Showing 1–50 of 290 results for author: Motome, Y

Weaving Hopfions from Emergent Monopoles in a Chiral Magnet

Authors: Shoya Kasai, Kotaro Shimizu, Shun Okumura, Yukitoshi Motome

Abstract: Recent advances in three-dimensional magnetization imaging techniques have opened new avenues for exploring topological spin textures beyond domain walls and skyrmions. Among them, magnetic hopfions are particularly promising, as their knotted topology is expected to give rise to unconventional dynamics and responses; however, their controlled creation remains challenging. Here we propose a simple… ▽ More Recent advances in three-dimensional magnetization imaging techniques have opened new avenues for exploring topological spin textures beyond domain walls and skyrmions. Among them, magnetic hopfions are particularly promising, as their knotted topology is expected to give rise to unconventional dynamics and responses; however, their controlled creation remains challenging. Here we propose a simple mechanism for generating hopfions from magnetic torons, three-dimensional textures hosting an emergent monopole-antimonopole pair. Using Landau-Lifshitz-Gilbert simulations, we show that an electric current drives the annihilation of this pair, converting a toron into a hopfion. The initial toron length determines the number of generated hopfions, while the current direction selects the sign of the Hopf invariant. We further find that the threshold current depends sensitively on material parameters, indicating a close connection to skyrmion dynamics. Our results establish an experimentally accessible route to hopfion creation and reveal a pathway from monopole defects to knotted topological textures. △ Less

Submitted 25 August, 2026; originally announced August 2026.

Comments: 9 pages, 5 figures

Two routes to quantum anomalous Hall states in altermagnets

Authors: Makoto Naka, Shuntaro Sumita, Yukitoshi Motome, Hitoshi Seo

Abstract: We theoretically propose two possible routes to realizing quantum anomalous Hall states in altermagnetic materials. We consider a minimal square-lattice Hubbard model with antisymmetric spin-orbit coupling associated with an orthorhombic crystal structure, which supports a topologically trivial altermagnetic state. By incorporating Rashba-type spin-orbit coupling and external perturbations, we dem… ▽ More We theoretically propose two possible routes to realizing quantum anomalous Hall states in altermagnetic materials. We consider a minimal square-lattice Hubbard model with antisymmetric spin-orbit coupling associated with an orthorhombic crystal structure, which supports a topologically trivial altermagnetic state. By incorporating Rashba-type spin-orbit coupling and external perturbations, we demonstrate that this trivial state can be turned into topological altermagnetic phases in two distinct ways. The first route is driven by a staggered potential that breaks the symmetry connecting crystallographically equivalent sublattices, leading to a topological altermagnetic ground state characterized by a quantized Hall conductivity $\left| σ_{xy} \right|=e^2/h$ and a Chern number $C=1$. The second route is realized by applying a magnetic field perpendicular to the two-dimensional plane. The resulting topological state appears as a metastable state in the magnetic hysteresis loop, exhibiting a quantized Hall conductivity $\left| σ_{xy} \right|=2e^2/h$ associated with a Chern number $C=2$. We show that these topological transitions are accompanied by characteristic gap closings at the Brillouin-zone boundary, with the number of gap-closing points determining the Chern number. Ribbon-geometry calculations reveal chiral edge states consistent with the bulk topological invariants and demonstrate distinct spin polarizations between the $C=1$ and $C=2$ states. Our results establish experimentally accessible routes to quantized anomalous Hall responses in altermagnets. △ Less

Submitted 12 August, 2026; originally announced August 2026.

Comments: 10 pages, 6 figures

Creating and Driving a Twist Soliton on a Magnetic Skyrmion Tube

Authors: Shoya Kasai, Kotaro Shimizu, Shun Okumura, Yukitoshi Motome

Abstract: A magnetic skyrmion tube is a three-dimensional topological soliton formed by stacking two-dimensional skyrmions along the out-of-plane direction. Recent real-space observations of skyrmion tubes have stimulated growing interest in their dynamics and emergent properties. Here, we go beyond simple skyrmion stacking and investigate how a ``twist" introduced along the tube direction affects the dynam… ▽ More A magnetic skyrmion tube is a three-dimensional topological soliton formed by stacking two-dimensional skyrmions along the out-of-plane direction. Recent real-space observations of skyrmion tubes have stimulated growing interest in their dynamics and emergent properties. Here, we go beyond simple skyrmion stacking and investigate how a ``twist" introduced along the tube direction affects the dynamics and emergent responses of skyrmion tubes. We find that such a twist can be created as a localized texture, termed a twist soliton, through thermal quench dynamics. By complementarily combining large-scale numerical simulations with analytical calculations based on collective coordinates, we clarify its current-driven nonlinear motions that depend on its twist chirality. Remarkably, its velocity can be substantially enhanced by a magnetic-field component perpendicular to the tube. Furthermore, the associated emergent electric field enables identification of the twist soliton, including the sign of its chirality, through Hall measurements. Our results reveal the twist degree of freedom as an essential ingredient of skyrmion-tube physics and pave the way for the development of spintronic devices exploiting the three-dimensional nature of spin textures. △ Less

Submitted 18 June, 2026; originally announced June 2026.

Comments: 9 pages, 5 figures

Phase diagram of the Kitaev-Heisenberg-$Γ$ model: Classical and quantum magnetism, frustration, and subdominant interactions

Authors: Kiyu Fukui, Yukitoshi Motome

Abstract: The Kitaev spin liquid provides a rare example of exactly solvable quantum spin liquid states. Intensive research over the past two decades has identified a variety of its candidate materials. In real materials, however, the Kitaev interaction is inevitably accompanied by additional magnetic interactions such as the Heisenberg and $Γ$ interactions. These interactions often induce magnetic ordering… ▽ More The Kitaev spin liquid provides a rare example of exactly solvable quantum spin liquid states. Intensive research over the past two decades has identified a variety of its candidate materials. In real materials, however, the Kitaev interaction is inevitably accompanied by additional magnetic interactions such as the Heisenberg and $Γ$ interactions. These interactions often induce magnetic ordering at low temperatures, making it essential to clarify their effects in the search for and design of Kitaev spin liquid candidate materials. In this study, we revisit the ground-state phase diagram of the Kitaev-Heisenberg-$Γ$ model from both classical and quantum perspectives, using state-of-the-art numerical techniques. In the classical case, we reveal a $zoo$ $of$ $noncollinear$ $orders$, where a variety of noncollinear multiple-$Q$ magnetic orders with and without incommensurate modulations emerge. In the quantum case, we unravel that quantum fluctuations suppress many of the competing orders found in the classical case, resulting in a reduced number of dominant incommensurate orders. We further identify $highly$ $frustrated$ regions, where spiral spin liquid states as well as new magnetically ordered states are potentially stabilized by other additional magnetic interactions. Our results provide a comprehensive perspective on the Kitaev-Heisenberg-$Γ$ model for both classical and quantum spins and offer a valuable guide not only for interpreting experimental results on candidate materials, but also for searching and designing new materials to realize the Kitaev spin liquid. △ Less

Submitted 24 August, 2026; v1 submitted 11 June, 2026; originally announced June 2026.

Comments: 19 pages, 13 figures

Journal ref: Phys. Rev. B 114, 074429 (2026)

Monte Carlo Study of the Phase Transition of the $XY$ Model on a Diamond Lattice

Authors: Sena Watanabe, Yukitoshi Motome, Haruki Watanabe

Abstract: We study the phase transition of the classical $XY$ model on a diamond lattice by Monte Carlo simulations using the Wolff cluster algorithm. Finite-size scaling (FSS) analysis of the Binder cumulant and the second-moment correlation length ratio $ξ_{2\rm nd}/L$ yields $T_c = 1.30036(1)$ and $ν= 0.671(6)$. Data collapse of both quantities confirms the three-dimensional $XY$ universality class. We study the phase transition of the classical $XY$ model on a diamond lattice by Monte Carlo simulations using the Wolff cluster algorithm. Finite-size scaling (FSS) analysis of the Binder cumulant and the second-moment correlation length ratio $ξ_{2\rm nd}/L$ yields $T_c = 1.30036(1)$ and $ν= 0.671(6)$. Data collapse of both quantities confirms the three-dimensional $XY$ universality class. △ Less

Submitted 30 July, 2026; v1 submitted 20 April, 2026; originally announced April 2026.

Comments: 2 pages, 1 figure

Journal ref: J. Phys. Soc. Jpn. 95, 085002 (2026)

Topological Phase Transitions and Their Thermodynamic Fate in Arbitrary-$S$ Pyrochlore Spin Ice

Authors: Sena Watanabe, Yukitoshi Motome, Haruki Watanabe

Abstract: We develop a self-contained theoretical framework that classifies the topological phases and critical phenomena of classical pyrochlore magnets with arbitrary spin $S$, subject to competing exchange and single-ion anisotropies. In the small-$w$ regime, where the single-ion term favors low spin amplitudes, exact dualities reveal a dichotomy: integer spins exhibit a continuous 3D $XY$ deconfinement… ▽ More We develop a self-contained theoretical framework that classifies the topological phases and critical phenomena of classical pyrochlore magnets with arbitrary spin $S$, subject to competing exchange and single-ion anisotropies. In the small-$w$ regime, where the single-ion term favors low spin amplitudes, exact dualities reveal a dichotomy: integer spins exhibit a continuous 3D $XY$ deconfinement transition, whereas half-integer spins remain in a $U(1)$ Coulomb liquid without any transition. In the large-$w$ regime, where the local spin amplitudes are maximized ($|S^z| = S$), the macroscopic flux is quantized to multiples of $2S$. By mapping the defect structure to topological loop gases, we prove that the compatibility between the physical ice rule and the emergent $\mathbb{Z}_{2S}$ flux conservation holds if and only if $S \le 3/2$. For $S=3/2$, this maps the system to the 3-state Potts model, whose symmetry-allowed cubic invariant drives a first-order transition. For $S \ge 2$, monopole contamination breaks the discrete clock mapping. Using an exact decomposition of the partition function, we show that the hierarchical string fusion cascade exponentially suppresses the discrete perturbations, which act as a dangerously irrelevant operator at the 3D $XY$ fixed point, protecting 3D $XY$ criticality. Finally, incorporating thermal monopoles, we show that they act as a symmetry-breaking effective magnetic field that severs defect strings. Consequently, the continuous transitions are rounded into crossovers, whereas the first-order $S=3/2$ transition is predicted to survive at finite temperatures, terminating at a critical endpoint. Classical Monte Carlo simulations for $S$ up to $7/2$ corroborate these analytical predictions. △ Less

Submitted 5 April, 2026; originally announced April 2026.

Comments: 20 + 17 pages, 6 figures, 4 tables

Bond-density-wave orders induced by geometric frustration in the kagome metal CeRu3Si2

Authors: Ryo Misawa, Shunsuke Kitou, Rinsuke Yamada, Xiaolong Feng, Ryota Nakano, Priya Ranjan Baral, Yuiga Nakamura, Leslie M. Schoop, Yukitoshi Motome, Taka-hisa Arima, Xiuzhen Yu, Max Hirschberger

Abstract: Geometric frustration gives rise to vast manifolds of degenerate ground states and competing orders in spin and charge systems. Typically, classical ground states are governed by a local ``zero-sum constraint" that relieves frustrated antiferromagnetic interactions or Coulomb repulsion. To date, the paradigm of geometric frustration has yielded a rich landscape of emergent phases, from spin ices a… ▽ More Geometric frustration gives rise to vast manifolds of degenerate ground states and competing orders in spin and charge systems. Typically, classical ground states are governed by a local ``zero-sum constraint" that relieves frustrated antiferromagnetic interactions or Coulomb repulsion. To date, the paradigm of geometric frustration has yielded a rich landscape of emergent phases, from spin ices and quantum spin liquids to charge glasses. However, an analogous phase rooted in chemical bonding has yet to be firmly demonstrated. Here we report the discovery of bond-density-wave orders induced by geometric frustration in the kagome metal CeRu$_3$Si$_2$ above room temperature. Through synchrotron X-ray diffraction, real-space transmission electron microscopy, and model calculations, we observe two distinct long-period superlattices with harmonic and anharmonic structural modulations. Crucially, interlayer bonds between kagome planes modulate in a sublattice-selective manner to fulfill the zero-sum constraint on the kagome lattice. We demonstrate the potential of kagome metals to host complex bond-ordered states constrained by geometric frustration and establish chemical bonding as a distinct pathway to frustration physics in quantum materials even above room temperature. △ Less

Submitted 2 April, 2026; originally announced April 2026.

Continuous crossover between high-pressure ice phases VII and X driven by monopole screening: a model study

Authors: Sena Watanabe, Yukitoshi Motome, Haruki Watanabe

Abstract: The proton-disordered molecular phase of water ice (ice-VII) and its ultrahigh-pressure non-molecular phase (ice-X) share identical macroscopic crystal symmetry (space group $Pn\bar{3}m$). This raises a fundamental thermodynamic question: are they distinct phases separated by a singularity, or are they adiabatically connected via a continuous crossover? To resolve this paradox, we investigate the… ▽ More The proton-disordered molecular phase of water ice (ice-VII) and its ultrahigh-pressure non-molecular phase (ice-X) share identical macroscopic crystal symmetry (space group $Pn\bar{3}m$). This raises a fundamental thermodynamic question: are they distinct phases separated by a singularity, or are they adiabatically connected via a continuous crossover? To resolve this paradox, we investigate the finite-temperature phase diagram of high-pressure ices VII and X, as well as VIII, the proton-ordered phase that emerges at lower temperatures, using an effective classical spin-$1$ Blume-Capel model on the pyrochlore lattice. Through Monte Carlo simulations, we demonstrate that within this model, the transformation between the states corresponding to ice-VII and ice-X lacks a thermodynamic singularity, as characterized by non-divergent and non-coinciding peaks in the specific heat and susceptibility associated with the $S^z=0$ occupation. We attribute this continuous crossover behavior to the topological fragility of the hydrogen-bond network: the thermal proliferation of point-like monopole excitations (violations of the ice rules) induces Debye-Hückel screening of the emergent gauge field, destroying the topological Coulomb phase at any finite temperature. In contrast, the destruction of the proton-ordered ice-VIII phase involves spontaneous symmetry breaking and remains a first-order phase transition. Our findings provide a microscopic rationale that reconciles the macroscopic crystallographic symmetries of dense ice with its underlying topological properties. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: 11 pages, 5 figures

Dualities and Topological Classification of the $S=1$ Pyrochlore Spin Ice

Authors: Sena Watanabe, Yukitoshi Motome, Haruki Watanabe

Abstract: We resolve the phase diagram of the $S=1$ pyrochlore spin ice, which exhibits trivial paramagnetic, U(1) Coulomb, and spin nematic phases. In the monopole-free limit, the system can be effectively mapped onto 3D $XY$ and Ising loop-gas models depending on the spin anisotropy, which provides theoretical estimates for the phase boundaries, while a macroscopic flux vector classifies the topological s… ▽ More We resolve the phase diagram of the $S=1$ pyrochlore spin ice, which exhibits trivial paramagnetic, U(1) Coulomb, and spin nematic phases. In the monopole-free limit, the system can be effectively mapped onto 3D $XY$ and Ising loop-gas models depending on the spin anisotropy, which provides theoretical estimates for the phase boundaries, while a macroscopic flux vector classifies the topological sectors via geometric parity rules. At finite temperatures, thermal monopoles act as a symmetry-breaking field in both 3D $XY$ and Ising loop-gas pictures, rounding the phase transitions into continuous crossovers. These theoretical findings are corroborated by classical Monte Carlo simulations. △ Less

Submitted 16 March, 2026; v1 submitted 4 March, 2026; originally announced March 2026.

Comments: 9 pages, 2 figures

Spin current generation via magnetic skyrmion, bimeron, and meron crystals

Authors: Aoi Kajihara, Shun Okumura, Yukitoshi Motome

Abstract: Spin current offers a promising route toward energy-efficient and high-speed information processing. Developing efficient methods for their generation remains a central challenge in spintronics. Here, we investigate spin current generation via two-dimensional topological spin textures: a skyrmion crystal (SkX) with out-of-plane magnetization, a bimeron crystal (BmX) with in-plane magnetization, an… ▽ More Spin current offers a promising route toward energy-efficient and high-speed information processing. Developing efficient methods for their generation remains a central challenge in spintronics. Here, we investigate spin current generation via two-dimensional topological spin textures: a skyrmion crystal (SkX) with out-of-plane magnetization, a bimeron crystal (BmX) with in-plane magnetization, and a meron crystal (MX) with zero net magnetization. We show that these distinct spin textures generate spin currents with characteristic spin polarization directions. In the absence of spin--orbit coupling, the SkX and BmX generate spin currents polarized along their magnetization directions, whereas the MX yields no spin current. Upon introducing spin--orbit coupling, while the behavior of the SkX does not qualitatively change, the BmX generates nonzero spin currents in multiple polarization directions. Notably, the MX, despite its zero net magnetization, exhibits a pronounced spin current with out-of-plane spin polarization, driven by an enhanced spin Berry curvature associated with characteristic band degeneracy. We further demonstrate that the electronic and spin transport properties of each texture are governed by their magnetic symmetries. Our results highlight the topological spin textures as efficient sources of spin current even without net magnetization, expanding the design for spintronics devices based on topological magnetic metals. △ Less

Submitted 5 February, 2026; originally announced February 2026.

Comments: 19 pages, 8 figures

Spiral-induced Anomalous Hall Effect from Odd-parity Spin-nodal Lines

Authors: Shun Okumura, Moritz M. Hirschmann, Yukitoshi Motome

Abstract: Spin spirals represent a fundamental class of noncollinear yet coplanar magnetic structures that give rise to diverse emergent phenomena reflecting spin chirality. We investigate metallic systems hosting commensurate spin spirals and uncover an unconventional anomalous Hall effect (AHE) induced by spiral magnetism. The spin spiral introduces odd-parity spin splitting with polarization perpendicula… ▽ More Spin spirals represent a fundamental class of noncollinear yet coplanar magnetic structures that give rise to diverse emergent phenomena reflecting spin chirality. We investigate metallic systems hosting commensurate spin spirals and uncover an unconventional anomalous Hall effect (AHE) induced by spiral magnetism. The spin spiral introduces odd-parity spin splitting with polarization perpendicular to the helical plane, forming spin-nodal lines in the electronic structure. In the presence of spin-orbit coupling, we find that these nodal lines become gapped by finite magnetization, concentrating the Berry curvature near the gap and generating a distinctive AHE. We identify the interplay among the spin-orbit coupling, helical plane orientation, and magnetization direction as the key ingredient for this spiral-induced AHE, which is expected to occur across a wide range of materials hosting commensurate spin spirals. △ Less

Submitted 15 December, 2025; originally announced December 2025.

Comments: 7 pages, 5 figures

Kitaev Meets Affleck-Kennedy-Lieb-Tasaki: Competing Quantum Disorder in Spin-3/2 Honeycomb Systems

Authors: Sogen Ikegami, Kiyu Fukui, Rico Pohle, Yukitoshi Motome

Abstract: We investigate an S=3/2 quantum spin model on a two-dimensional honeycomb lattice that continuously interpolates between two paradigmatic quantum disordered states with distinct entanglement structures: the Kitaev quantum spin liquid and the Affleck-Kennedy-Lieb-Tasaki (AKLT) valence bond solid. Combining classical, semi-classical, and exact diagonalization approaches, we map out the ground-state… ▽ More We investigate an S=3/2 quantum spin model on a two-dimensional honeycomb lattice that continuously interpolates between two paradigmatic quantum disordered states with distinct entanglement structures: the Kitaev quantum spin liquid and the Affleck-Kennedy-Lieb-Tasaki (AKLT) valence bond solid. Combining classical, semi-classical, and exact diagonalization approaches, we map out the ground-state phase diagram and elucidate the role of quantum fluctuations across the entire parameter range. While classical and semi-classical frameworks predict noncoplanar orders competing with a collinear Néel state, we find these phases to be fragile: once full quantum fluctuations are included, they melt into a quantum-entangled state characterized by suppressed spin correlations and enhanced entanglement entropy. Our findings highlight how competition between qualitatively different quantum disordered phases provides a fertile playground for unconventional phases emerging from their interplay and quantum fluctuations. △ Less

Submitted 25 April, 2026; v1 submitted 6 December, 2025; originally announced December 2025.

Comments: 6+4pages, 7 Figures

Journal ref: Phys. Rev. B 113, 144433 (2026)

Nonequilibrium dynamics of magnetic hopfions driven by spin-orbit torque

Authors: Shoya Kasai, Shun Okumura, Yukitoshi Motome

Abstract: Hopfions--three-dimensional topological solitons with knotted spin texture--have recently garnered attention in topological magnetism due to their unique topology characterized by the Hopf number $H$, a topological invariant derived from knot theory. In contrast to two-dimensional skyrmions, which are typically limited to small topological invariants, i.e., skyrmion numbers, hopfions can, in princ… ▽ More Hopfions--three-dimensional topological solitons with knotted spin texture--have recently garnered attention in topological magnetism due to their unique topology characterized by the Hopf number $H$, a topological invariant derived from knot theory. In contrast to two-dimensional skyrmions, which are typically limited to small topological invariants, i.e., skyrmion numbers, hopfions can, in principle, be stabilized with arbitrary Hopf numbers. However, the nonequilibrium dynamics, especially interconversion between different Hopf numbers, remain poorly understood. Here, we theoretically investigate the nonequilibrium dynamics of hopfions with various Hopf numbers by numerically solving the Landau-Lifshitz-Gilbert equation with spin-orbit torque (SOT). For $H=1$, we show that SOT induces both translational and precessional motion, with dynamics sensitive to the initial orientation. For $H=2$, we find that intermediate SOT strengths can forcibly split the hopfion into two $H = 1$ hopfions. This behavior is explained by an effective tension picture, derived from the dynamics observed in the $H=1$ case. By comparing the splitting dynamics across different $H$, we identify a hierarchical structure governing SOT-driven behavior and use it to predict the dynamics of hopfions with general $H$. Furthermore, we show that by appropriately scheduling the time dependence of the SOT, it is possible to repeatedly induce both splitting and recombination of hopfions. These results demonstrate the controllability of hopfion topology via SOT and suggest a pathway toward multilevel spintronic devices based on topology switching. △ Less

Submitted 28 November, 2025; originally announced November 2025.

Comments: 17 pages, 12 figures, We also submit a concise version entitled "Controlling Knot Topology in Magnetic Hopfions via Spin-orbit Torque" to Physical Review Letters as Joint Submission

Controlling Knot Topology in Magnetic Hopfions via Spin-orbit Torque

Authors: Shoya Kasai, Shun Okumura, Yukitoshi Motome

Abstract: Knots, characterized by topological invariants called the Hopf number $H$, arise from the intertwining of strings and exhibit diverse configurations. The knot structures have recently been observed in condensed matters, as examplified by a magnetic hopfion, sparking interest in controlling their topology. Here, we show that spin-orbit torque (SOT) enables dynamic manipulation of the Hopf number of… ▽ More Knots, characterized by topological invariants called the Hopf number $H$, arise from the intertwining of strings and exhibit diverse configurations. The knot structures have recently been observed in condensed matters, as examplified by a magnetic hopfion, sparking interest in controlling their topology. Here, we show that spin-orbit torque (SOT) enables dynamic manipulation of the Hopf number of magnetic hopfions. We investigate the SOT-driven evolution of hopfions, revealing the splitting of a high-$H$ hopfion into multiple lower-$H$ ones, a process that can be quantified by an effective tension picture. Comparative analysis across different $H$ uncovers a hierarchy of instabilities that dictates these dynamical topological transitions. These findings establish SOT as a powerful tool for controlling hopfion topology, paving the way for potential applications in topological memory devices. △ Less

Submitted 28 November, 2025; originally announced November 2025.

Comments: 6 pages, 3 figures, We also submit an extended version entitled "Nonequilibrium dynamics of magnetic hopfions driven by spin-orbit torque" to Physical Review B as Joint Submission

Electric-field-induced magnetic toroidal moment and nonlinear magnetoelectric effect in antiferromagnetic olivines

Authors: Yasuyuki Kato, Takeshi Hayashida, Koei Matsumoto, Tsuyoshi Kimura, Yukitoshi Motome

Abstract: Beyond conventional electric and magnetic monopoles, electric and magnetic toroidal monopoles, which are rank-0 multipoles distinguished by opposite parities under spatial inversion and time reversal, can exist in nature. The recent observation of electric-field-induced directional dichroism in antiferromagnetic olivine Co$_2$SiO$_4$ has provided the first concrete example of a magnetic toroidal m… ▽ More Beyond conventional electric and magnetic monopoles, electric and magnetic toroidal monopoles, which are rank-0 multipoles distinguished by opposite parities under spatial inversion and time reversal, can exist in nature. The recent observation of electric-field-induced directional dichroism in antiferromagnetic olivine Co$_2$SiO$_4$ has provided the first concrete example of a magnetic toroidal monopole; however, its microscopic origin remains elusive. Here, we propose a minimal spin model that incorporates magnetoelectric coupling via the $d$-$p$ hybridization mechanism and analyze it within the mean-field approximation. The model qualitatively reproduces the experimentally observed temperature dependence of the dielectric constant and its pronounced sensitivity to the direction of the applied electric field. Furthermore, it elucidates the temperature evolution of the magnetic toroidal monopole and the strong electric-field-direction dependence of the magnetic toroidal moment. Our calculations also predict a second-order nonlinear magnetoelectric response, consistent with the symmetry classification of Co$_2$SiO$_4$ as an altermagnet. Additionally, we demonstrate that the same framework is applicable to other antiferromagnetic olivines with analogous magnetic order, indicating the robustness and generality of the toroidal-type magnetoelectric response in this material family. △ Less

Submitted 27 November, 2025; originally announced November 2025.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. B 113, 094411 (2026)

Topological transition induced by selective random defects on a honeycomb lattice

Authors: Sogen Ikegami, Kiyu Fukui, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome

Abstract: We investigate how the spectral and topological properties of electron systems evolve on a lattice that interpolates between the honeycomb and its 1/6-depleted structures through the introduction of selective random defects. We find that in certain parameter regimes, the topological properties of the two lattice systems are smoothly connected, whereas in other regimes, selective random defects ind… ▽ More We investigate how the spectral and topological properties of electron systems evolve on a lattice that interpolates between the honeycomb and its 1/6-depleted structures through the introduction of selective random defects. We find that in certain parameter regimes, the topological properties of the two lattice systems are smoothly connected, whereas in other regimes, selective random defects induce a topological transition. Analysis based on an effective model reveals that the effect of selective random defects can be understood as a modulation of hopping amplitudes. Our results highlight the potential for designing and controlling the spectral and even topological properties of electronic systems across a wide range of material platforms. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Comments: 11 pages, 9 Figures

Journal ref: Phys. Rev. B 113, 144203 (2026)

Lessons from $α$-RuCl3 for pursuing quantum spin liquid physics in atomically thin materials

Authors: Claudia Ojeda-Aristizabal, Xiaohu Zheng, Changsong Xu, Zohar Nussinov, Yukitoshi Motome, Arnab Banerjee, Adam W. Tsen, Michael Knap, Rui-Rui Du, Gajadhar Joshi, Andy Mounce, Youngwook Kim, Benjamin M. Hunt, Dmitry Shcherbakov, Boyi Zhou, Ran Jing, Mengkun Liu, Hui Zhao, Bolin Liao, Martin Claassen, Onur Erten, Yong P. Chen, Erik A. Henriksen

Abstract: Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from… ▽ More Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from the field of atomically thin materials. We primarily focus on the antiferromagnetic Mott insulator $α$-RuCl3, which exhibits Kitaev couplings and is readily exfoliated to single- or few-layer sheets, and thus serves as a test bed for developing probes of Kitaev phenomena in atomically thin materials and devices. We introduce the Kitaev model and how it is realized in $α$-RuCl3 and other material candidates; and cover $α$-RuCl3 synthesis and fabrication into van der Waals heterostructure devices. A key discovery is a work-function-mediated charge transfer that heavily dopes both the $α$-RuCl3 and proximate materials, and can enhance Kitaev interactions by up to 50%. We further discuss a wide range of recent results in electronic transport and optical and tunneling spectroscopies of $α$-RuCl3 devices. The experimental techniques and theoretical insights developed for $α$-RuCl3 establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Comments: 53 pages, 27 figures

High-field NMR study of field-induced states in Pb(TiO)Cu$_4$(PO$_4$)$_4$

Authors: Y. Ihara, T. Kanda, Y. Kato, Y. Motome, K. Matsui, K. Kindo, Y. Kohama, T. Kimura, K. Kimura

Abstract: The square cupola antiferromagnet Pb(TiO)Cu$_4$(PO$_4$)$_4$ exhibits the intriguing magnetoelectric responses arising from the consecutive change in the magnetic quadrupolar-type configuration of magnetic moments under external magnetic fields higher than 15 T. To clarify the high-field magnetic structures in Pb(TiO)Cu$_4$(PO$_4$)$_4$, an NMR measurement was performed in pulsed fields up to 32.2 T… ▽ More The square cupola antiferromagnet Pb(TiO)Cu$_4$(PO$_4$)$_4$ exhibits the intriguing magnetoelectric responses arising from the consecutive change in the magnetic quadrupolar-type configuration of magnetic moments under external magnetic fields higher than 15 T. To clarify the high-field magnetic structures in Pb(TiO)Cu$_4$(PO$_4$)$_4$, an NMR measurement was performed in pulsed fields up to 32.2 T significantly extending the field range accessible by superconducting magnets. The double-peak structure of NMR spectra emerging above 29 T applied along the [001] direction evidences the successive magnetic transitions. The field dependence of NMR spectra was analyzed on the basis of cluster mean-field theory, which allows us to propose possible magnetic structures for the high-field magnetic states. △ Less

Submitted 18 August, 2025; originally announced August 2025.

Comments: 9 pages, 6 figures

Spin-liquid and spin-glass behavior in quantum spin models with all-to-all p-spin interactions

Authors: Shusei Wadashima, Yukitoshi Motome

Abstract: Spin-liquid and spin-glass states represent two distinct phases of disordered quantum spin systems. These states are, in principle, distinguished by quantum-entangled fluctuations and spin freezing, but identifying each state and characterizing the transition between them remains challenging. Here, we systematically explore the relationship between the spin-liquid and spin-glass states using a mod… ▽ More Spin-liquid and spin-glass states represent two distinct phases of disordered quantum spin systems. These states are, in principle, distinguished by quantum-entangled fluctuations and spin freezing, but identifying each state and characterizing the transition between them remains challenging. Here, we systematically explore the relationship between the spin-liquid and spin-glass states using a model with all-to-all random interactions among p spins, which interpolates between the Ising-like one-component, XY-like two-component, and isotropic three-component cases. By analyzing the system-size N dependence of the Edwards-Anderson order parameter and the density of states, we identify the transition from the spin liquid to the spin glass for various values of p. We show that the phase diagrams for different p can be unified through a scaling with N/p2, revealing that increasing anisotropy in the interactions systematically suppresses the spin-liquid phase and extends the spin-glass regime. Furthermore, we examine the competition between multiple-spin interactions and anisotropy under an external magnetic field in the isotropic case, and find that the spin-liquid phase transitions into the spin-glass phase before entering a quantum paramagnetic phase. Our findings provide insights into quantum disordered phases and the transitions between them. △ Less

Submitted 7 December, 2025; v1 submitted 11 August, 2025; originally announced August 2025.

Comments: 12 pages, 12 figures

Journal ref: Phys. Rev. B 112, 214204 (2025)

Electron-phonon-coupled Langevin dynamics for strongly-correlated insulators

Authors: Rico Pohle, Yukitoshi Motome, Terumasa Tadano, Shintaro Hoshino

Abstract: The Landau-Lifshitz-Gilbert (LLG) equations are widely used to study spin dynamics in Mott insulators. However, because energy damping is typically introduced phenomenologically, their validity for describing nonequilibrium processes and their connection to the microscopic origin of dissipation in real materials remains unclear. In this paper, we derive generalized stochastic LLG equations from fi… ▽ More The Landau-Lifshitz-Gilbert (LLG) equations are widely used to study spin dynamics in Mott insulators. However, because energy damping is typically introduced phenomenologically, their validity for describing nonequilibrium processes and their connection to the microscopic origin of dissipation in real materials remains unclear. In this paper, we derive generalized stochastic LLG equations from first principles for spin-orbital coupled Mott insulators, explicitly incorporating the coupling between electronic degrees of freedom and lattice vibrations. Our approach is based on a path-integral formalism formulated along the Keldysh contour, which naturally accounts for dissipation and thermal fluctuations through interactions with a phonon bath and emergent stochastic noise. We benchmark our theoretical framework by numerically integrating the equations of motion for a two-orbital spin chain coupled to Einstein phonons. The resulting energy relaxation mimics realistic cooling dynamics, exhibits nontrivial transient behavior during thermalization, and accurately reproduces thermodynamic properties upon equilibration. We further demonstrate how electron-phonon coupling induces hybridization between electronic and phononic modes in the excitation spectrum and show that the conventional LLG equations are recovered as a limiting case of our microscopic theory. These results establish a robust and reliable framework for capturing dissipative spin dynamics in strongly correlated systems, both in and out of equilibrium. △ Less

Submitted 12 June, 2026; v1 submitted 25 July, 2025; originally announced July 2025.

Comments: 28 pages, 8 figures

Thermal Hall transport in Kitaev spin liquids

Authors: Tsuyoshi Okubo, Joji Nasu, Takahiro Misawa, Yukitoshi Motome

Abstract: We investigate the thermal Hall conductivity in the Kitaev model with additional interactions under a magnetic field, employing a finite-temperature tensor network method benchmarked by a thermal pure quantum state technique. We find that the thermal Hall conductivity divided by temperature, $κ_{xy}/T$, significantly overshoots the value of the half-integer quantization and exhibits a pronounced h… ▽ More We investigate the thermal Hall conductivity in the Kitaev model with additional interactions under a magnetic field, employing a finite-temperature tensor network method benchmarked by a thermal pure quantum state technique. We find that the thermal Hall conductivity divided by temperature, $κ_{xy}/T$, significantly overshoots the value of the half-integer quantization and exhibits a pronounced hump while decreasing temperature. Moreover, we show that the field-direction dependence of $κ_{xy}/T$ is consistent with the sign of the Chern number associated with the Majorana fermions across a wide range of magnetic fields. We also demonstrate that the additional off-diagonal interactions, known as the $Γ$ and $Γ^{\prime}$ terms, considerably affect $κ_{xy}/T$. In particular, we show that positive $Γ$ and negative $Γ^{\prime}$ lead to a remarkable enhancement in the intermediate temperature region. From the comparison with the classical counterpart, we reveal that the effects of the $Γ$ term go beyond the classical picture, indicating significant quantum fluctuation effects, while those of the $Γ^\prime$ term are well captured at the classical level. These comprehensive analyses indicate that the enhanced thermal Hall response is consistently explained by dominant contributions from topological Majorana fermions, even within the polarized regime beyond the critical field. Our approach not only establishes a robust theoretical framework for understanding the thermal Hall transport in Kitaev materials such as $α$-RuCl$_{3}$, but also offers a promising pathway to bridge the gap between theories and experiments across a wide range of strongly correlated materials. △ Less

Submitted 2 September, 2025; v1 submitted 22 July, 2025; originally announced July 2025.

Comments: 26 pages, 26 figures

Topological Majorana flat bands in the Kitaev model on a Bishamon-kikko lattice

Authors: Kiyu Fukui, Yukitoshi Motome

Abstract: We unveil an interesting example of topological flat bands of Majorana fermions in quantum spin liquids. We study the Kitaev model on a periodically depleted honeycomb lattice, under a magnetic field within the perturbation theory. The model can be straightforwardly extended while maintaining the exact solvability, and its ground state is a quantum spin liquid as on the honeycomb lattice. As fract… ▽ More We unveil an interesting example of topological flat bands of Majorana fermions in quantum spin liquids. We study the Kitaev model on a periodically depleted honeycomb lattice, under a magnetic field within the perturbation theory. The model can be straightforwardly extended while maintaining the exact solvability, and its ground state is a quantum spin liquid as on the honeycomb lattice. As fractionalized excitations, there are unpaired localized Majorana fermions in addition to the itinerant Majorana fermions and $\mathbb{Z}_2$ fluxes. We show that in the absence of the magnetic field the Majorana fermions have completely flat bands at zero energy, and by applying the magnetic field, they turn into topological flat bands with nonzero Chern number. By varying the anisotropy of the interactions and the magnitude of the magnetic field, we clarify that the system exhibits a variety of topological phases that do not appear in the original model. We emphasize that the topological flat bands that give this rich topology come from the hybridization of the Majorana flat bands and unpaired Majorana fermions, which is unique to the flat bands of fractionalized excitations in quantum spin liquids. Our findings would stimulate the exploration of a new type of Kitaev materials exhibiting rich topology from topological Majorana flat bands. △ Less

Submitted 18 July, 2025; originally announced July 2025.

Comments: 14 pages, 10 figures

Nonrelativistic Piezomagnetic Effect in an Organic Altermagnet

Authors: Makoto Naka, Yukitoshi Motome, Tsuyoshi Miyazaki, Hitoshi Seo

Abstract: We theoretically study the piezomagnetic effect on the altermagnetic state in $κ$-type molecular conductors, focusing on its nonrelativistic mechanism. By introducing shear stress as a monoclinic distortion, we evaluate variations in the effective tight-binding model using first-principles calculations. Using the derived parameters, we investigate the Hubbard model and its effective Heisenberg mod… ▽ More We theoretically study the piezomagnetic effect on the altermagnetic state in $κ$-type molecular conductors, focusing on its nonrelativistic mechanism. By introducing shear stress as a monoclinic distortion, we evaluate variations in the effective tight-binding model using first-principles calculations. Using the derived parameters, we investigate the Hubbard model and its effective Heisenberg model on the two-dimensional (distorted) $κ$-type lattice within mean-field approximation. We show that the system exhibits the piezomagnetic effect, i.e., a net magnetization induced at finite temperatures in the undoped insulating state and both in the ground state and at finite temperatures upon doping. In a real-space picture, this uniform magnetization arises from the ferrimagnetic spin structure due to inequivalent spin sites induced by lattice distortion. Meanwhile, in a momentum-space picture, it stems from the {\it s}-wave spin splitting of the electron and magnon bands, independent of spin-orbit coupling. We find that this nonrelativistic piezomagnetism remains finite, but becomes smaller in the limit of strong dimerization where the energy gap between the bonding and antibonding orbitals is infinitely large and the {\it d}-wave altermagnetic spin splitting is absent, highlighting the importance of the multi-orbital nature. △ Less

Submitted 12 May, 2025; originally announced May 2025.

Comments: 5 pages, 3 figures, 2 tables

Metal-insulator transitions in pyrochlore oxides

Authors: Yoshinori Tokura, Yukitoshi Motome, Kentaro Ueda

Abstract: Pyrochlore oxides with chemical formula of A2B2O7 exhibit a diverse range of electronic properties as a representative family of quantum materials. These properties mostly stem from strong electron correlations at the transition metal B site and typical geometrical frustration effects on the pyrochlore lattice. Furthermore, the coupling between the magnetic moments of the rare-earth A site and the… ▽ More Pyrochlore oxides with chemical formula of A2B2O7 exhibit a diverse range of electronic properties as a representative family of quantum materials. These properties mostly stem from strong electron correlations at the transition metal B site and typical geometrical frustration effects on the pyrochlore lattice. Furthermore, the coupling between the magnetic moments of the rare-earth A site and the conduction electrons at the B site, along with the relativistic spin-orbit coupling particularly affecting the 4d/5d electrons at the B site, gives rise to the topological characteristics of the correlated electrons. This review paper focuses on the metal-insulator transitions in pyrochlore oxides as evidence of the strong electron correlation, which is highlighted as a rich source of intriguing charge dynamics coupled with frustrated spin-orbital entangled magnetism. △ Less

Submitted 24 April, 2025; originally announced April 2025.

Comments: Submitted to Reports on Progress in Physics (under review)

Journal ref: Rep. Prog. Phys. 88, 056001 (2025)

Metallic $p$-wave magnet with commensurate spin helix

Authors: Rinsuke Yamada, Max T. Birch, Priya R. Baral, Shun Okumura, Ryota Nakano, Shang Gao, Motohiko Ezawa, Takuya Nomoto, Jan Masell, Yuki Ishihara, Kamil K. Kolincio, Ilya Belopolski, Hajime Sagayama, Hironori Nakao, Kazuki Ohishi, Takashi Ohhara, Ryoji Kiyanagi, Taro Nakajima, Yoshinori Tokura, Taka-hisa Arima, Yukitoshi Motome, Moritz M. Hirschmann, Max Hirschberger

Abstract: Antiferromagnetic states with spin-split electronic structure give rise to novel spintronic, magnonic, and electronic phenomena despite (near-) zero net magnetization. The simplest odd-parity spin splitting - $p$-wave - was originally proposed to emerge from a collective instability in interacting electron systems. Recent theory identifies a distinct route to realise $p$-wave spin-split electronic… ▽ More Antiferromagnetic states with spin-split electronic structure give rise to novel spintronic, magnonic, and electronic phenomena despite (near-) zero net magnetization. The simplest odd-parity spin splitting - $p$-wave - was originally proposed to emerge from a collective instability in interacting electron systems. Recent theory identifies a distinct route to realise $p$-wave spin-split electronic bands without strong correlations, termed $p$-wave magnetism. Here we demonstrate an experimental realisation of a metallic $p$-wave magnet. The odd-parity spin splitting of delocalised conduction electrons arises from their coupling to an antiferromagnetic texture of localised magnetic moments: a coplanar spin helix whose magnetic period is an even multiple of the chemical unit cell, as revealed by X-ray scattering experiments. This texture breaks space inversion symmetry but preserves time-reversal ($T$) symmetry up to a half-unit-cell translation - thereby fulfilling the symmetry conditions for $p$-wave magnetism. Consistent with theoretical predictions, our $p$-wave magnet exhibits a characteristic anisotropy in the electronic conductivity. Relativistic spin-orbit coupling and a tiny spontaneous net magnetization further break $T$ symmetry, resulting in a giant anomalous Hall effect (AHE, $σ_{xy}>600\,$S/cm, Hall angle $>3\,\%$), for an antiferromagnet. Our model calculations show that the spin nodal planes found in the electronic structure of $p$-wave magnets are readily gapped by a small perturbation to induce the AHE. △ Less

Submitted 9 September, 2025; v1 submitted 14 February, 2025; originally announced February 2025.

Comments: 24 pages, 4 figures

Journal ref: Nature 646, 837-842 (2025)

Altermagnetic Perovskites

Authors: Makoto Naka, Yukitoshi Motome, Hitoshi Seo

Abstract: Altermagnet is a class of antiferromagnets, which shows a staggered spin ordering with wave vector ${\bm q}=0$, while its net magnetization is canceled out in the limit of zero relativistic spin-orbit coupling. The simplest case is when the up and down spins are ordered on two crystallographically equivalent sublattice sites within the unit cell that are not connected by translation, and consequen… ▽ More Altermagnet is a class of antiferromagnets, which shows a staggered spin ordering with wave vector ${\bm q}=0$, while its net magnetization is canceled out in the limit of zero relativistic spin-orbit coupling. The simplest case is when the up and down spins are ordered on two crystallographically equivalent sublattice sites within the unit cell that are not connected by translation, and consequently, the system breaks the macroscopic time-reversal symmetry. Accordingly, it exhibits non-relativistic spin splitting in the energy band and characteristic cross-correlation phenomena between spin, charge, and lattice (orbital) degrees of freedom. This is in contrast to conventional Néel-type antiferromagnets with ${\bm q} \neq 0$ conserving the macroscopic time-reversal symmetry, where the time-reversal operation flipping of spins combined with translation can make the system identical to the original state. Altermagneticsm is universally latent in various magnetic materials that have been considered as simple collinear-type antiferromagnets. In this article, we focus on perovskites with chemical formula {\it ABX}$_3$, which are typical playgrounds for strongly correlated electron systems, and overview their altermagnetic aspects that have been overlooked in the past researches, based on microscopic model studies revealing the mechanisms of their properties. We display that a combination of a variety of antiferromagnetic ordering and the commonly-seen lattice distortions in perovskites gives rise to a non-relativistic spin splitting whose mechanism does not rely on the spin-orbit coupling and its consequent spin current generation, and the anomalous Hall effect in the presence of the spin-orbit coupling. △ Less

Submitted 17 November, 2024; originally announced November 2024.

Comments: 10 pages, 6 figures, 3 tables

Three-dimensional Topological Superstructure of Magnetic Hopfions Threaded by Meron Strings in Easy-plane Magnets

Authors: Shoya Kasai, Kotaro Shimizu, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome

Abstract: Topological spin textures exhibit a hierarchical nature. For instance, magnetic skyrmions, which possess a particle-like nature, can aggregate to form superstructures such as skyrmion strings and skyrmion lattices. Magnetic hopfions are also regarded as superstructures constructed from closed loops of twisted skyrmion strings, which behave as another independent particles. However, it remains elus… ▽ More Topological spin textures exhibit a hierarchical nature. For instance, magnetic skyrmions, which possess a particle-like nature, can aggregate to form superstructures such as skyrmion strings and skyrmion lattices. Magnetic hopfions are also regarded as superstructures constructed from closed loops of twisted skyrmion strings, which behave as another independent particles. However, it remains elusive whether such magnetic hopfions can also aggregate to form higher-level superstructures. Here, we report a stable superstructure with three-dimensional periodic arrangement of magnetic hopfions in a frustrated spin model with easy-plane anisotropy. By comprehensively examining effective interactions between two hopfions, we construct the hopfion superstructure by a staggered arrangement of one-dimensional hopfion chains with Hopf number $H=+1$ and $H=-1$ running perpendicular to the easy plane. Each hopfion chain is threaded by a magnetic meron string, resulting in a nontrivial topological texture with skyrmion number $N_{\rm sk}=2$ per magnetic unit cell on any two-dimensional cut parallel to the easy plane. We show that the hopfion superstructure remains robust as a metastable state across a range of the hopfion density. Furthermore, we demonstrate that superstructures with higher Hopf number can also be stabilized. Our findings extend the existing hierarchy of topological magnets and pave the way for exploring new quantum phenomena and spin dynamics. △ Less

Submitted 1 November, 2024; originally announced November 2024.

Comments: 15 pages, 9 figures

Soliton penetration from edges in a monoaxial chiral magnet

Authors: Kotaro Shimizu, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome

Abstract: 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… ▽ More 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 exhibiting periodic arrangements of solitons, has been limited thus far. Here, we theoretically perform the systematic analysis for the dynamical control of the number of chiral solitons in monoaxial chiral magnets by effectively utilizing the edge modes whose excitation is localized near the edges. By studying the dynamical process associated with this edge mode in an applied rotating magnetic field by using the Landau-Lifshitz-Gilbert equation, we show that multiple soliton penetrations can take place until the system reaches the nonequilibrium steady state, and the number of infiltrated solitons successively increases with the amplitude of the rotating magnetic field after surpassing the threshold. We also clarify that the threshold amplitude of the rotating magnetic field can be reduced through the static magnetic field. Our results reveal that the desired number of solitons can be added within a certain range by taking advantage of the edge modes that appear without any special processing at the edges of the system. These results contribute to the development of an experimental way to control the number of solitons and are expected to be further applied to a wide range of magnetic solitons, not limited to chiral solitons. △ Less

Submitted 16 September, 2024; originally announced September 2024.

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

Authors: Kotaro Shimizu, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome

Abstract: 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 topolo… ▽ More 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. △ Less

Submitted 3 July, 2024; originally announced July 2024.

SP-STM study of the multi-Q phases in GdRu2Si2

Authors: Jonas Spethmann, Nguyen Duy Khanh, Haruto Yoshimochi, Rina Takagi, Satoru Hayami, Yukitoshi Motome, Roland Wiesendanger, Shinichiro Seki, Kirsten von Bergmann

Abstract: The two stable surfaces of GdRu2Si2 are studied using spin-polarized scanning tunneling microscopy (SP-STM). Depending on the applied magnetic field different magnetic phases have been found and the presented measurements are in agreement with the respective previously proposed multi-Q spin textures. In particular the multi-Q nature of the zero magnetic field state, for which previous experiments… ▽ More The two stable surfaces of GdRu2Si2 are studied using spin-polarized scanning tunneling microscopy (SP-STM). Depending on the applied magnetic field different magnetic phases have been found and the presented measurements are in agreement with the respective previously proposed multi-Q spin textures. In particular the multi-Q nature of the zero magnetic field state, for which previous experiments could not rule out the coexistence of single-Q states, can be confirmed by our spin-resolved measurements on the Si-terminated surface. The surfaces of GdRu2Si2 exhibit strong magnetism-induced modulations of the spin-averaged density of states. We find that while the magnetic contribution to the tunnel signal can be clearly identified for the Si-terminated surface this proves to be much more difficult for the Gd-terminated surface. However, the magnetic field dependent spatial modulations on the Gd-terminated surface demonstrate that additional magnetic phase transitions occur for the surface layer compared to those identified for bulk GdRu2Si2 and possible spin textures are presented. △ Less

Submitted 28 May, 2024; originally announced May 2024.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. Materials 8, 064404 (2024)

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

Authors: Sogen Ikegami, Kiyu Fukui, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome

Abstract: 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 unexp… ▽ More 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. △ Less

Submitted 10 May, 2024; originally announced May 2024.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 110, 245107 (2024.)

Exchange Interactions in Rare-earth Magnets $A_2$PrO$_3$ ($A$= alkali metals): Revisited

Authors: Seong-Hoon Jang, Yukitoshi Motome

Abstract: Rare-earth materials hold promise to realize exotic magnetic states owing to synergy between electron correlations and spin-orbit coupling. Recently, quasi-two-dimensional honeycomb magnets $A_2$PrO$_3$ ($A$ = alkali metals) were predicted to be good candidates for Kitaev quantum spin liquids, as the Kitaev-type bond-dependent anisotropic interactions dominate over bond-independent isotropic Heise… ▽ More Rare-earth materials hold promise to realize exotic magnetic states owing to synergy between electron correlations and spin-orbit coupling. Recently, quasi-two-dimensional honeycomb magnets $A_2$PrO$_3$ ($A$ = alkali metals) were predicted to be good candidates for Kitaev quantum spin liquids, as the Kitaev-type bond-dependent anisotropic interactions dominate over bond-independent isotropic Heisenberg ones. However, experimental observations are negative, questioning the energy hierarchy in Pr$^{4+}$ ions assumed in the conjecture on the basis of the conventional Russell-Saunders coupling scheme. We here revisit the exchange interactions in these Pr compounds, by explicitly taking into account the ionic states beyond the assumption. We show that, while increasing the octahedral crystal field splitting, which was assumed to be negligibly small, relative to the spin-orbit coupling, the Kitaev-type interactions are suppressed to be subdominant compared to prevailing Heisenberg ones. Our finding compromises the contradiction as arising from the peculiar ionic state of the high valence Pr$^{4+}$. △ Less

Submitted 25 April, 2024; originally announced April 2024.

Journal ref: Phys. Rev. B 110, 155124 (2024)

Eight-color chiral spin liquid in the $S=1$ bilinear-biquadratic model with Kitaev interactions

Authors: Rico Pohle, Nic Shannon, Yukitoshi Motome

Abstract: Multipolar spin systems provide a rich ground for the emergence of unexpected states of matter due to their enlarged spin degree of freedom. In this study, with a specific emphasis on $S=1$ magnets, we explore the interplay between spin nematic states and spin liquids. Based on the foundations laid in the prior work [R. Pohle et al., Phys. Rev. B 107, L140403 (2023)], we investigate the $S=1$ Kita… ▽ More Multipolar spin systems provide a rich ground for the emergence of unexpected states of matter due to their enlarged spin degree of freedom. In this study, with a specific emphasis on $S=1$ magnets, we explore the interplay between spin nematic states and spin liquids. Based on the foundations laid in the prior work [R. Pohle et al., Phys. Rev. B 107, L140403 (2023)], we investigate the $S=1$ Kitaev model with bilinear-biquadratic interactions, which stabilizes, next to Kitaev spin liquid, spin nematic and triple-$q$ phases, also an exotic chiral spin liquid. Through a systematic reduction of the spin degree of freedom -- from $\mathbb{CP}^{2}$ to $\mathbb{CP}^{1}$ and ultimately to a discrete eight-color model -- we provide an intuitive understanding of the nature and origin of this chiral spin liquid. We find that the chiral spin liquid is characterized by an extensive ground-state degeneracy, bound by a residual entropy, extremely short-ranged correlations, a nonzero scalar spin chirality marked by $\mathbb{Z}_{2}$ flux order, and a gapped continuum of excitations. Our work contributes not only to the specific exploration of $S=1$ Kitaev magnets but also to the broader understanding of the importance of multipolar spin degree of freedom on the ground state and excitation properties in quantum magnets. △ Less

Submitted 3 August, 2024; v1 submitted 11 April, 2024; originally announced April 2024.

Comments: 30 pages, 16 figures

Journal ref: Phys. Rev. Research 6, 033077 (2024)

Spin-Orbit Coupled Insulators and Metals on the Verge of Kitaev Spin Liquids in Ilmenite Heterostructures

Authors: Yi-Feng Zhao, Seong-Hoon Jang, Yukitoshi Motome

Abstract: Competition and cooperation between electron correlation and relativistic spin-orbit coupling give rise to diverse exotic quantum phenomena in solids. An illustrative example is spin-orbit entangled quantum liquids, which exhibit remarkable features such as topological orders and fractional excitations. The Kitaev honeycomb model realizes such interesting states, called the Kitaev spin liquids, bu… ▽ More Competition and cooperation between electron correlation and relativistic spin-orbit coupling give rise to diverse exotic quantum phenomena in solids. An illustrative example is spin-orbit entangled quantum liquids, which exhibit remarkable features such as topological orders and fractional excitations. The Kitaev honeycomb model realizes such interesting states, called the Kitaev spin liquids, but its experimental feasibility is still challenging. Here we theoretically investigate hexagonal heterostructures including a candidate for the Kitaev magnets, MgIrO$_3$, to actively manipulate the electronic and magnetic properties toward realizing the Kitaev spin liquids. For three different structure types of ilmenite bilayers MgIrO$_3$/$A$TiO$_3$ with $A$ = Mn, Fe, Co, and Ni, we obtain the optimized lattice structures, the electronic band structures, the stable magnetic orders, and the effective magnetic couplings. We find that the spin-orbital coupled bands characterized by the pseudospin $j_{\rm eff}=$ 1/2 are retained in the MgIrO$_3$ layer for all the heterostructures, but the magnetic state and the band gap depend on the types of heterostructures as well as the $A$ atoms. In particular, one type becomes metallic irrespective of $A$, while the other two are mostly insulating. We show that the insulating cases provide spin-orbit coupled Mott insulating states with dominant Kitaev-type interactions, accompanied by different combinations of subdominant interactions depending on the heterostructural type and $A$, while the metallic cases realize spin-orbit coupled metals with various doping rates. Our results indicate that these hexagonal heterostructures are a good platform for engineering electronic and magnetic properties of the spin-orbital coupled correlated materials, including the possibility of Majorana Fermi surfaces and topological superconductivity. △ Less

Submitted 14 March, 2024; originally announced March 2024.

Exploring rare-earth Kitaev magnets by massive-scale computational analysis

Authors: Seong-Hoon Jang, Yukitoshi Motome

Abstract: The Kitaev honeycomb model plays a pivotal role in the quest for quantum spin liquids, in which fractional quasiparticles would provide applications in decoherence-free topological quantum computing. The key ingredient is the bond-dependent Ising-type interactions, dubbed the Kitaev interactions, which require strong entanglement between spin and orbital degrees of freedom. This study investigates… ▽ More The Kitaev honeycomb model plays a pivotal role in the quest for quantum spin liquids, in which fractional quasiparticles would provide applications in decoherence-free topological quantum computing. The key ingredient is the bond-dependent Ising-type interactions, dubbed the Kitaev interactions, which require strong entanglement between spin and orbital degrees of freedom. This study investigates the identification and design of rare-earth materials displaying robust Kitaev interactions. We scrutinize all possible $4f$ electron configurations, which require up to $6+$ million intermediate states in the perturbation processes, by developing a parallel computational program designed for massive scale calculations. Our analysis reveals a predominant interplay between the isotropic Heisenberg $J$ and anisotropic Kitaev $K$ interactions across all realizations of the Kramers doublets. Remarkably, instances featuring $4f^3$ and $4f^{11}$ configurations showcase the prevalence of $K$ over $J$, presenting unexpected prospects for exploring the Kitaev QSLs in compounds including Nd$^{3+}$ and Er$^{3+}$, respectively. Beyond the Kitaev model, our computational program also proves adaptable to a wide range of $4f$-electron magnets. △ Less

Submitted 28 February, 2024; originally announced February 2024.

Journal ref: Commun. Mater. 5, 192 (2024)

Topological transitions by magnetization rotation in kagome monolayers of ferromagnetic Weyl semimetal Co-based shandite

Authors: Kazuki Nakazawa, Yasuyuki Kato, Yukitoshi Motome

Abstract: Co-based shandite Co$_3$Sn$_2$S$_2$ is a ferromagnet hosting Weyl fermions in the layered Co kagome structure. The band topology as well as the magnetism is predicted to vary drastically in the atomically thin films depending on the thickness and surface termination, and as an extreme case, the quantum anomalous Hall state is expected in a monolayer of the Co kagome lattice. Given that the bulk We… ▽ More Co-based shandite Co$_3$Sn$_2$S$_2$ is a ferromagnet hosting Weyl fermions in the layered Co kagome structure. The band topology as well as the magnetism is predicted to vary drastically in the atomically thin films depending on the thickness and surface termination, and as an extreme case, the quantum anomalous Hall state is expected in a monolayer of the Co kagome lattice. Given that the bulk Weyl gap depends on the magnetization direction, here we theoretically study how the topological nature and transport properties vary with the magnetization direction in the systems with kagome monolayer with both Sn and S surface terminations. By using $ab \ initio$ calculations, we find that in the Sn-end monolayer the anomalous Hall conductivity shows successive discrete changes between different quantized values by rotating the magnetization, indicating several topological transitions between the anomalous quantum Hall insulators with different Chern numbers. Notably, when the magnetization is oriented in-plane and perpendicular to the Co-Co bond, the system exhibits a planar quantized anomalous Hall effect. We clarify that these peculiar behaviors are due to topological changes in the band structures associated with gap closing of the Weyl nodes. In contrast, the S-end monolayer shows rather continuous changes in the transport properties since the system is metallic, although the band structure contains many Weyl nodes. Our results pave the way for controlling Weyl fermions in atomically thin films of Co-based shandite, where the topological nature associated with the Weyl nodes appears more clearly than the bulk. △ Less

Submitted 25 February, 2024; originally announced February 2024.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 110, 085112 (2024)

Inverse Hamiltonian design of highly-entangled quantum systems

Authors: Koji Inui, Yukitoshi Motome

Abstract: Solving inverse problems to identify Hamiltonians with desired properties holds promise for the discovery of fundamental principles. In quantum systems, quantum entanglement plays a pivotal role in not only characterizing the quantum nature but also developing quantum technology like quantum computing. Nonetheless, the design principles of the quantum entanglement are yet to be clarified. Here we… ▽ More Solving inverse problems to identify Hamiltonians with desired properties holds promise for the discovery of fundamental principles. In quantum systems, quantum entanglement plays a pivotal role in not only characterizing the quantum nature but also developing quantum technology like quantum computing. Nonetheless, the design principles of the quantum entanglement are yet to be clarified. Here we apply an inverse design framework using automatic differentiation to quantum spin systems, aiming to construct Hamiltonians with large quantum entanglement. We show that the method automatically finds the Kitaev model with bond-dependent anisotropic interactions, whose ground state is a quantum spin liquid, on both honeycomb and square-octagon lattices. On triangular and maple-leaf lattices with geometrical frustration, it generates numerous solutions with spatially inhomogeneous interactions rather than converging to a specific model, but it still helps to construct unprecedented models. The comparative study reveals that bond-dependent anisotropic interactions, rather than isotropic Heisenberg interactions, amplify quantum entanglement, even in systems with geometrical frustration. The present study paves the way for the automatic design of new quantum systems with desired quantum nature and functionality. △ Less

Submitted 24 February, 2024; originally announced February 2024.

Journal ref: Phys. Rev. Research 6, 033080 (2024)

Quantum reservoir probing of quantum phase transitions

Authors: Kaito Kobayashi, Yukitoshi Motome

Abstract: Quantum phase transitions are highly remarkable phenomena manifesting in quantum many-body systems. However, their precise identifications in equilibrium systems pose significant theoretical and experimental challenges. Thus far, dynamical detection protocols employing global quantum quenches have been proposed, wherein transitions are discerned via global nonequilibrium excitations. In this work,… ▽ More Quantum phase transitions are highly remarkable phenomena manifesting in quantum many-body systems. However, their precise identifications in equilibrium systems pose significant theoretical and experimental challenges. Thus far, dynamical detection protocols employing global quantum quenches have been proposed, wherein transitions are discerned via global nonequilibrium excitations. In this work, we demonstrate that quantum phase transitions can be detected through localized out-of-equilibrium excitations induced by local quantum quenches. While the resulting dynamics after the quench is influenced by both the local quench operation and the intrinsic dynamics of the quantum system, the effects of the former are exclusively extracted using the cutting-edge framework called quantum reservoir probing (QRP). Through the QRP, we find that the impacts of the local quenches vary across different quantum phases and are significantly suppressed by quantum fluctuations amplified near quantum critical points; consequently, phase boundaries are precisely delineated. We demonstrate that the QRP can detect quantum phase transitions in the paradigmatic integrable and nonintegrable quantum spin systems, and even topological quantum phase transitions, all within the identical framework employing local quantum quenches and single-site observables. △ Less

Submitted 23 March, 2025; v1 submitted 10 February, 2024; originally announced February 2024.

Comments: 15 pages, 9 figures

Magnetic field effects on the Kitaev model coupled to environment

Authors: Kiyu Fukui, Yasuyuki Kato, Yukitoshi Motome

Abstract: Open quantum systems display unusual phenomena not seen in closed systems, such as new topological phases and unconventional phase transitions. An interesting example was studied for a quantum spin liquid in the Kitaev model [K. Yang, S. C. Morampudi, and E. J. Bergholtz, Phys. Rev. Lett. ${\bf 126}$, 077201 (2021)]; an effective non-Hermitian Kitaev model, which incorporates dissipation effects,… ▽ More Open quantum systems display unusual phenomena not seen in closed systems, such as new topological phases and unconventional phase transitions. An interesting example was studied for a quantum spin liquid in the Kitaev model [K. Yang, S. C. Morampudi, and E. J. Bergholtz, Phys. Rev. Lett. ${\bf 126}$, 077201 (2021)]; an effective non-Hermitian Kitaev model, which incorporates dissipation effects, was shown to give rise to a gapless spin liquid state with exceptional points in the Majorana dispersions. Given that an external magnetic field induces a gapped Majorana topological state in the Hermitian case, the exceptional points may bring about intriguing quantum phenomena under a magnetic field. Here we investigate the non-Hermitian Kitaev model perturbed by the magnetic field. We show that the exceptional points remain gapless up to a finite critical magnetic field, in stark contrast to the Hermitian case where an infinitesimal field opens a gap. The gapless state is stable over a wide range of the magnetic field for some particular parameter sets, and in special cases, undergoes topological transitions to another gapless state with different winding number around the exceptional points without opening a gap. In addition, in the system with edges, we find that the non-Hermitian skin effect is induced by the magnetic field, even for the parameters where the skin effect is absent at zero field. The chirality of edge states is switched through the exceptional points, similarly to the surface Fermi arcs connected by the Weyl points in three-dimensional Weyl semimetals. Our results provide a new possible route to stabilize topological gapless quantum spin liquids under the magnetic field in the presence of dissipation. △ Less

Submitted 27 June, 2024; v1 submitted 8 February, 2024; originally announced February 2024.

Comments: 16 pages, 10 figures

Spin Seebeck Effect as a Probe for Majorana Fermions in Kitaev Spin Liquids

Authors: Yasuyuki Kato, Joji Nasu, Masahiro Sato, Tsuyoshi Okubo, Takahiro Misawa, Yukitoshi Motome

Abstract: Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids (QSLs) provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we th… ▽ More Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids (QSLs) provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we theoretically investigate the spin Seebeck effect, which is a thermoelectric response via a spin current, as an efficient probe of the fractional quasiparticles in QSLs, focusing on the Kitaev honeycomb model. By comprehensive studies using the real-time dynamics, the perturbation theory, and the linear spin-wave theory based on the tunnel spin-current theory, we find that the spin current is induced by thermal gradient in the Kitaev spin liquid, via the low-energy fractional Majorana excitations. This underscores the ability of Majorana fermions to carry spin current, despite lacking spin angular momentum. Furthermore, we find that the induced spin current changes its sign depending on the sign of the Kitaev interaction, indicating that the Majorana fermions contribute to the spin current with (up-)down-spin like nature when the exchange coupling is (anti)ferromagnetic. Thus, in contrast to the negative spin current already found in a one-dimensional QSL, our finding reveals that the spin Seebeck effect can exhibit either positive or negative signals, contingent upon the nature of fractional excitations in the QSLs. We also clarify contrasting field-angle dependence between the Kitaev spin liquid in the low-field limit and the high-field ferromagnetic state, which is useful for the experimental identification. Our finding suggests that the spin Seebeck effect could be used not only to detect fractional quasiparticles emerging in QSLs but also to generate and control them. △ Less

Submitted 7 March, 2025; v1 submitted 23 January, 2024; originally announced January 2024.

Comments: 18 pages, 10 figures

Journal ref: Phys. Rev. X 15, 011050 (2025)

Video recognition by physical reservoir computing in magnetic materials

Authors: Kaito Kobayashi, Yukitoshi Motome

Abstract: Nonlinear spin dynamics in magnetic materials offers a promising avenue for implementing physical reservoir computing, one of the most accomplished brain-inspired frameworks for information processing. In this study, we investigate the practical utility of magnetic physical reservoirs by assessing their performance in a video recognition task. Leveraging a recently developed spatiotemporal paralle… ▽ More Nonlinear spin dynamics in magnetic materials offers a promising avenue for implementing physical reservoir computing, one of the most accomplished brain-inspired frameworks for information processing. In this study, we investigate the practical utility of magnetic physical reservoirs by assessing their performance in a video recognition task. Leveraging a recently developed spatiotemporal parallelization scheme, our reservoir achieves accurate classifications of previously provided images. Our findings pave the way for the development of visual sensors based on the magnetic physical reservoir computing. △ Less

Submitted 10 October, 2023; originally announced October 2023.

Anomalous charge transport upon quantum melting of chiral spin order

Authors: Y. Fujishiro, C. Terakura, A. Miyake, N. Kanazawa, K. Nakazawa, N. Ogawa, H. Kadobayashi, S. Kawaguchi, T. Kagayama, M. Tokunaga, Y. Kato, Y. Motome, K. Shimizu, Y. Tokura

Abstract: A plethora of correlated and exotic metallic states have been identified on the border of itinerant magnetism, where the long-range spin texture is melted by tuning the magnetic transition temperature (T$_C$) towards zero, referred to as the quantum phase transition (QPT). So far, the study of QPT in itinerant magnets has mainly focused on low-T$_C$ materials (i.e., typically T$_C$ ~ 10 K) where t… ▽ More A plethora of correlated and exotic metallic states have been identified on the border of itinerant magnetism, where the long-range spin texture is melted by tuning the magnetic transition temperature (T$_C$) towards zero, referred to as the quantum phase transition (QPT). So far, the study of QPT in itinerant magnets has mainly focused on low-T$_C$ materials (i.e., typically T$_C$ ~ 10 K) where the modification of electronic band structure is subtle, and only makes a small contribution to the QPT. Here we report a distinct example of a magnetic QPT accompanied by a gigantic modification of the electronic structure in FeGe, i.e., a well-studied itinerant chiral magnet hosting near-room-temperature (T$_C$ = 278 K) helical/skyrmion spin texture. The pressure-driven modification of the band structure (e.g., reduction of exchange splitting) is evidenced by magneto-transport study, suggesting a Fermi-surface reconstruction around the magnetic QPT (P ~19 GPa), in stark contrast to the case of typical metallic ferromagnets. Further application of pressure leads to a metal-to-insulator transition above P > 30 GPa, as also corroborated by our density-functional theory (DFT) calculation. Of particular interest is the occurrence of anomalous magneto-transport in the inhomogeneous short-range chiral-spin ground state (P = 20-30 GPa) above the QPT, with longitudinal fluctuations of magnetization. The unexpected observation of spontaneous anomalous Hall effect in this exotic quantum regime suggests macroscopic time-reversal symmetry (TRS) breaking, even in the absence of long-range magnetic order. Our findings mark the large body of unexplored high-T$_C$ itinerant magnets with broken inversion-symmetry as promising candidates of novel ground state formation near QPT. △ Less

Submitted 7 October, 2023; originally announced October 2023.

Possible Realization of Kitaev Spin Liquids in van der Waals Heterostructures of $α$-RuCl$_3$ and Cr$X_3$ ($X$=Cl and I)

Authors: Lingzhi Zhang, Yukitoshi Motome

Abstract: Despite the presence of the exact solution and well-established recipe for its realization, the quest for the Kitaev spin liquid in real materials remains exceptionally challenging. Among many magnets, $α$-RuCl$_3$ emerges as a prime candidate, albeit the hallmarks of the spin liquid manifest only within the specific region where the zigzag-type antiferromagnetic order is suppressed by an applied… ▽ More Despite the presence of the exact solution and well-established recipe for its realization, the quest for the Kitaev spin liquid in real materials remains exceptionally challenging. Among many magnets, $α$-RuCl$_3$ emerges as a prime candidate, albeit the hallmarks of the spin liquid manifest only within the specific region where the zigzag-type antiferromagnetic order is suppressed by an applied magnetic field. Here, we propose the possible realization of the Kitaev spin liquid at zero field by making van der Waals heterostructures of $α$-RuCl$_3$ and a ferromagnet Cr$X_3$ ($X$=Cl and I). Using {\it ab initio} calculations, we find that in the case of $X$=Cl the zigzag order is suppressed by the proximity effect of the ferromagnetic CrCl$_3$ layer, while the Kitaev interaction is still relevant in the $α$-RuCl$_3$ layer. Notably, the induced Ru moment is close to the value observed in the spin liquid region of the bulk material, signifying the possibility of the Kitaev spin liquid at zero field. In contrast, in the case of $X$=I, the system is on the verge of an insulator-metal transition by carrier doping through interlayer hybridization. Our results indicate that van der Waals heterostructures provide a new platform for studying not only magnetic but also electronic properties of the Kitaev magnets. △ Less

Submitted 25 December, 2024; v1 submitted 2 October, 2023; originally announced October 2023.

Quantum reservoir probing: an inverse paradigm of quantum reservoir computing for exploring quantum many-body physics

Authors: Kaito Kobayashi, Yukitoshi Motome

Abstract: Quantum reservoir computing (QRC) is a brain-inspired computational paradigm, exploiting natural dynamics of a quantum system for information processing. To date, a multitude of quantum systems have been utilized in the QRC, with diverse computational capabilities demonstrated accordingly. This study proposes a reciprocal research direction: probing quantum systems themselves through their informa… ▽ More Quantum reservoir computing (QRC) is a brain-inspired computational paradigm, exploiting natural dynamics of a quantum system for information processing. To date, a multitude of quantum systems have been utilized in the QRC, with diverse computational capabilities demonstrated accordingly. This study proposes a reciprocal research direction: probing quantum systems themselves through their information processing performance in the QRC framework. Building upon this concept, here we develop quantum reservoir probing (QRP), an inverse extension of the QRC. The QRP establishes an operator-level linkage between physical properties and performance in computing. A systematic scan of this correspondence reveals intrinsic quantum dynamics of the reservoir system from computational and informational perspectives. Unifying quantum information and quantum matter, the QRP holds great promise as a potent tool for exploring various aspects of quantum many-body physics. In this study, we specifically apply it to analyze information propagation in a one-dimensional quantum Ising chain. We demonstrate that the QRP not only distinguishes between ballistic and diffusive information propagation, reflecting the system's dynamical characteristics, but also identifies system-specific information propagation channels, a distinct advantage over conventional methods. △ Less

Submitted 24 July, 2024; v1 submitted 1 August, 2023; originally announced August 2023.

Comments: 14 pages, 9 figures

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

Authors: Kotaro Shimizu, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome

Abstract: 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 a… ▽ More 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. △ Less

Submitted 16 July, 2023; originally announced July 2023.

Unveiling the orbital-selective electronic band reconstruction through the structural phase transition in TaTe$_2$

Authors: Natsuki Mitsuishi, Yusuke Sugita, Tomoki Akiba, Yuki Takahashi, Masato Sakano, Koji Horiba, Hiroshi Kumigashira, Hidefumi Takahashi, Shintaro Ishiwata, Yukitoshi Motome, Kyoko Ishizaka

Abstract: Tantalum ditelluride TaTe$_2$ belongs to the family of layered transition metal dichalcogenides but exhibits a unique structural phase transition at around 170 K that accompanies the rearrangement of the Ta atomic network from a "ribbon chain" to a "butterfly-like" pattern. While multiple mechanisms including Fermi surface nesting and chemical bonding instabilities have been intensively discussed,… ▽ More Tantalum ditelluride TaTe$_2$ belongs to the family of layered transition metal dichalcogenides but exhibits a unique structural phase transition at around 170 K that accompanies the rearrangement of the Ta atomic network from a "ribbon chain" to a "butterfly-like" pattern. While multiple mechanisms including Fermi surface nesting and chemical bonding instabilities have been intensively discussed, the origin of this transition remains elusive. Here we investigate the electronic structure of single-crystalline TaTe$_2$ with a particular focus on its modifications through the phase transition, by employing core-level and angle-resolved photoemission spectroscopy combined with first-principles calculations. Temperature-dependent core-level spectroscopy demonstrates a splitting of the Ta $4f$ core-level spectra through the phase transition indicative of the Ta-dominated electronic state reconstruction. Low-energy electronic state measurements further reveal an unusual kink-like band reconstruction occurring at the Brillouin zone boundary, which cannot be explained by Fermi surface nesting or band folding effects. On the basis of the orbital-projected band calculations, this band reconstruction is mainly attributed to the modifications of specific Ta $5d$ states, namely the $d_{XY}$ orbitals (the ones elongating along the ribbon chains) at the center Ta sites of the ribbon chains. The present results highlight the strong orbital-dependent electronic state reconstruction through the phase transition in this system and provide fundamental insights towards understanding complex electron-lattice-bond coupled phenomena. △ Less

Submitted 10 February, 2024; v1 submitted 27 June, 2023; originally announced June 2023.

Comments: The final version is available in Phys. Rev. Res. (Open Acess)

Journal ref: Physical Review Research 6, 013155 (2024)

Metallic ruthenium ilmenites: first-principles study of MgRuO$_3$ and CdRuO$_3$

Authors: Seong-Hoon Jang, Yukitoshi Motome

Abstract: Ilmenites $AB$O$_3$ provide a platform for electron correlation and magnetism on alternatively stacked honeycomb layers of edge-sharing $A$O$_6$ or $B$O$_6$ octahedra. When $A$ and $B$ are $3d$ transition metals, strong electron correlation makes the systems Mott insulators showing various magnetic properties, while when $B$ is Ir with $5d$ electrons, competition between electron correlation and s… ▽ More Ilmenites $AB$O$_3$ provide a platform for electron correlation and magnetism on alternatively stacked honeycomb layers of edge-sharing $A$O$_6$ or $B$O$_6$ octahedra. When $A$ and $B$ are $3d$ transition metals, strong electron correlation makes the systems Mott insulators showing various magnetic properties, while when $B$ is Ir with $5d$ electrons, competition between electron correlation and spin-orbit coupling realizes a spin-orbital coupled Mott insulator as a potential candidate for quantum spin liquids. Here we theoretically investigate intermediate $4d$ ilmenites, $A$RuO$_3$ with $A$=Mg and Cd, which were recently synthesized and shown to be metallic, unlike the $3d$ and $5d$ cases. By using first-principles calculations, we optimize the lattice structures and obtain the electronic band structures. We show that MgRuO$_3$ exhibits strong dimerization on RuO$_6$ honeycomb layers, leading to the formation of bonding and anti-bonding bands for one of three $t_{2g}$ orbitals; the lattice symmetry is lowered from $R\bar{3}$ to $P\bar{1}$, and the Fermi surfaces are composed of the other two $t_{2g}$ orbitals. In contrast, we find that CdRuO$_3$ has a lattice structure close to $R\bar{3}$, and all three $t_{2g}$ orbitals contribute almost equally to the Fermi surfaces. Comparison of our results with other Ru honeycomb materials such as Li$_2$RuO$_3$ indicates that the metallic ruthenium ilmenites stand on a subtle balance among electron correlation, spin-orbit coupling, and electron-phonon coupling. △ Less

Submitted 20 June, 2023; originally announced June 2023.

Journal ref: AIP Advances 14, 015229 (2024)

High-field phase diagram of the chiral-lattice antiferromagnet Sr(TiO)Cu4(PO4)4

Authors: Toshihiro Nomura, Yasuyuki Kato, Yukitoshi Motome, Atsushi Miyake, Masashi Tokunaga, Yoshimitsu Kohama, Sergei Zherlitsyn, Joachim Wosnitza, Shojiro Kimura, Tsukasa Katsuyoshi, Tsuyoshi Kimura, Kenta Kimura

Abstract: High-field phase diagram of a chiral-lattice antiferromagnet Sr(TiO)Cu4(PO4)4 is studied by means of the ultrasound, dielectric, and magnetocaloric-effect measurements. These experimental techniques reveal two new phase transitions at high fields, which have not been resolved by the previous magnetization experiments. Specifically, the c66 acoustic mode shows drastic changes with hysteresis with a… ▽ More High-field phase diagram of a chiral-lattice antiferromagnet Sr(TiO)Cu4(PO4)4 is studied by means of the ultrasound, dielectric, and magnetocaloric-effect measurements. These experimental techniques reveal two new phase transitions at high fields, which have not been resolved by the previous magnetization experiments. Specifically, the c66 acoustic mode shows drastic changes with hysteresis with applied fields along the c axis, indicating a strong magneto-elastic coupling. Combined with the cluster mean-field theory, we discuss the origins of these phase transitions. By considering the chiral-twist effect of Cu4O12 cupola units, which is inherent to the chiral crystal structure, the phase diagram is reasonably reproduced. The agreement between the experiment and theory suggests that this material is a unique quasi two-dimensional spin system with competing exchange interactions and chirality, leading to the rich phase diagram. △ Less

Submitted 1 December, 2023; v1 submitted 4 June, 2023; originally announced June 2023.

Comments: 12 pages, 11 figures, 1 table

Journal ref: Phys. Rev. B 108, 054434 (2023)

Interedge spin resonance in the Kitaev quantum spin liquid

Authors: Takahiro Misawa, Joji Nasu, Yukitoshi Motome

Abstract: The Kitaev model offers a platform for quantum spin liquids (QSLs) with fractional excitations, itinerant Majorana fermions and localized fluxes. Since these fractional excitations could be utilized for quantum computing, how to create, observe, and control them through the spin degree of freedom is a central issue. Here, we study dynamical spin transport in a wide range of frequency for the Kitae… ▽ More The Kitaev model offers a platform for quantum spin liquids (QSLs) with fractional excitations, itinerant Majorana fermions and localized fluxes. Since these fractional excitations could be utilized for quantum computing, how to create, observe, and control them through the spin degree of freedom is a central issue. Here, we study dynamical spin transport in a wide range of frequency for the Kitaev-Heisenberg model, by applying an AC magnetic field to an edge of the system. We find that, in the Kitaev QSL phase, spin polarizations at the other edge are resonantly induced in a specific spin component, even though the static spin correlations are vanishingly small. This interedge spin resonance appears around the input frequency over the broad frequency range. Comparing with the dynamical spin correlations, we clarify that the resonance is governed by the itinerant Majorana fermions with a broad continuum excitation spectrum, which can propagate over long distances, although it vanishes for the pure Kitaev model because of accidental degeneracy and requires weak Heisenberg interactions. We also find that the spin polarizations in the other spin components are weakly induced at an almost constant frequency close to the excitation gap of the localized fluxes, irrespective of the input frequency. These results demonstrate that the dynamical spin transport is a powerful probe of the fractional excitations in the Kitaev QSL. Possible experimental realization of the interedge spin resonance is discussed. △ Less

Submitted 7 September, 2023; v1 submitted 2 April, 2023; originally announced April 2023.

Comments: 12 pages, 9 figure

Journal ref: Phys. Rev. B 108, 115117 (2023)

Ground-State Phase Diagram of the Kitaev-Heisenberg Model on a Three-dimensional Hyperhoneycomb Lattice

Authors: Kiyu Fukui, Yasuyuki Kato, Yukitoshi Motome

Abstract: The Kitaev model, which hosts a quantum spin liquid (QSL) in the ground state, was originally defined on a two-dimensional honeycomb lattice, but can be straightforwardly extended to any tricoordinate lattices in any spatial dimensions. In particular, the three-dimensional (3D) extensions are of interest as a realization of 3D QSLs, and some materials like $β$-Li$_{2}$IrO$_{3}$, $γ$-Li$_2$IrO$_3$,… ▽ More The Kitaev model, which hosts a quantum spin liquid (QSL) in the ground state, was originally defined on a two-dimensional honeycomb lattice, but can be straightforwardly extended to any tricoordinate lattices in any spatial dimensions. In particular, the three-dimensional (3D) extensions are of interest as a realization of 3D QSLs, and some materials like $β$-Li$_{2}$IrO$_{3}$, $γ$-Li$_2$IrO$_3$, and $β$-ZnIrO$_{3}$ were proposed for the candidates. However, the phase diagrams of the models for those candidates have not been fully elucidated, mainly due to the limitation of numerical methods for 3D frustrated quantum spin systems. Here we study the Kitaev-Heisenberg model defined on a 3D hyperhoneycomb lattice, by using the pseudofermion functional renormalization group method. We show that the ground-state phase diagram contains the QSL phases in the vicinities of the pristine ferromagnetic and antiferromagnetic Kitaev models, in addition to four magnetically ordered phases, similar to the two-dimensional honeycomb case. Our results respect the four-sublattice symmetry inherent in the model, which was violated in the previous study. Moreover, we also show how the phase diagram changes with the anisotropy in the interactions. The results provide a reference for the search of the hyperhoneycomb Kitaev materials. △ Less

Submitted 16 March, 2023; originally announced March 2023.

Comments: 8 pages, 6 figures