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Motohiko Ezawa

Publications and source records attributed to Motohiko Ezawa.

At least 37 records · Page 2Linked to original sources
arXiv2025

Out-of-plane Edelstein effects: Electric-field induced magnetization in $p$-wave magnets

In-plane magnetization is induced by the Edelstein effect in the Rashba spin-orbit interaction system. However, out-of-plane magnetization is more useful for switching a ferromagnetic memory. We study analytically and numerically electric-field induced magnetization in $p$-wave magnets with the aid of the Rashba interaction based on a simple two-band model. The out-of-plane magnetization is induced when the Néel vector of the $p$-wave magnet is along the $z$ direction. We also show that no magnetization is induced in the absence of the Rashba interaction. The electric-field induced magnetization will be useful for future switching technology of ferromagnetic memories based on the $p$-wave magnet.

cond-mat.mes-hall↗
arXiv2024

Realization of a one-dimensional topological insulator in ultrathin germanene nanoribbons

Realizing a one-dimensional (1D) topological insulator and identifying the lower dimensional limit of two-dimensional (2D) behavior are crucial steps toward developing high-density quantum state networks, advancing topological quantum computing, and exploring dimensionality effects in topological materials. Although 2D topological insulators have been experimentally realized, their lower dimensional limit and 1D counterparts remain elusive. Here, we fabricated and characterized arrays of zigzag-terminated germanene nanoribbons, a 2D topological insulator with a large topological bulk gap. The electronic properties of these nanoribbons strongly depend on their width, with topological edge states persisting down to a critical width (approx. 2 nm), defining the limit of 2D topology. Below this threshold, contrary to the tenfold way classification, we observe zero-dimensional (0D) states localized at the ends of the ultrathin nanoribbons. These end states, topologically protected by time-reversal and mirror symmetries, mark the first realization of a 1D topological insulator with strong spin-orbit coupling. Our findings establish germanene nanoribbons as a platform for investigating 1D topology and dimensionality effects in topological materials.

cond-mat.mes-hall↗
arXiv2024

Analytic approach to quantum metric and optical conductivity in Dirac models with parabolic mass in arbitrary dimensions

The imaginary part of the quantum geometric tensor is the Berry curvature, while the real part is the quantum metric. Dirac fermions derived from a tight-binding model naturally contains a mass term $m(k)$ with parabolic dispersion, $m(k)=$ $m+uk^{2}$. However, in the Chern insulator based on Dirac fermions, only the sign of the mass $m$ is relevant. Recently, it was reported that the quantum metric is observable by means of the optical conductivity, which is significantly affected by the parabolic coefficient $% u $. We analytically obtain the quantum metric and the optical conductivity in the Dirac Hamiltonian in arbitrary dimensions, where the Dirac mass has parabolic dispersion. The optical conductivity at the band-edge frequency significantly depends on the dimensions. We also make an analytical study on the quantum metric and the optical conductivity in the Su-Schrieffer-Heeger model, the Qi-Wu-Zhang model and the Haldane model. The optical conductivity is found to be quite different between the topological and trivial phases even when the gap is taken identical.

cond-mat.mes-hall↗
arXiv2024

Intrinsic nonlinear conductivity induced by quantum geometry in altermagnets and measurement of the in-plane Néel vector

The $z$-component of the Néel vector is measurable by the anomalous Hall conductivity in altermagnets because time reversal symmetry is broken. On the other hand, it is a nontrivial problem how to measure the in-plane component of the Néel vector. We study the second-order nonlinear conductivity of a system made of the $d$-wave altermagnet with the Rashba interaction. It is shown that the quantum-metric induced nonlinear conductivity and the nonlinear Drude conductivity are proportional to the in-plane component of the Néel vector, and hence, the in-plane component of the Néel vector is measurable. We obtain analytic formulas of the quantum-metric induced nonlinear conductivity and the nonlinear Drude conductivity both for the longitudinal and transverse conductivities. The quantum-metric induced nonlinear conductivity diverges at the Dirac point, while the nonlinear Drude conductivity is always finite. Hence, the quantum-metric induced nonlinear conductivity is dominant at the Dirac point irrespective of the relaxation time.

cond-mat.mes-hall↗
arXiv2024

Topological insulators and superconductors based on $p$-wave magnets,electrical control and detection of a domain wall

Altermagnets are time-reversal broken antiferromagnets, where the $z$ component of the Néel vector is detectable by anomalous Hall effects. On the other hand, recently proposed $p$-wave magnets are time-reversal preserved antiferromagnets, and it is a highly nontrivial problem how to detect and control a domain wall. We study a one-dimensional hybrid system made of a $p$-wave magnet and a metal possessing the orbital degree of freedom. The hybrid system is a topological insulator without the spin-orbit interaction. There emerge two edge states per one edge, because the system is mapped to a set of two copies of a topological insulator. Each copy resembles the long-range Su-Schrieffer-Heeger model but it is topologically different. Topological interface states emerge at a domain wall in the $p$-wave magnet, which are charged due to the Jackiw-Rebbi mechanism. Consequently, a domain wall in the $p$-wave magnet will be controllable and detectable purely by electrical means. We also study Majorana fermions induced by proximity coupling of $s$-wave superconductivity and $p$-wave magnet.

cond-mat.mes-hall↗
arXiv2024

Moiré-modulated band gap and van Hove singularities in twisted bilayer germanene

Twisting bilayers of two-dimensional topological insulators has the potential to create unique quantum states of matter. Here, we successfully synthesized a twisted bilayer of germanene on Ge2Pt(101) with a 21.8$^o$ degrees twist angle, corresponding to a commensurate ($\sqrt{7} \times \sqrt{7}$) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moiré-modulated band gap and a well-defined edge state. This band gap opens at AB/BA stacked sites and closes at AA stacked sites, a phenomenon attributed to the electric field induced by the scanning tunneling microscopy tip. Our study further revealed two van Hove singularities at -0.8 eV and +1.04 eV, resulting in a Fermi velocity of $(8 \pm 1) \times 10^5$ m/s. Our tight-binding results uncover a unique quantum state, where the topological properties could be regulated through an electric field, potentially triggering two topological phase transitions.

cond-mat.mes-hall↗
arXiv2024

Detecting the Néel vector of altermagnet by attaching a topological insulator and crystalline valley-edge insulator

In order to detect the Néel vector of an altermagnet, we investigate topological phases in a bilayer system composed of an altermagnet and a two-dimensional topological insulator described by the Bernevig-Hughes-Zhang model. A topological phase transition occurs from a first-order topological insulator to a trivial insulator at a certain critical altermagnetization if the Néel vector of altermagnet is along the $x$ axis or the $y$ axis. It is intriguing that valley-protected edge states emerge along the Néel vector in this trivial insulator, which are as stable as the topological edge states. We name it a crystalline valley-edge insulator. On the other hand, the system turns out to be a second-order topological insulator when the Néel vector is along the $z$ axis. The tunneling conductance has a strong dependence on the Néel vector. In addition, the band gap depends on the Néel vector, which is measurable by optical absorption. Hence, it is possible experimentally to detect the Néel vector by measuring tunneling conductance and optical absorption.

cond-mat.mes-hall↗
arXiv2024

Braiding and fusion of Majorana fermions in minimal Kitaev spin liquid on a single hexagon with $5$ qubits

We investigate the minimal Kitaev spin liquid on a single hexagon with three Ising-type exchange interactions proportional to $K_{x}$, $K_{y}$ and $K_{z}$. In the limit $K_{z}=0$, we find 32-fold zero-energy states, leading to 10 free Majorana fermions, and hence, 5 qubits are constructed. These qubits are protected by particle-hole symmetry even for $K_{z}\neq 0$. Braiding of these Majorana fermions is possible by temporally controlling a spin-correlation Hamiltonian. In addition, the fusion is possible by measuring the spin correlation. By switching on the Heisenberg interaction together with magnetic field, only one zero-energy state persists, which can be used as an initialization of qubits. Furthermore, it is shown that $3L+2$ qubits are constructed on the Kitaev spin liquid model on connected $L$ hexagons. All the processes of initialization, operation and readout of qubits are executable in terms of spin operators.

cond-mat.str-el↗
arXiv2023

Chiral Skyrmions Interacting with Chiral Flowers

The chiral nature of active matter plays an important role in the dynamics of active matter interacting with chiral structures. Skyrmions are chiral objects, and their interactions with chiral nanostructures can lead to intriguing phenomena. Here, we explore the random-walk dynamics of a thermally activated chiral skyrmion interacting with a chiral flower-like obstacle in a ferromagnetic layer, which could create topology-dependent outcomes. It is a spontaneous mesoscopic order-from-disorder phenomenon driven by the thermal fluctuations and topological nature of skyrmions that exists only in ferromagnetic and ferrimagnetic systems. The interactions between the skyrmions and chiral flowers at finite temperatures can be utilized to control the skyrmion position and distribution without applying any external driving force or temperature gradient. The phenomenon that thermally activated skyrmions are dynamically coupled to chiral flowers may provide a new way to design topological sorting devices.

cond-mat.mes-hall↗
arXiv2023

Robustness of Majorana edge states of short-length Kitaev chains coupled with environment

Recently, the two-site Kitaev model hosting Majorana edge states was experimentally realized based on double quantum dots. In this context, we construct two-band effective models describing Majorana edge states of a finite-length Kitaev chain by using the isospectral matrix reduction method. We analytically estimate the robustness of Majorana edge states as a function of model parameters. We also study effects of coupling to an environment based on non-Hermitian Hamiltonians derived from the Lindblad equation. We study three types of dissipation, a local one, an adjacent one and a global one. It is found that the Majorana zero-energy edge states acquire nonzero energy such as $E\propto \pm \left( iγ\right) ^{L}$ for the local dissipation, where $γ$ is the magnitude of the dissipation and $L$ is the length of the chain. On the other hand, the Majorana zero-energy edge states acquire nonzero energy such as $E\propto \pm iγ$ irrespective of the length $L$ for the global dissipation. Hence, the Majorana edge states are robust against the local dissipation but not against the global one. Our results will be useful for future studies on Majorana edge states based on quantum dots.

cond-mat.supr-con↗
arXiv2023

Nonadiabatic nonlinear non-Hermitian quantized pumping

We analyze a quantized pumping in a nonlinear non-Hermitian photonic system with nonadiabatic driving. The photonic system is made of a waveguide array, where the distances between adjacent waveguides are modulated. It is described by the Su-Schrieffer-Heeger model together with a saturated nonlinear gain term and a linear loss term. A topological interface state between the topological and trivial phases is stabilized by the combination of a saturated nonlinear gain term and a linear loss term. We study the pumping of the topological interface state. We define the transfer-speed ratio $ω/Ω$ by the ratio of the pumping speed $% ω$ of the center of mass of the wave packet to the driving speed $ Ω$ of the topological interface. It is quantized as $ω/Ω=1$ in the adiabatic limit. It remains to be quantized for slow driving even in the nonadiabatic regime, which is a nonadiabatic quantized pump. On the other hand, there is almost no pump for fast driving. We find a transition in pumping as a function of the driving speed.

cond-mat.mes-hall↗
arXiv2023

Experimental demonstration of position-controllable topological interface states in high-frequency topological integrated circuits

Topological integrated circuits are integrated-circuit realizations of topological systems. Here we show an experimental demonstration by taking the case of the Kitaev topological superconductor model. An integrated-circuit implementation enables us to realize high resonant frequency as high as 13GHz. We explicitly observe the spatial profile of a topological edge state. In particular, the topological interface state between a topological segment and a trivial segment is the Majorana-like state. We construct a switchable structure in the integrated circuit, which enables us to control the position of a Majorana-like interface state arbitrarily along a chain. Our results contribute to the development of topological electronics with high frequency integrated circuits.

cond-mat.mes-hall↗
arXiv2023

Laminar and transiently disordered dynamics of a magnetic skyrmion pipe flow

The world is full of fluids that flow. The fluid nature of flowing skyrmionic quasiparticles is of fundamental physical interest and plays an essential role in the transport of many skyrmions. Here, we report the laminar and transiently disordered dynamic behaviors of many magnetic skyrmions flowing in a pipe channel. The skyrmion flow driven by a uniform current may show a lattice structural transition. The skyrmion flow driven by a non-uniform current shows a dynamically varying lattice structure. A large uniform current could result in the compression of skyrmions toward the channel edge, leading to the transition of the skyrmion pipe flow into an open-channel flow with a free surface. Namely, the width of the skyrmion flow could be adjusted by the driving current. Skyrmions on the free surface may form a single shear layer adjacent to the main skyrmion flow. In addition, although we focus on the skyrmion flow dynamics in a clean pipe channel without any pinning or defect effect, we also show that a variation of magnetic anisotropy in the pipe channel could lead to more complicated skyrmion flow dynamics and pathlines. Our results reveal the fluid nature of skyrmionic quasiparticles that may motivate future research on the complex flow physics of magnetic textures.

cond-mat.mes-hall↗
arXiv2023

Systematic construction of topological-nontopological hybrid universal quantum gates based on many-body Majorana fermion interactions

Topological quantum computation by way of braiding of Majorana fermions is not universal quantum computation. There are several attempts to make universal quantum computation by introducing some additional quantum gates or quantum states. However, there is an embedding problem that $M$-qubit gates cannot be embedded straightforwardly in $N$ qubits for $N>M$. This problem is inherent to the Majorana system, where logical qubits are different from physical qubits because braiding operations preserve the fermion parity. By introducing $2N$-body interactions of Majorana fermions, topological-nontopological hybrid universal quantum computation is shown to be possible. Especially, we make a systematic construction of the C$^{n}$Z gate, C$^{n}$NOT gate and the C$^{n}$SWAP gate.

quant-ph↗
arXiv2023

Cavity-induced topological edge and corner states

We investigate a two-level system with alternating XX coupling in a photon cavity. It is mapped to a free boson model equally coupled to a photon, whose interaction is highly nonlocal. Some intriguing topological phenomena emerge as a function of the photon coupling. The photon energy level anticrosses the zero-energy topological edges at a certain photon coupling, around which the symmetric edge state acquires nonzero energy due to the mixing with the photon. Furthermore, the photon state is transformed into the topological zero-energy edge or corner state when the photon coupling is strong enough. It is a cavity-induced topological edge or corner state. On the other hand, the other topological edge or corner states do not couple with the photon and remains at zero energy even in the presence of the cavity. We analyze a cavity-induced topological edge state in the Su-Schrieffer-Heeger model and a cavity-induced topological corner state in the breathing Kagome model.

cond-mat.mes-hall↗
arXiv2023

Universal quantum computing based on magnetic domain wall qubits

Quantum computers allow to solve efficiently certain problems that are intractable for classical computers. For the realization of a quantum computer, a qubit design as the basic building block is a nontrivial starting point. We propose the utilization of nanoscale magnetic domain walls, which are stabilized by achiral energy, as the building blocks for a universal quantum computer made of ferromagnetic racetracks. In contrast to the domain walls stabilized by conventional Dzyaloshinskii-Moriya interactions, these achiral domain walls are bistable and show two degenerate chirality forms. When the domain wall is extremely small, it can be viewed as a quantum mechanical object and the two degenerate chiralities of the domain walls can be used to encode the qubit states $\lvert 0 \rangle$ and $\lvert 1 \rangle$. We show that the single-qubit quantum gates are regulated by magnetic and electric fields, while the Ising exchange coupling facilitates the two-qubit gates. The integration of these quantum gates allows for a universal quantum computation. Our findings demonstrate a promising approach for achieving quantum computing through spin textures that exist in ferromagnetic materials.

cond-mat.mes-hall↗
arXiv2023

Nonlinearity-induced topological phase transition characterized by the nonlinear Chern number

As first demonstrated by the characterization of the quantum Hall effect by the Chern number, topology provides a guiding principle to realize robust properties of condensed matter systems immune to the existence of disorder. The bulk-boundary correspondence guarantees the emergence of gapless boundary modes in a topological system whose bulk exhibits nonzero topological invariants. Although some recent studies have suggested a possible extension of the notion of topology to nonlinear systems such as photonics and electrical circuits, the nonlinear counterpart of topological invariant has not yet been understood. Here, we propose the nonlinear extension of the Chern number based on the nonlinear eigenvalue problems in two-dimensional systems and reveal the bulk-boundary correspondence beyond the weakly nonlinear regime. Specifically, we find the nonlinearity-induced topological phase transitions, where the existence of topological edge modes depends on the amplitude of oscillatory modes. We propose and analyze a minimal model of a nonlinear Chern insulator whose exact bulk solutions are analytically obtained and indicate the amplitude dependence of the nonlinear Chern number, for which we confirm the nonlinear counterpart of the bulk-boundary correspondence in the continuum limit. Thus, our result reveals the existence of genuinely nonlinear topological phases that are adiabatically disconnected from the linear regime, showing the promise for expanding the scope of topological classification of matter towards the nonlinear regime.

cond-mat.mes-hall↗
arXiv2023

Reversible magnetic domain reorientation induced by magnetic field pulses with fixed direction

Nanoscale magnetic domains with controllable configurations could be used for classical and quantum applications, where the switching of magnetization configurations is an essential operation for information processing. Here, we report that the magnetic domain reorientation in a notched ferromagnetic nanotrack can be realized and effectively controlled by applying uniform magnetic field pulses in a fixed in-plane direction perpendicular to the nanotrack. Our micromagnetic simulation results show that the configurations of magnetic domains in the notched nanotrack can be switched between a head-to-head state and a tail-to-tail state in a reversible manner driven by magnetic field pulses, while it is unnecessary to reverse the direction of the magnetic field. Such a unique magnetic domain reorientation dynamics is found to depend on magnetic parameters and nanotrack geometries. The reorientation dynamics of magnetic domains also depends on the strength and length of the applied magnetic field pulse. In addition, we point out that the notches at the center of the nanotrack play an important role for the stabilization of the head-to-head and tail-to-tail states during the magnetic domain reorientation. We also qualitatively explain the field-induced reorientation phenomenon with a simplified two-dimensional macrospin model. Our results may make it possible to build spintronic devices driven by a fixed magnetic field. Our findings may also motivate future studies to investigate the classical and quantum applications based on nanoscale magnetic domains.

cond-mat.mes-hall↗