arXiv subjects

Shun Okumura

Publications and source records attributed to Shun Okumura.

At least 37 records · Page 2Linked to original sources

arXiv2024

High harmonic generation from electrons moving in topological spin textures

High harmonic generation (HHG) is a striking phenomenon, which reflects the ultrafast dynamics of electrons. Recently, it has been demonstrated that HHG can be used to reconstruct not only the energy band structure but also the geometric structure characterized by the Berry curvature. Here, we numerically investigate HHG arising from electrons coupled with a topological spin texture in a spin scalar chiral state where time reversal symmetry is broken. In this system, a sign change in scalar chirality alters the sign of the Berry curvature while keeping the energy band structure unchanged, allowing us to discuss purely geometrical effects on HHG. Notably, we found that, when the optical frequency is significantly lower than the energy gap, the sign of scalar chirality largely affects the longitudinal response parallel to the optical field rather than the transverse response. Our analysis suggests that this can be attributed to interband currents induced by the recombination of electron-hole pairs whose real-space trajectories are modulated by the anomalous velocity term.

cond-mat.mes-hall↗

arXiv2024

Current-induced motion of nanoscale magnetic torons over the wide range of the Hall angle

Current-driven dynamics of spin textures plays a pivotal role in potential applications for electronic devices. While two-dimensional magnetic skyrmions with topologically nontrivial spin textures have garnered significant interest, their practical use is hindered by the skyrmion Hall effect $\unicode{x2014}$ a transverse motion to the current direction that occurs as a counteraction to the topological Hall effect of electrons by an emergent magnetic field arising from the Berry phase effect. Here, we explore current-driven dynamics of three-dimensional topological spin textures known as magnetic torons, composed of layered skyrmions with two singularities called Bloch points at their ends. Through extensive numerical simulations, we show that the torons also exhibit a Hall motion, but surprisingly over a wide range spanning from the zero Hall effect, a purely longitudinal motion, to the perfect Hall effect, a purely transverse motion accompanied by no longitudinal motion. Such flexible and controllable behaviors stem from anisotropic potential barriers on the discrete lattice, which can be particularly relevant for nanoscale torons recently discovered. Our results not only provide an experimental method to probe topology of three-dimensional magnetic textures but also pave the way for future developments in topological spintronics beyond the realm of skyrmions.

cond-mat.str-el↗

arXiv2024

Application of time-series quantum generative model to financial data

Despite proposing a quantum generative model for time series that successfully learns correlated series with multiple Brownian motions, the model has not been adapted and evaluated for financial problems. In this study, a time-series generative model was applied as a quantum generative model to actual financial data. Future data for two correlated time series were generated and compared with classical methods such as long short-term memory and vector autoregression. Furthermore, numerical experiments were performed to complete missing values. Based on the results, we evaluated the practical applications of the time-series quantum generation model. It was observed that fewer parameter values were required compared with the classical method. In addition, the quantum time-series generation model was feasible for both stationary and nonstationary data. These results suggest that several parameters can be applied to various types of time-series data.

quant-ph↗

arXiv2024

Topological phase diagram of the Haldane model on a Bishamon-kikko--honeycomb lattice

Topological flat bands have gained extensive interest as a platform for exploring the interplay between nontrivial band topology and correlation effects. In recent studies, strongly correlated phenomena originating from a topological flat band were discussed on a periodically 1/6-depleted honeycomb lattice, but the fundamental topological nature associated with this lattice structure remains unexplored. Here we study the band structure and topological phase diagram for the Haldane model on this lattice, which we call the Bishamon-kikko lattice. We also extend our study to the model connecting the Bishamon-kikko and honeycomb lattices. We show that these models exhibit richer topological characteristics compared to the original Haldane model on the honeycomb lattice, such as topological insulating states with higher Chern numbers, metallic states with nontrivial band topology even at commensurate electron fillings, and metal-insulator transitions between them.7 Our findings offer a playground of correlated topological phenomena and stimulate their realization in a variety of two-dimensional systems, such as van der Waals materials, graphene nanostructures, and photonic crystals.

cond-mat.mes-hall↗

arXiv2024

Non-coplanar helimagnetism in the layered van-der-Waals metal DyTe$_3$

Magnetic materials with highly anisotropic chemical bonding can be exfoliated to realize ultrathin sheets or interfaces with highly controllable optical or spintronics responses, while also promising novel cross-correlation phenomena between electric polarization and the magnetic texture. The vast majority of these van-der-Waals magnets are collinear ferro-, ferri-, or antiferromagnets, with a particular scarcity of lattice-incommensurate helimagnets of defined left- or right-handed rotation sense, or helicity. Here we use polarized neutron scattering to reveal cycloidal, or conical, magnetic structures in DyTe$_3$, with coupled commensurate and incommensurate order parameters, where covalently bonded double-slabs of dysprosium square nets are separated by highly metallic tellurium layers. Based on this ground state and its evolution in a magnetic field as probed by small-angle neutron scattering (SANS), we establish a one-dimensional spin model with off-diagonal on-site terms, spatially modulated by the unconventional charge order in DyTe$_3$. The CDW-driven term couples to antiferromagnetism, or to the net magnetization in applied magnetic field, and creates a complex magnetic phase diagram indicative of competing interactions in an easily cleavable helimagnet. Our work paves the way for twistronics research, where helimagnetic layers can be combined to form complex spin textures on-demand, using the vast family of rare earth chalcogenides and beyond.

cond-mat.mtrl-sci↗

arXiv2023

Emergent electric field from magnetic resonances in a one-dimensional chiral magnet

The emergent electric field (EEF) is a fictitious electric field acting on conduction electrons through the Berry phase mechanism. The EEF is generated by the dynamics of noncollinear spin configurations and becomes nonzero even in one dimension. Although the EEF has been studied for several one-dimensional chiral magnets, most of the theoretical studies were limited with respect to the strength and direction of the magnetic fields. Furthermore, the effect of edges of the system has not been clarified, whereas it can be crucial in nano- and micro-scale samples. Here, we perform a theoretical study on the momentum-frequency profile of the EEF in a one-dimensional chiral magnet while changing the strength and direction of the magnetic field for both bulk and finite-size chains with edges. As the bulk contributions, we find that the EEF is resonantly enhanced at the magnetic resonance frequencies; interestingly, the higher resonance modes are more clearly visible in the EEF response than in the magnetic one. Furthermore, we show that the EEF is amplified along with the solitonic feature of the spin texture introduced by the static magnetic field perpendicular to the chiral axis. We also show that the static magnetic field parallel to the chiral axis drives the EEF in the field direction, in addition to much slower drift motion in the opposite direction associated with the Archimedean screw dynamics, suggesting a DC electric current generation. As the edge contributions, we find additional resonance modes localized at the edges of the system that are also more clearly visible in the EEF response than the magnetic one. Our results reveal that the emergent electric phenomena in one-dimensional chiral magnets can be tuned by the magnetic field and the sample size, and provide not only a good probe of the magnetic resonances but also a platform for the applications to electronic devices.

cond-mat.mes-hall↗

arXiv2023

Instability of magnetic skyrmion strings induced by longitudinal spin currents

It is well established that spin-transfer torques exerted by in-plane spin currents give rise to a motion of magnetic skyrmions resulting in a skyrmion Hall effect. In films of finite thickness or in three-dimensional bulk samples the skyrmions extend in the third direction forming a string. We demonstrate that a spin current flowing longitudinally along the skyrmion string instead induces a Goldstone spin wave instability. Our analytical results are confirmed by micromagnetic simulations of both a single string as well as string lattices suggesting that the instability eventually breaks the strings. A longitudinal current is thus able to melt the skyrmion string lattice via a dynamical phase transition. For films of finite thickness or in the presence of disorder a threshold current will be required, and we estimate the latter assuming weak collective pinning.

cond-mat.mes-hall↗

arXiv2023

Anomalous Hall effect of light-driven three-dimensional Dirac electrons in bismuth

Recent advancement in laser technology has opened the path toward the manipulation of functionalities in quantum materials by intense coherent light. Here, we study three-dimensional (3D) Dirac electrons driven by circularly polarized light (CPL), when the photon energy lies within the Dirac bands. As an experimental realization of this setup, we irradiate a thin film sample of elemental bismuth, which is a well-known semimetal hosting 3D Dirac electrons, with mid-infrared CPL. We successfully observe the emergence of the anomalous Hall effect (AHE) via terahertz Faraday rotation that is both pump-helicity-dependent and instantaneous. We compare our experimental findings with the results of Floquet theory, which is a powerful framework for analyzing the electronic band structure driven by coherent light. The contribution from the band structures near the one-photon resonant positions to the AHE shows a field-strength dependence consistent with our experimental results. The effective Hamiltonian on which we base our model calculations also implies that a pair of "double Weyl points" emerge due to the CPL-induced hybridization between the occupied and unoccupied 3D Dirac bands. Our findings shed light on ultrafast control of material properties in nonlinear topological optics.

cond-mat.mes-hall↗

arXiv2022

TEM at Millikelvin Temperatures: Observing and Utilizing Superconducting Qubits

We present a case for developing a millikelvin-temperature transmission electron microscope (TEM). We start by reviewing known reasons for such development, then present new possibilities that have been opened up by recent progress in superconducting quantum circuitry, and finally report on our ongoing experimental effort. Specifically, we first review possibilities to observe a quantum mechanically superposed electromagnetic field around a superconducting qubit. This is followed by a new idea on TEM observation of microwave photons in an unusual quantum state in a resonator. We then proceed to review potential applications of these phenomena, which include low dose electron microscopy beyond the standard quantum limit. Finally, anticipated engineering challenges, as well as the authors' current ongoing experimental effort towards building a millikelvin TEM are described. In addition, we provide a brief introduction to superconducting circuitry in the Appendix for the interested reader who is not familiar with the subject.

quant-ph↗

arXiv2022

Magnetic Hedgehog Lattice in a Centrosymmetric Cubic Metal

The hedgehog lattice (HL) is a three-dimensional topological spin texture hosting a periodic array of magnetic monopoles and antimonopoles. It has been studied theoretically for noncentrosymmetric systems with the Dzyaloshinskii-Moriya interaction, but the stability, as well as the magnetic and topological properties, remains elusive in the centrosymmetric case. We here investigate the ground state of an effective spin model with long-range bilinear and biquadratic interactions for a centrosymmetric cubic metal by simulated annealing. We show that our model stabilizes a HL composed of two pairs of left- and right-handed helices, resulting in no net scalar spin chirality, in stark contrast to the noncentrosymmetric case. We find that the HL turns into topologically-trivial conical states in an applied magnetic field. From the detailed analyses of the constituent spin helices, we clarify that the ellipticity and angles of the helical planes change gradually while increasing the magnetic field. We discuss the results in comparison with the experiments for a centrosymmetric cubic metal SrFeO$_3$.

cond-mat.str-el↗

arXiv2022

Phase degree of freedom and topology in multiple-$Q$ spin textures

A periodic array of topological spin textures, such as skyrmions and hedgehogs, is called the multiple-$Q$ spin texture, as it is represented by a superposition of multiple spin density waves. Depending on the way of superposition, not only the magnetic but also the topological properties are modified, leading to a variety of quantum transport and optical phenomena caused by the emergent electromagnetic fields through the Berry phase. Among others, the phase degree of freedom of the superposed waves is potentially important for such modifications, but its effect has not been fully elucidated thus far. Here we perform systematic theoretical analyses of magnetic and topological properties of the multiple-$Q$ spin textures with the phase degree of freedom. By introducing a hyperspace with an additional dimension corresponding to the phase degree of freedom, we establish a generic framework to deal with the phase shift in the multiple-$Q$ spin textures. Applying the framework to the two-dimensional 3$Q$ spin textures, we clarify the complete topological phase diagram while changing the phase and magnetization, which depends on the types of the superposed waves. We also study the three-dimensional 4$Q$ spin textures and clarify even richer topological phase diagrams. In particular, we find novel topological phase transitions associated with the previously unidentified Dirac strings on which the hedgehogs and antihedgehogs cause pair creation and fusion. Moreover, we demonstrate that phase shifts are caused by an external magnetic field in both 3$Q$ and 4$Q$ cases by analyzing the numerical data in the previous studies. Our results illuminate the topological aspects of the skyrmion and hedgehog lattices with the phase degree of freedom, which would be extended to other multiple-$Q$ textures and useful for the exploration of topologically nontrivial magnetic phases and exotic quantum phenomena.

cond-mat.str-el↗

arXiv2021

Electrical conductivity in helical and conical magnetic states

We theoretically study the electrical conductivity in a one-dimensional helimagnet whose spin texture changes from helimagnetic to conical magnetic, and to forced ferromagnetic state while increasing the magnetic field along the helical axis. We find that the conductivity in the helimagnetic state at zero field depends on the electron filling and the coefficient of the spin-charge coupling. We also find that the conductivity in the conical magnetic state changes nonlinearly to the applied field, and the magnetoresistance becomes negative and positive depending on the model parameters.

cond-mat.str-el↗

arXiv2021

Phase shift, ellipticity, angle, and topological number in skyrmion lattices

We theoretically study skyrmion lattices realized in a Kondo lattice model on a triangular lattice, focusing on the phase, ellipticity, and angle of the constituent multiple-$Q$ waves. Analyzing the numerical data obtained in the previous study [Ozawa R, Hayami S and Motome Y 2017 Phys. Rev. Lett. 118 147205], we extract these parameters for the two types of skyrmion lattices with the skyrmion number of $1$ and $2$. We show that the topological transition between the two skyrmion lattices driven by an external magnetic field is accompanied by significant modulations of all three parameters.

cond-mat.str-el↗

arXiv2021

Electric polarization and nonlinear optical effects in noncentrosymmetric magnets

We study electric polarization and nonlinear optical effects in spin systems with broken inversion symmetry. We apply strong coupling expansion to the underlying electronic Hamiltonians, and systematically derive expressions for electric polarization in spin systems that are represented in terms of spin operators. The magnon representation of the obtained electric polarization operator allows us to compute linear and nonlinear optical responses by the standard diagrammatic method. We apply our formalism to Heisenberg model with alternating coupling constants and $J_1$-$J_2$ model with inversion symmetry breaking. We demonstrate that these inversion broken spin systems support dc current flow upon magnon excitations which arises from the shift current mechanism.

cond-mat.str-el↗

arXiv2021

Quadratic Optical Responses in a Chiral Magnet

Chiral magnets, which break both spatial inversion and time reversal symmetries, carry a potential for quadratic optical responses. Despite the possibility of enhanced and controlled responses through the magnetic degree of freedom, the systematic understanding remains yet to be developed. We here study nonlinear optical responses in a prototypical chiral magnetic state with a one-dimensional conical order by using the second-order response theory. We show that the photovoltaic effect and the second harmonic generation are induced by asymmetric modulation of the electronic band structure under the conical magnetic order, and the coefficients, including the sign, change drastically depending on the frequency of incident lights, the external magnetic field, and the strength of spin-charge coupling. We find that both effects can be enormously large compared to those in the conventional nonmagnetic materials. Our results would pave the way for next-generation optical electronic devices, such as unconventional solar cells and optical sensors, based on chiral magnets.

cond-mat.str-el↗

arXiv2021

Spin moiré engineering of topological magnetism and emergent electromagnetic fields

A superposition of spin helices can yield topological spin textures, such as skyrmion and hedgehog lattices. Based on the analogy with the moiré in optics, we study the magnetic and topological properties of such superpositions in a comprehensive way by modulating the interference pattern continuously. We find that the control of the angles between the superposed helices and the net magnetization yields successive topological transitions associated with pair annihilation of hedgehogs and antihedgehogs. Accordingly, emergent electromagnetic fields, magnetic monopoles and antimonopoles, and Dirac strings arising from the noncoplanar spin textures show systematic evolution. In addition, we also show how the system undergoes the magnetic transitions with dimensional reduction from the three-dimensional hedgehog lattice to a two-dimensional skyrmion lattice or a one-dimensional conical state. The results indicate that the concept of "spin moiré" provides an efficient way of engineering the emergent electromagnetism and topological nature in magnets.

cond-mat.str-el↗

arXiv2020

Phase transitions between helices, vortices, and hedgehogs driven by spatial anisotropy in chiral magnets

Superpositions of spin helices can yield topological spin textures, such as two-dimensional vortices and skyrmions, and three-dimensional hedgehogs. Their topological nature and spatial dimensionality depend on the number and relative directions of the constituent helices. This allows mutual transformation between the topological spin textures by controlling the spatial anisotropy. Here we theoretically study the effect of anisotropy in the magnetic interactions for an effective spin model for chiral magnetic metals. By variational calculations for both cases with triple and quadruple superpositions, we find that the hedgehog lattices, which are stable in the isotropic case, are deformed by the anisotropy, and eventually changed into other spin textures with reduced dimension, such as helices and vortices. We also clarify the changes of topological properties by tracing the real-space positions of magnetic monopoles and antimonopoles as well as the emergent magnetic field generated by the noncoplanar spin textures. Our results suggest possible control of the topological spin textures, e.g., by uniaxial pressure and chemical substitution in chiral materials.

cond-mat.str-el↗

arXiv2020

Tracing Monopoles and Anti-monopoles in a Magnetic Hedgehog Lattice

The magnetic hedgehog lattice (HL), which was recently discovered in the $B$20-type chiral magnet MnSi$_{1-x}$Ge$_x$, is a topological spin texture with a periodic array of magnetic monopoles and anti-monopoles. Within the continuum approximation, the monopoles and anti-monopoles are predicted to move, collide, and pair annihilate in an applied magnetic field, but it remains unclear how the lattice discretization affects their motions. Here, we study the trajectories of monopoles and anti-monopoles in a lattice system by simulated annealing with field sweep. We show that the monopoles and anti-monopoles move and repel before pair annihilations. We also clarify that their motions are closely related with the field dependence of the scalar spin chirality.

cond-mat.str-el↗