arXiv subjects

Motohiko Ezawa

Publications and source records attributed to Motohiko Ezawa.

At least 73 records · Page 4Linked to original sources

arXiv2021

Dynamics of ferrimagnetic skyrmionium driven by spin-orbit torque

Magnetic skyrmionium is a skyrmion-like spin texture with nanoscale size and high mobility. It is a topologically trivial but dynamically stable structure, which can be used as a non-volatile information carrier for next-generation spintronic storage and computing devices. Here, we study the dynamics of a skyrmionium driven by the spin torque in a ferrimagnetic nanotrack. It is found that the direction of motion is jointly determined by the internal configuration of a skyrmionium and the spin polarization vector. Besides, the deformation of a skyrmionium induced by the intrinsic skyrmion Hall effect depends on both the magnitude of the driving force and the net angular momentum. The ferrimagnetic skyrmionium is most robust at the angular momentum compensation point, whose dynamics is quite similar to the skyrmionium in antiferromagnet. The skyrmion Hall effect is perfectly prohibited, where it is possible to observe the position of the skyrmionium by measuring the magnetization. Furthermore, the current-induced dynamics of a ferrimagnetic skyrmionium is compared with that of a ferromagnetic and antiferromagnetic skyrmionium. We also make a comparison between the motion of a ferrimagnetic skyrmionium and a skyrmion. Our results will open a new field of ferrimagnetic skyrmioniums for future development of ferrimagnetic spintronics devices.

cond-mat.mes-hall↗

arXiv2021

Nonlinear non-Hermitian higher-order topological laser

We investigate topological lasers in combination of nonlinear, non-Hermitian and topological lattice systems based on a quench dynamics starting from one site. We consider explicitly the topological laser in the Su-Schrieffer-Heeger (SSH) model with two topological edge states and the second-order topological laser in the breathing Kagome lattice with three topological corner states. Once we stimulate any one site, after a delay, all sites belonging to the topological edge or corner states begin to emit stable laser light depending on the density of states, although no wave propagation is observed from the stimulated site. It is intriguing that the profile of topological edge or corner states is observable by measuring the intensity of lasing. The phenomenon occurs due to a combinational effect of linear non-Hermitian loss terms and nonlinear non-Hermitian gain terms in the presence of the topological edge or corner states.

cond-mat.mes-hall↗

arXiv2021

Nonlinear topological phase diagram in dimerized sine-Gordon model

We investigate the topological physics and the nonlinearity-induced trap phenomenon in a coupled system of pendulums. It is described by the dimerized sine-Gordon model, which is a combination of the sine-Gordon model and the Su-Schrieffer-Heeger model. The initial swing angle of the left-end pendulum may be regarded as the nonlinearity parameter. The topological number is well defined as far as the pendulum is approximated by a harmonic oscillator. The emergence of the topological edge state is clearly observable in the topological phase by solving the quench dynamics starting from the left-end pendulum. A phase diagram is constructed in the space of the swing angle $ξπ$ with $|ξ|\leq 1$ and the dimerization parameter $λ$ with $|λ|\leq 1$. It is found that the topological phase boundary is rather insensitive to the swing angle for $% |ξ|\lesssim 1/2$. On the other hand, the nonlinearity effect becomes dominant for $|ξ|\gtrsim 1/2$, and eventually the system turns into the nonlinearity-induced trap phase. Furthermore, when the system is almost dimerized ($λ\simeq 1$), coupled standing waves appear and are trapped to a few pendulums at the left-end, forming the dimer phase. Its dynamical origin is the cooperation of the dimerization and the nonlinear term.

cond-mat.mes-hall↗

arXiv2021

Nonlinearity-induced transition in nonlinear Su-Schrieffer-Heeger model and nonlinear higher-order topological system

We study the topological physics in nonlinear Schrödinger systems on lattices. We employ the quench dynamics to explore the phase diagram, where a pulse is given to a lattice point and we analyze its time evolution. There are two system parameters $λ$ and $ξ$, where $λ$ controls the hoppings between the neighboring links and $ξ$ controls the nonlinearity. The dynamics crucially depends on these system parameters. Based on analytical and numerical studies, we derive the phase diagram of the nonlinear Su-Schrieffer-Heeger (SSH) model in the ($λ,ξ$) plane. It consists of four phases. The topological and trivial phases emerge when the nonlinearity $ξ$ is small. The nonlinearity-induced localization phase emerges when $ξ$ is large. We also find a dimer phase as a result of a cooperation between the hopping and nonlinear terms. A similar analysis is made of the nonlinear second-order topological system on the breathing Kagome lattice, where a trimer phase appears instead of the dimer phase.

cond-mat.mes-hall↗

arXiv2021

Nonlinear Anderson localization in Toda lattices

We study the Anderson localization in nonlinear systems by taking a nonlinear transmission line realizing the Toda lattice. It is found that the randomness in inductance induces the Anderson localization in the voltage propagation. Furthermore, the nonlinearity enhances the Anderson localization. They are understood in terms of attractive interactions to form a soliton in a nonlinear system. Our results will be applicable in general to the Anderson localization in nonlinear systems that have solitons.

cond-mat.dis-nn↗

arXiv2021

Quench dynamics and bulk-edge correspondence in nonlinear mechanical systems

We study a topological physics in a one-dimensional nonlinear system by taking an instance of a mechanical rotator model with alternating spring constants. This nonlinear model is smoothly connected to an acoustic model described by the Su-Schrieffer-Heeger model in the linear limit. We numerically show that quench dynamics of the kinetic and potential energies for the nonlinear model is well understood in terms of the topological and trivial phases defined in the associated linearized model. It indicates phenomenologically the emergence of the edge state in the topological phase even for the nonlinear system, which may be the bulk-edge correspondence in nonlinear system.

cond-mat.mes-hall↗

arXiv2021

Topological Toda lattice and nonlinear bulk-edge correspondence

The Toda lattice is a model of nonlinear wave equations allowing exact soliton solutions. It is realized by an electric circuit made of a transmission line with variable capacitance diodes and inductors. It has been generalized to the dimerized Toda lattice by introducing alternating bondings specified by a certain parameter $λ$, where it is reduced to the Toda lattice at $λ=0$. In this work, we investigate the topological dynamics of the voltage along the transmission line. It is demonstrated numerically that the system is topological for $λ<0$, while it is trivial for $λ>0$ with the phase transition point given by the original Toda lattice ($λ=0$). These topological behaviors are well explained by the chiral index familiar in the Su-Schrieffer-Heeger model. The topological phase transition is observable by a significant difference between the dynamics of voltages in the two phases, which is explained by the emergence of the topological edge states. This is a bulk-edge correspondence in nonlinear systems. The dimerized Toda lattice is adiabaticaly connected to a linear system, which would be the reason why the topological arguments are valid. Furthermore, we show that the topological edge state is robust against randomness.

cond-mat.mes-hall↗

arXiv2021

Antiferromagnetic Skyrmion-Based Logic Gates Controlled by Electric Currents and Fields

Antiferromagnets are promising materials for future spintronic applications due to their unique properties including zero stray fields, robustness versus external magnetic fields and ultrafast dynamics, which have attracted extensive interest in recent years. In this work, we investigate the dynamics of isolated skyrmions in an antiferromagnetic nanotrack with a voltage-gated region. It is found that the skyrmion can be jointly controlled by the driving current and the voltage-controlled magnetic anisotropy gradient. We further propose a design of logic computing gates based on the manipulation of antiferromagnetic skyrmions, which is numerically realized combining several interactions and phenomena, including the spin Hall effect, voltage-controlled magnetic anisotropy effect, skyrmion-skyrmion interaction, and skyrmion-edge interaction. The proposed logic gates can perform the basic Boolean operations of the logic AND, OR, NOT, NAND and NOR gates. Our results may have a great impact on fundamental physics, and be useful for designing future non-volatile logic computing devices with ultra-low energy consumption and ultra-high storage density.

cond-mat.mes-hall↗

arXiv2021

Non-Hermitian non-Abelian topological insulators with $PT$ symmetry

We study a non-Hermitian non-Abelian topological insulator preserving $PT$ symmetry, where the non-Hermitian term represents nonreciprocal hoppings. As it increases, a spontaneous $PT$ symmetry breaking transition occurs in the perfect-flat band model from a real-line-gap topological insulator into an imaginary-line-gap topological insulator. By introducing a band bending term, we realize two phase transitions, where a metallic phase emerges between the above two topological insulator phases. We discuss an electric-circuit realization of non-Hermitian non-Abelian topological insulators. We find that the spontaneous $PT$ symmetry breaking as well as the edge states are well observed by the impedance resonance.

cond-mat.mes-hall↗

arXiv2021

Current-induced dynamics of skyrmion tubes in synthetic antiferromagnetic multilayers

Topological spin textures can be found in both two-dimensional and three-dimensional nanostructures, which are of great importance to advanced spintronic applications. Here we report the current-induced skyrmion tube dynamics in three-dimensional synthetic antiferromagnetic (SyAF) bilayer and multilayer nanostructures. It is found that the SyAF skyrmion tube made of thinner sublayer skyrmions is more stable during its motion, which ensures that a higher speed of the skyrmion tube can be reached effectively at larger driving current. In the SyAF multilayer with a given total thickness, the current-induced deformation of the SyAF skyrmion tube decreases with an increasing number of interfaces; namely, the rigidity of the SyAF skyrmion tube with a given thickness increases with the number of ferromagnetic (FM) layers. For the SyAF multilayer with an even number of FM layers, the skyrmion Hall effect can be eliminated when the thicknesses of all FM layers are identical. Larger damping parameter leads to smaller deformation and slower speed of the SyAF skyrmion tube. Larger fieldlike torque leads to larger deformation and a higher speed of the SyAF skyrmion tube. Our results are useful for understanding the dynamic behaviors of three-dimensional topological spin textures and may provide guidelines for building SyAF spintronic devices.

cond-mat.mes-hall↗

arXiv2021

Topological Micro-Electro-Mechanical Systems

We explore the topological aspect of dynamics in a micro-electro-mechanical system (MEMS), which is a combination of an electric-circuit system and a mass-spring system. A simplest example is a sequential chain of capacitors and springs. It is shown that such a chain exhibits novel topological dynamics with respect to its oscillation modes. On one hand, when it undergoes free oscillation, the system is governed by the Su-Schrieffer-Heeger model, and the topological charge is given by a winding number. Topological and trivial phases are differentiated by measuring the dynamics of the outermost plate. On the other hand, when it undergoes periodical motion in time, the system is governed by an inversion-symmetric-trimer model, and the topological phases are characterized by an inversion-symmetry indicator. There emerge topological edge states, which are well signaled by measuring electromechanical-impedance resonance. Our results will open an attractive field of topological MEMS.

cond-mat.mes-hall↗

arXiv2021

Topological parafermion corner states in clock-symmetric non-Hermitian second-order topological insulator

Parafermions are a natural generalization of Majorana fermions. We consider a breathing Kagome lattice with complex hoppings by imposing $\mathbb{Z}_{3}$ clock symmetry in the complex energy plane. It is a non-Hermitian generalization of the second-order topological insulator characterized by the emergence of topological corner states. We demonstrate that the topological corner states are parafermions in the present $\mathbb{Z}_{3}$ clock-symmetric model. It is also shown that the model is realized in electric circuits properly designed, where the parafermion corner states are observed by impedance resonance. We also construct $\mathbb{Z}_{4}$ and $\mathbb{Z}_{6}$ parafermions on breathing square and honeycomb lattices, respectively.

cond-mat.mes-hall↗

arXiv2021

A Frustrated Bimeronium: Static Structure and Dynamics

We show a topological spin texture called "bimeronium" in magnets with in-plane magnetization. It is a topological counterpart of skyrmionium in perpendicularly magnetized magnets and can be seen as a combination of two bimerons with opposite topological charges. We report the static structure and spin-orbit-torque-induced dynamics of an isolated bimeronium in a magnetic monolayer with frustrated exchange interactions. We study the anisotropy and magnetic field dependences of a static bimeronium. We also explore the bimeronium dynamics driven by the damping-like spin-orbit torque. We find that the bimeronium shows steady rotation when the spin polarization direction is parallel to the easy axis. Moreover, we demonstrate the annihilation of the bimeronium when the spin polarization direction is perpendicular to the easy axis. Our results are useful for understanding fundamental properties of bimeronium structures and may offer an approach to build bimeronium-based spintronic devices.

cond-mat.mes-hall↗

arXiv2021

Variational Quantum Support Vector Machine based on $Γ$ matrix expansion and Variational Universal-Quantum-State Generator

We analyze a binary classification problem by using a support vector machine based on variational quantum-circuit model. We propose to solve a linear equation of the support vector machine by using a $Γ$ matrix expansion. In addition, it is shown that an arbitrary quantum state is prepared by optimizing a universal quantum circuit representing an arbitrary $U(2^N)$ based on the steepest descent method. It may be a quantum generalization of Field-Programmable-Gate Array (FPGA).

quant-ph↗

arXiv2021

Topological Euler insulators

The Euler number is a new topological number recently debuted in the topological physics. Unlike the Chern number defined for a band, it is defined for interbands. We propose a simple model realizing the topological Euler insulator for the first time. We utilize the fact that the Euler number in a three-band model in two dimensions is reduced to the Pontryagin number. A skyrmion structure appears in momentum phase, yielding a nontrivial Euler number. Topological edge states emerge when the Euler number is nonzero. We discuss how to realize this model in electric circuits. We show that topological edge states are well signaled by impedance resonances.

cond-mat.mes-hall↗

arXiv2020

Universal quantum gates, artificial neurons and pattern recognition simulated by \textit{LC} resonators

We propose to simulate quantum gates by \textit{LC} resonators, where the amplitude and the phase of the voltage describe the quantum state. By controlling capacitance or inductance of resonators, it is possible to control the phase of the voltage arbitrarily. A set of resonators acts as the phase-shift, the Hadamard and the CNOT gates. They constitute a set of universal quantum gates. We also discuss an application to an artificial neuron. As an example, we study a pattern recognition of numbers and alphabets by evaluating the similarity between an input and the reference pattern. We also study a colored pattern recognition by using a complex neural network.

quant-ph↗

arXiv2020

Signal detection based on the chaotic motion of an antiferromagnetic domain wall

The antiferromagnetic domain wall dynamics is currently a hot topic in mesoscopic magnetic systems. In this work, it is found that, based on the Thiele approach, the motion of an antiferromagnetic domain wall is described by the Duffing equation. Numerical simulations demonstrate that the antiferromagnetic domain wall can be used as a Duffing oscillator, and the transition between the periodic and chaotic motion can be used to detect the periodic signal in the presence of the white noise. Furthermore, we calculate the bifurcation diagram and Lyapunov exponents to study the chaotic behavior of an antiferromagnetic domain wall. The numerical simulations are in good agreement with the analytical solutions. Our results may be useful for building spintronic detection devices based on antiferromagnetic domain walls.

cond-mat.mes-hall↗

arXiv2020

Non-Abelian braiding of Majorana-like edge states and topological quantum computations in electric circuits

Majorana fermions subject to the non-Abelian braid group are believed to be the basic ingredients of future topological quantum computations. In this work, we propose to simulate Majorana fermions of the Kitaev model in electric circuits based on the observation that the circuit Laplacian can be made identical to the Hamiltonian. A set of AC voltages along the chain plays a role of the wave function. We generate an arbitrary number of topological segments in a Kitaev chain. A pair of topological edge states emerge at the edges of a topological segment. Its wave function is observable by the position and the phase of a peak in impedance measurement. It is possible to braid any pair of neighboring edge states with the aid of T-junction geometry. By calculating the Berry phase acquired by their eigenfunctions, the braiding is shown to generate one-qubit and two-qubit unitary operations. We explicitly construct Clifford quantum gates based on them. We also present an operator formalism by regarding a topological edge state as a topological soliton intertwining the trivial segment and the topological segment. Our analysis shows that the electric-circuit approach can simulate the Majorana-fermion approach to topological quantum computations.

cond-mat.mes-hall↗