.mes-hall↗

arXiv2024

Higher-order topology in honeycomb lattice with Y-Kekulé distortions

We investigate higher-order topological states in honeycomb lattice with Y-Kekulé distortions that preserve $C_{6v}$ crystalline symmetry. The gapped states in expanded and shrunken distortions are adiabatically connected to isolated hexamers and Y-shaped tetramer states, respectively, where the former possesses nontrivial higher-order topology characterized by a $\mathbb{Z}_6$ invariant. Topological corner states exist in a flake structure with expanded distortion where the hexamers are broken at the corners. Our work reveals that honeycomb lattice with Y-Kekulé distortions serves as a promising platform to study higher-order topological states.

cond-mat.mes-hall↗

arXiv2023

Possible gapless helical edge states in hydrogenated graphene

Electronic band structures in hydrogenated graphene are theoretically investigated by means of first-principle calculations and an effective tight-binding model. It is shown that regularly designed hydrogenation to graphene gives rise to a large band gap about 1 eV. Remarkably, by changing the spatial pattern of the hydrogenation, topologically distinct states can be realized, where the topological nontriviality is detected by $C_2$ parity indices in bulk and confirmed by the existence of gapless edge/interface states as protected by the mirror and sublattice symmetries. The analysis of the wave functions reveals that the helical edge states in hydrogenated graphene with the appropriate design carry pseudospin currents that are reminiscent of the quantum spin Hall effect. Our work shows the potential of hydrogenated graphene in pseudospin-based device applications.

cond-mat.mes-hall↗

arXiv2023

Topological electronic states in holey graphyne

We unveil that the holey graphyne (HGY), a two-dimensional carbon allotrope where benzene rings are connected by two $-$C$\equiv$C$-$ bonds fabricated recently in a bottom-up way, exhibits topological electronic states. Using first-principles calculations and Wannier tight-binding modeling, we discover a higher-order topological invariant associated with $C_2$ symmetry of the material, and show that the resultant corner modes appear in nanoflakes matching to the structure of precursor reported previously, which are ready for direct experimental observations. In addition, we find that a band inversion between emergent $g$-like and $h$-like orbitals gives rise to a nontrivial topology characterized by $\mathbb{Z}_2$ invariant protected by an energy gap as large as 0.52 eV, manifesting helical edge states mimicking those in the prominent quantum spin Hall effect, which can be accessed experimentally after hydrogenation in HGY. We hope these findings trigger interests towards exploring the topological electronic states in HGY and related future electronics applications.

cond-mat.mes-hall↗

arXiv2022

Twisted bilayer BC$_3$: Valley interlocked anisotropic flat bands

Here we propose BC$_3$, a graphene derivative that has been synthesized, as a platform to realize exotic quantum phases by introducing a moiré pattern with mismatched stacking. In twisted bilayer BC$_3$, it is shown that a crossover from two-dimensional to quasi one-dimensional band structure takes place with the twist angle as a control parameter. This is a typical manifestation of the guiding principle in van der Waals stacked systems: the quantum interference between Bloch wave functions in adjacent layers has a striking effect on the effective interlayer tunneling. Interestingly, quasi one-dimensionalization happens in a valley dependent manner. Namely, there is interlocking between the valley index and the quasi-1D directionality, which makes BC$_3$ a plausible candidate for valleytronics devices. In addition, the strongly correlated regime of the valley interlocked quasi-1D state reduces to an interesting variant of the Kugel-Khomskii model where intertwined valley and spin degrees of freedom potentially induces exotic quantum phases. Notably, this variant of the Kugel-Khomskii model cannot be realized in conventional solids due to the three fold valley degeneracy.

cond-mat.mes-hall↗

arXiv2021

Topological magnon modes on honeycomb lattice with coupling textures

Topological magnon modes are expected to be useful for novel applications such as robust information propagation, since they are immune to backscattering and robust against disorder. Although there are several of theoretical proposals for topological magnon modes and growing experimental efforts for realizing them by now, it is still desirable to add complementary insights on this important phenomenon. Here, we propose a new scheme to achieve topological magnon where only nearest-neighbour exchange couplings on honeycomb lattice are necessary. In both ferromagnets and antiferromagnets, tuning exchange couplings between and inside hexagonal unit cells induces a topological state accompanied by a band inversion between p-orbital and d-orbital like magnon modes. Topological magnon modes appear at the interface between a topological domain and a trivial domain with magnon currents, which counterpropagate depending on pseudospins originated from orbital angular momenta of magnon modes. This mimics the spin-momentum locking phenomenon in the quantum spin Hall effect.

cond-mat.mes-hall↗

arXiv2021

Disentangling Orbital Magnetic Susceptibility with Wannier Functions

Orbital magnetic susceptibility involves rich physics such as interband effects despite of its conceptual simplicity. In order to appreciate the rich physics related to the orbital magnetic susceptibility, it is essential to derive a formula to decompose the susceptibility into the contributions from each band. Here, we propose a scheme to perform this decomposition using the modified Wannier functions. The derived formula nicely decomposes the susceptibility into intraband and interband contributions, and from the other aspect, into itinerant and local contributions. The validity of the formula is tested in a couple of simple models. Interestingly, it is revealed that the quality of the decomposition depends on the degree of localization of the used Wannier functions. The formula here complements another formula using Bloch functions, or the formula derived in the semiclassical theory, which deepens our understanding of the orbital magnetic susceptibility and may serve as a foundation of a better computational method. The relationship to the Berry curvature in the present scheme is also clarified.

cond-mat.mes-hall↗

arXiv2021

Effective continuum model of twisted bilayer GeSe and origin of emerging one-dimensional mode

The electric structure of twisted bilayer GeSe, which shows a rectangular moiré pattern, is analyzed using a $\bm{k}\cdot\bm{p}$ type effective continuum model. The effective model is constructed on the basis of the the local approximation method, where the local lattice structure of a twisted bilayer system is approximated by its untwisted bilayer with parallel displacement, and the required parameters are fixed with the help of the first-principles method. By inspecting the twist angle dependence of the physical properties, we reveal a relation between the effective potential under moiré pattern and the alignment of the Ge atoms, and also the resultant one-dimensional flat band, where the band is flattened stronger in a specific direction than the perpendicular direction. Due to the relatively large effective mass of the original monolayers, the flat band with its band width as small as a few meV appear in a relatively large angle. This gives us an opportunity to explore the dimensional crossover in the twisted bilayer platform.

cond-mat.mes-hall↗

arXiv2020

Selective branching, quenching, and converting of topological modes

A salient feature of topological phases are surface states and many of the widely studied physical properties are directly tied to their existence. Although less explored, a variety of topological phases can however similarly be distinguished by their response to localized flux defects, resulting in the binding of modes whose stability can be traced back to that of convectional edge states. The reduced dimensionality of these objects renders the possibility of arranging them in distinct geometries, such as arrays that branch or terminate in the bulk. We show that the prospect of hybridizing the modes in such new kinds of channels poses profound opportunities in a dynamical context. In particular, we find that creating junctions of $π$-flux chains or extending them as function of time can induce transistor and stop-and-go effects. Pending controllable initial conditions certain branches of the extended defect array can be actively biased. Discussing these physical effects within a generally applicable framework that relates to a variety of established artificial topological materials, such as mass-spring setups and LC circuits, our results offer an avenue to explore and manipulate new transport effects that are rooted in the topological characterization of the underlying system.

cond-mat.mes-hall↗

arXiv2019

Flat band in twisted bilayer Bravais lattices

Band engineering in twisted bilayers of the five generic two-dimensional Bravais networks is demonstrated. We first derive symmetry-based constraints on the interlayer coupling, which helps us to predict and understand the shape of the potential barrier for the electrons under the influence of the moiré structure without reference to microscopic details. It is also pointed out that the generic constraints becomes best relevant when the typical length scale of the microscopic interlayer coupling is moderate. The concepts are numerically demonstrated in simple tight-binding models to show the band flattening due to the confinement into the potential profile fixed by the generic constraints. On the basis of the generic theory, we propose the possibility of anisotropic band flattening, in which quasi one-dimensional band dispersion is generated from relatively isotropic original band dispersion. In the strongly correlated regime, anisotropic band flattening leads to a spin-orbital model where intertwined magnetic and orbital ordering can give rise to rich physics.

cond-mat.str-el↗

arXiv2019

Many-body Chern number without integration

The celebrated work of Niu, Thouless, and Wu demonstrated the quantization of Hall conductance in the presence of many-body interactions by revealing the many-body counterpart of the Chern number. The generalized Chern number is formulated in terms of the twisted angles of the boundary condition, instead of the single particle momentum, and involves an integration over all possible twisted angles. However, this formulation is physically unnatural, since topological invariants directly related to observables should be defined for each Hamiltonian under a fixed boundary condition. In this work, we show via numerical calculations that the integration is indeed unnecessary - the integrand itself is effectively quantized and the error decays exponentially with the system size. This implies that the numerical cost in computing the many-body Chern number could, in principle, be significantly reduced as it suffices to compute the Berry connection for a single value of the twisted boundary condition if the system size is sufficiently large.

cond-mat.str-el↗