The University of Tokyo · Physics and Astronomy
Professor Kotaro Shimizu's research lab specializes in topological quantum phenomena in condensed matter systems, with a focus on emergent electromagnetic fields, spin textures, and their interplay with electronic transport and optical responses. The lab investigates the design and control of complex magnetic structures—such as skyrmions, hedgehogs, and vortex lattices—through the superposition of spin helices and modulation of magnetic anisotropy. By combining theoretical modeling, variational calculations, and Berry phase effects, the group explores how spin topology can be engineered for novel quantum devices and topological electronics.
Figures are computed from collected data and may differ slightly.
A superposition of spin helices can yield topological spin textures, such as skyrmion and hedgehog lattices. Based on the analogy with the moir\'e 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 antih
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 of the magnetic interactions in an effective spin model for ch
The C, P and T transformations in higher dimensions are studied for the massive spin-1/2 field. It is shown that two different fileds with the sign of their masses opposite to each other are necessary in order to have a C- or P- (P- or T-) invariant Lagrangian for 4n+1 (4n+3) dimensions. It is also pointed out that Weyl spinors in even dimensions can be dealt with in the same way.
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 w
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 it becomes nonzero even in one dimension. Although the EEF has been studied for several one-dimensional chiral magnets, most of the theoretical studies were performed in limited situations with respect to the strength and direction of the magnetic fields. Furthermore, the effect of the edge
Current-driven dynamics of topological spin textures plays a pivotal role in potential applications for electronic devices. While two-dimensional magnetic skyrmions have garnered significant interest, their practical use is hindered by the skyrmion Hall effect—a transverse motion to the current direction that occurs as a counteraction to the topological Hall effect of electrons arising from the Berry phase effect. Here, we explore current-driven dynamics of three-dimensional topological spin tex
We develop a theory of designing slit experiments in two-dimensional electron systems with the Rashba spin-orbit interaction. By investigating the spatiotemporal dynamics of electrons passing through a single slit or a double slit both analytically and numerically, we find that the interference fringes of the electron probability density attain specific spin orientations via the precession of spins around effective magnetic fields mediated by the Rashba spin-orbit interaction the directions of w
Abstract 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 d
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. He
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 cla
We investigate the phase ordering kinetics of skyrmion lattice (SkL) in a metallic magnet. The SkL can be viewed as a superposition of magnetic stripes whose periods are determined by the quasi-nesting wave vectors of the underlying Fermi surface. An effective magnetic Hamiltonian that describes the electron-mediated spin-spin interaction is obtained for a two-dimensional s-d model with the Rashba spin-orbit coupling. Large-scale Landau-Lifshitz-Gilbert dynamics simulations based on the effectiv
The magnetic solitons such as chiral solitons, magnetic skyrmions, and magnetic hopfions, exhibiting particlelike nature widely emerge in magnets depending on spatial dimension. As their number directly gives rise to an impact on magnetic properties and electronic properties, it is of great importance to control the number of solitons. However, a systematic study on dynamical processes to control the number of solitons, particularly by adding the desired number of solitons to the ground state ex
We present a comprehensive approach to characterizing labyrinthine structures that often emerge as a final steady state in pattern forming systems. We employ advanced machine learning based pattern recognition techniques to identify the types and locations of topological defects of the local stripe ordering. Applying this method to single-crystal Bi-substituted Yttrium Iron Garnet films, we uncover a distinct morphological transition between two zero-field labyrinthine structures. Crucially, the
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