arXiv subjects

Takuto Kawakami

Publications and source records attributed to Takuto Kawakami.

At least 19 recordsLinked to original sources

arXiv2026

Up/down-conversion of infrared light by few-layer graphene polytypes

Optical nonlinearity of materials with broken inversion symmetry enables two-photon processes where an incoming pair of photons can generate an up-converted photon with the combined frequency, or a high-energy photon can be split into a correlated pair of down-converted photons. Here, we identify few-layer graphene films that offer up/down-conversion capability in the infrared spectral range. For mixed stacking tetralayers (ABCB), which are non-centrosymmetric 2D crystals, we find highly efficient nearly resonant up/down-conversion of [$ω_1,ω_2$] photon pairs into/from a photon with $0.7$eV$<Ω=ω_1+ω_2<1.1$eV. We also note that a pronounced second-order nonlinearity in the spectral range of $0.7$eV$<Ω<0.9$eV can be promoted in rhombohedral tri- and tetralayers by asymmetrical encapsulation. Potentially, for fibers coated with few-layer graphene, this opens the door for ``in fiber" production of photon pairs with correlated polarizations.

cond-mat.mes-hall↗

arXiv2026

One-Dimensional Electronic States in a Moiré Superlattice of Twisted Bilayer WTe2

One-dimensional (1D) moiré superlattices provide a new route to engineering reduced-dimensional electronic states in van der Waals materials, yet their electronic structure and microscopic origin remain largely unexplored. Here, we investigate the structural relaxation and electronic properties of a 1D moiré superlattice formed in twisted bilayer 1T$'$-WTe$_2$ using density functional theory calculations, complemented by high-angle annular dark-field scanning transmission electron microscopy. We show that lattice relaxation strongly reconstructs the moiré stripes, leading to stacking-dependent stripe widths that are in excellent agreement with experimental observations. The relaxed structure hosts quasi-one-dimensional electronic bands near the Fermi level, characterized by strong dispersion along the stripe direction and nearly flat dispersion in the perpendicular direction. By comparing the full bilayer with isolated relaxed layers, we establish that these 1D electronic states are governed predominantly by an intralayer moiré potential induced by in-plane lattice relaxation, rather than by interlayer hybridization. We extract this position-dependent moiré potential directly from DFT calculations and construct an effective tight-binding model that reproduces both the band dispersion and the real-space localization of the electronic wave functions. Our results identify lattice relaxation as the key mechanism underlying 1D electronic states in 1D moiré superlattices. %and establish twisted bilayer WTe$_2$ as a promising platform for exploring emergent one-dimensional moiré physics. The framework developed here provides a unified theoretical basis for realizing and exploring one-dimensional moiré physics in a broad class of anisotropic two-dimensional materials.

cond-mat.mes-hall↗

arXiv2026

Second-harmonic generation in twisted double bilayer graphene: Double-resonant enhancement from moiré flat bands

We theoretically investigate second-harmonic generation (SHG) in twisted double bilayer graphene (TDBG) with AB--AB and AB--BA stacking configurations using a perturbative approach based on an effective continuum Hamiltonian. We present a systematic analysis of the SHG response as a function of twist angle, vertical bias voltage, Fermi energy, and stacking configuration. We find that the SHG signal is strongly enhanced at small twist angles due to the emergence of moiré flat bands and the associated increase in the joint density of states. Beyond this conventional enhancement mechanism, we demonstrate that the reduced bandwidth enables a pronounced double-resonant process, in which optical transitions at both $ω$ and $2ω$ are simultaneously satisfied over extended regions of the moiré Brillouin zone. This mechanism leads to a substantial amplification of the SHG response, analogous to the enhancement observed in systems with discrete energy levels, but realised here in a tuneable moiré band structure. Furthermore, we show that the AB--AB and AB--BA configurations exhibit a systematic $π$ phase shift in the SHG response at large bias voltages, reflecting their distinct symmetry and electronic structure. Our results identify double-resonance effects as a generic mechanism for enhancing SHG in moiré systems and provide a unified framework for understanding and controlling nonlinear optical responses in tuneable flat-band materials.

cond-mat.mes-hall↗

arXiv2026

Twist-configured moire-moire reconstruction governs diverse commensurate double-moire phases in twisted bilayer graphene on h-BN

The coexistence of multiple moire lattices in van der Waals heterostructures raises a fundamental question: how do distinct moire patterns interact and reconstruct? Here, we investigate twisted bilayer graphene (tBG) on hexagonal boron nitride (h-BN), where tBG and graphene/h-BN moire structures coexist, using conductive atomic force microscopy combined with continuum-model simulations. We show that reconstruction between these moire lattices-moire-moire reconstruction-manifests across multiple length scales, giving rise to diverse commensurate double-moire phases. Locally, the stacking registry between the two moire lattices is uniquely selected by the global twist configuration (helical or alternate), mediated by rotational relaxation of the shared graphene layer. This registry, together with twist angle and strain, governs commensurate domains from C3z-symmetric to strained symmetry-modified structures. These results establish moire-moire reconstruction as a general framework for engineering structural and electronic order -- including theoretically predicted topological flat bands below the magic angle -- in multilayer moire materials.

cond-mat.mtrl-sci↗

arXiv2026

Ferroelectricity in a magnon Bose-Einstein condensate: Nonreciprocal superfluidity, exceptional points, and Majorana bosons

We investigate a ferroelectric instability of a magnon Bose-Einstein condensate, mediated by its interaction with an electric field through a geometric Aharonov-Casher (AC) phase. A distinct feature of the system is the positive feedback loop in which an electric field induces magnon orbital motion via the AC phase, generating electric polarization that in turn enhances the original field. Based on bosonic Bogoliubov-de Gennes (BdG) mean-field theory, we show that this feedback drives a spontaneous ferroelectric transition in the magnon superfluid, accompanied by a persistent magnon supercurrent. In the resulting ferroelectric phase, the quasiparticle excitation spectrum becomes nonreciprocal, reflecting spontaneous breaking of spatial inversion symmetry. At the critical point of the transition, the bosonic BdG Hamiltonian exhibits a global coalescence of both eigenvalues and eigenvectors, forming exceptional points throughout the entire Brillouin zone. The corresponding eigenvector is an equally weighted superposition of bosonic quasiparticle and quasihole states and is invariant under particle-hole transformation, allowing it to be interpreted as a bosonic analog of a Majorana fermion.

cond-mat.mes-hall↗

arXiv2026

Landau levels and magneto-optics in 30$^\circ$ quasi-periodic twisted bilayer graphene

We develop a theoretical framework for Landau levels in quasi-periodic twisted bilayer graphene at a $30^\circ$ twist angle, a system without translational symmetry but possessing 12-fold rotational symmetry. Using a quasi-band formalism, we incorporate the magnetic field through a conventional momentum substitution in the zero-field Hamiltonian. This approach provides a transparent physical interpretation by directly relating the Landau levels to the quasi-band structure, allowing them to be understood as quantized orbits of quasi-band pockets. By using this method, we reveal distinctive spectral features, including nearly flat bands with weak magnetic-field dependence and highly degenerate levels arising from twelve off-center pockets. The resulting Landau levels are classified by two quantum numbers: the Landau-level index and the angular momentum associated with the underlying quasicrystalline symmetry. We also compute the magneto-optical conductivity and show that optical transitions follow angular-momentum selection rules enforced by the 12-fold symmetry. Our approach provides a symmetry-based and computationally efficient framework for bulk quantum magneto-optics in quasicrystalline van der Waals systems, predicting spectroscopic signatures accessible in high-field infrared and THz experiments.

cond-mat.mes-hall↗

arXiv2026

Quantum higher-spin Hall insulators

We develop a theory of quantum spin Hall insulators with arbitrary spin $J$. Our analysis demonstrates that such systems support $J+\tfrac{1}{2}$ pairs of helical edge modes protected by nontrivial mirror Chern numbers. We establish that the corresponding edge theory is described by a generalized Dirac fermion with higher-order dispersion. These modes produce unique transport responses that are non-linear with voltage. An in-plane magnetic field opens a mass gap in the edge spectrum, and magnetic domain walls host $(J+\tfrac{1}{2})$-fold degenerate bound states characterized by nontrivial winding numbers. Our results extend quantum spin Hall physics to higher-spin systems and suggest possible realizations in ultracold atomic gases.

cond-mat.mes-hall↗

arXiv2026

Non-Abelian Chern band in rhombohedral graphene multilayers

Moiré flat bands in rhombohedral multilayer graphene provide a platform for exploring interaction-driven topological phases, where a single isolated band often forms a Chern band. However, non-Abelian degenerate Chern bands with internal symmetries such as $\mathrm{SU}(N)$ have so far been realized only in highly engineered systems. Here, we show that a doubly degenerate non-Abelian Chern band with Chern number $|C|=1$ emerges spontaneously at filling $ν=2$ in rhombohedral 3-, 4-, and 5-layer graphene, regardless of the presence of an hBN substrate. Using self-consistent Hartree-Fock calculations, we map out phase diagrams as functions of displacement field and electronic periodicity, and analytically demonstrate that the Fock term drives spontaneous symmetry breaking and generates non-Abelian Berry curvature. We further show that this non-Abelian topology is characterized by $\mathrm{SU}(2)$ gauge flux threading the noncontractible cycles of the Brillouin zone, leading to a global non-Abelian holonomy. Our findings unveil a new class of interaction-driven non-Abelian topological phases, distinct from quantum anomalous Hall and fractional Chern phases.

cond-mat.mes-hall↗

arXiv2026

Resonant Structure of Second Harmonic Generation in Multilayer Graphene Polytypes

Second harmonic generation (SHG) is a powerful optical tool for identifying non-centrosymmetric crystalline structures. Here, we analyze SHG in multilayer graphenes (MLG), with a focus on its dependence on the stacking order, encapsulation environment and biasing which break inversion symmetry in multilayers, as well as the SHG sensitivity to the electron-hole asymmetry in the MLG spectra and doping. In particular, we identify stacking-order-dependent resonant features in the SHG spectra for trilayers and tetralayers, suggesting that infra-red range SHG offers a non-invasive characterization method for distinguishing between MLG polytypes, as well as optical identification of crystallographic direction in MLG films.

cond-mat.mes-hall↗

arXiv2026

Singular flat bands in three dimensions: Landau level spreading, quantum geometry, and Weyl reconstruction

We theoretically investigate three-dimensional singular flat band systems, focusing on their quantum geometric properties and response to external magnetic fields. As a representative example, we study the pyrochlore lattice, which hosts a pair of degenerate flat bands touching a dispersive band. We derive a three-orbital effective continuum model that captures the essential features near the band-touching point. Within this framework, we identify the point-like topological singularity on a planar manifold defined by the degenerate flat band eigenvectors. This singularity strongly influences the quantum geometry and results in a characteristic Landau level structure, where the levels spread over a finite energy range. We show that this structure reflects the underlying band reconstruction due to the orbital Zeeman effect, which lifts the flat band degeneracy and induces the Weyl-semimetal-like dispersion near the singularity. Our analysis reveals that the range of Landau level spreading is proportional to the quantum metric of each Zeeman-split band. We further demonstrate that adding a small dispersion via longer-range . Finally, we show that our approach extends naturally to systems with higher orbital angular momentum, indicating the robustness of these features in a broad class of three-dimensional flat band models.

cond-mat.mes-hall↗

arXiv2026

Twist-angle tunable Josephson junctions in three-dimensional superconductors

We theoretically investigate the superconducting phase and perpendicular Josephson supercurrent in twisted three-dimensional (3D) superconductors, where two layered 3D materials are stacked with a relative twist. We formulate the Bogoliubov-de Gennes Hamiltonian and develop a self-consistent method to calculate the superconducting order parameter and the resulting supercurrent. Applying this framework to a toy model with Fermi surfaces located near the Brillouin zone corners, we demonstrate a phase discontinuity at the twisted interface, indicating that a Josephson junction is formed purely by the twist. Our calculations reveal that the interface supports a finite critical current even when the Fermi surfaces of the two superconductors are completely separated, unlike in the case of a twisted normal-metal interface. We further show that the critical current can be effectively controlled by the twist angle, transitioning from a high-transparency regime at small angles to a low-transparency regime at larger angles.

cond-mat.mes-hall↗

arXiv2025

Novel coupling between charge order and time-reversal-symmetry-breaking superconductivity

The interplay between charge-density waves (CDWs), which break translational symmetry, and spatially homogeneous superconductivity, which breaks global U(1) gauge symmetry, can give rise to an intriguing phenomenon: the pair-density wave, characterized by a spatial modulation of the superconducting order parameter. Yet how CDWs couple to unconventional superconducting states-particularly those with time-reversal symmetry breaking (TRSB)-remains largely unexplored. Here, using scanning tunneling microscopy on heavily hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, which hosts an s $\pm$ is superconducting state, we reveal a previously unobserved coupling between a surface CDW and TRSB superconductivity. Experimentally, the TRSB superconductivity imparts "chirality" to the CDW, which manifests as commensurate domains separated by domain walls with $π$-phase slips-forming what we term a bipolar CDW. The domain walls delineate TRSB domains of opposite chirality, consistent with spontaneous breaking of U(1) $\times$ Z2. Supported by theoretical modelling, we construct a framework in which a hidden interfacial pair-density modulation (PDM) mediates a linear coupling between the surface CDW and interband Josephson currents of TRSB superconductivity. Crucially, the theory shows that realizing this linear coupling requires a controlled global phase difference $δ$ $ϕ$ = $π$/2 between the PDM and CDW states. Our results uncover a previously overlooked connection between charge ordering and TRSB superconductivity, opening a pathway to explore intertwined quantum orders in iron-based superconductors and other strongly correlated systems.

cond-mat.supr-con↗

arXiv2025

One-dimensional moiré engineering in zigzag graphene nanoribbons on hBN

We study the structural relaxation and electronic properties of a one-dimensional (1D) moiré system composed of a zigzag graphene nanoribbon (GNR) placed on a hexagonal boron nitride (hBN) substrate. Using an effective grid model derived from continuum elasticity theory, we calculate the relaxed atomic structure of the GNR/hBN system for various twist angles and ribbon widths. The relaxation gives rise to a characteristic 1D domain structure consisting of alternating commensurate AB$'$ regions and two distinct types of domain boundaries. At finite twist angles, the ribbon adopts a wavy shape, locally tracing the hBN zigzag direction but occasionally sliding to adjacent atomic rows. The resulting moiré potential strongly modulates the electronic structure: the zero-energy zigzag edge states are modulated by the local stacking, leading to densely packed subbands in the AB$'$ domains and sharply localized domain-wall states in the energy gaps between domain plateaus, which together realize gate-tunable one-dimensional arrays of quantum-confined electronic states. Our results demonstrate that moiré modulation in GNR/hBN heterostructures provides a versatile platform for electronic structure engineering and the design of 1D moiré nanodevices.

cond-mat.mes-hall↗

arXiv2025

Shift current response in twisted double bilayer graphenes

We calculate the shift current response in twisted double bilayer graphenes (TDBG) by applying the perturbative approach to the effective continuum Hamiltonian. We have performed a systematic study of the shift current in AB-AB and AB-BA stacked TDBG, where we have investigated the dependence of the signal on the twist angle, the vertical bias voltage and the Fermi level. The numerical analyses demonstrate that the signal is greatly enhanced as the twist angle is reduced. Notably, we also found that there is a systematic sign reversal of the signal in the two stacking configurations below the charge neutrality point for large bias voltages. We qualitatively explain the origin of this sign reversal by studying the shift current response in AB-stacked bilayer graphene.

cond-mat.mes-hall↗

arXiv2025

Moiré Band Engineering in Twisted Trilayer WSe2

We present a systematic theoretical study on the structural and electronic properties of twisted trilayer transition metal dichalcogenide (TMD) WSe$_2$, where two independent moiré patterns form between adjacent layers. Using a continuum approach, we investigate the optimized lattice structure and the resulting energy band structure, revealing fundamentally different electronic behaviors between helical and alternating twist configurations. In helical trilayers, lattice relaxation induces $αβ$ and $βα$ domains, where the two moiré patterns shift to minimize overlap, while in alternating trilayers, $αα'$ domains emerge with aligned moiré patterns. A key feature of trilayer TMDs is the summation of moiré potentials from the top and bottom layers onto the middle layer, effectively doubling the potential depth. In helical trilayers, this mechanism generates a Kagome lattice potential in the $αβ$ domains, giving rise to flat bands characteristic of Kagome physics. In alternating trilayers, the enhanced potential confinement forms deep triangular quantum wells, distinct from those found in bilayer systems. Furthermore, we demonstrate that a moderate perpendicular electric field can switch the layer polarization near the valence band edge, providing an additional degree of tunability. In particular, it enables tuning of the hybridization between orbitals on different layers, allowing for the engineering of diverse and controllable electronic band structures. Our findings highlight the unique role of moiré potential summation in trilayer systems, offering a broader platform for designing moiré-based electronic and excitonic phenomena beyond those achievable in bilayer TMDs.

cond-mat.mes-hall↗

arXiv2025

Low-temperature thermal transport in moiré superlattices

We calculate the phonon thermal conductivity of various moiré bilayer systems using a continuum approach and the semiclassical transport theory. When the twist angle is close to 0, we observe a significant reduction of thermal conductivity in a particular low-temperature regime. This reduction is attributed to a moiré-induced reconstruction of acoustic phonon bands and associated decrease of the group velocity. Conversely, in the zero temperature limit, the thermal conductivity is enhanced by moiré effect, surpassing the original values in non-moiré counterparts. These changes result in a characteristic temperature dependence which deviates from the quadratic behavior in intrinsic two-dimensional systems.

cond-mat.mes-hall↗

arXiv2025

Electromagnetic response in dipole superfluids: vortex lattices and singular domain walls

Among the most significant macroscopic quantum phenomena in condensed matter physics is the Meissner effect observed in superconductivity, which arises from the unique interaction between superfluids of charged particles and electromagnetic fields. However, superfluids can also emerge from particles possessing distinct electromagnetic properties. In particular, there has been growing interest in superfluids composed of charge-neutral particles with magnetic or electric dipole moments, such as Bose-Einstein condensates of magnons or excitons. Despite this interest, the electromagnetic response of dipole superfluids, including potential analogs or contrasts to the Meissner effect, remains poorly understood. In this work, we develop a Ginzburg-Landau phenomenological theory to describe magnetic and electric dipole superfluids subjected to pseudo-magnetic fields induced by geometric phases. For magnetic dipole superfluids interacting with the Aharonov-Casher (AC) phase, we find that they form vortex lattices with sharply localized pseudo-magnetic fields along hexagonal domain walls, leading to singular and discontinuous change of physical variables at these boundaries. For electric dipole superfluids influenced by the He-McKellar-Wilkens (HMW) phase, in contrast, we identify vortex lattices where the pseudo-magnetic field and supercurrent are concentrated at vortex cores, resembling superconductors. These results reveal strikingly different electromagnetic responses in dipole superfluids, opening new directions for exploring superfluid systems with unconventional electromagnetic responses.

cond-mat.supr-con↗

arXiv2023

Perpendicular electronic transport and moiré-induced resonance in twisted interfaces of three-dimensional graphite

We calculate the perpendicular electrical conductivity in twisted three-dimensional graphite (rotationally stacked graphite pieces) by using the effective continuum model and the recursive Green's function method. In the low twist angle regime $(θ\lesssim 2^\circ)$, the conductivity shows a nonmonotonic dependence with a peak and dip structure as a function of the twist angle. By analyzing the momentum-resolved conductance and the local density of states, this behavior is attributed to the Fano resonance between continuum states of bulk graphite and interface-localized states, which is a remnant of the flat band in the magic-angle twisted bilayer graphene. We also apply the formulation to the high-angle regime near the commensurate angle $θ\approx 21.8^\circ$, and reproduce the conductance peak observed in the experiment.

cond-mat.mes-hall↗