arXiv subjects

Yukitoshi Motome

Publications and source records attributed to Yukitoshi Motome.

At least 19 recordsLinked to original sources

arXiv2026

Weaving Hopfions from Emergent Monopoles in a Chiral Magnet

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.

cond-mat.mes-hall↗

arXiv2026

Two routes to quantum anomalous Hall states in altermagnets

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 $\left| \sigma_{xy} \right|=e^2/h$ 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 $\left| \sigma_{xy} \right|=2e^2/h$ 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.

cond-mat.str-el↗

arXiv2026

Creating and Driving a Twist Soliton on a Magnetic Skyrmion Tube

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.

cond-mat.str-el↗

arXiv2026

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

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 $\Gamma$ 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-$\Gamma$ 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-$\Gamma$ 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.

cond-mat.str-el↗

arXiv2026

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

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 $\xi_{2\rm nd}/L$ yields $T_c = 1.30036(1)$ and $\nu = 0.671(6)$. Data collapse of both quantities confirms the three-dimensional $XY$ universality class.

cond-mat.str-el↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2026

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

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-H\"{u}ckel 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.

cond-mat.str-el↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2025

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

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.

cond-mat.mes-hall↗

arXiv2025

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

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 N\'eel 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.

cond-mat.str-el↗

arXiv2025

Controlling Knot Topology in Magnetic Hopfions via Spin-orbit Torque

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.

cond-mat.mes-hall↗

arXiv2025

Nonequilibrium dynamics of magnetic hopfions driven by spin-orbit torque

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.

cond-mat.mes-hall↗

arXiv2025

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

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.

cond-mat.str-el↗

arXiv2025

Topological transition induced by selective random defects on a honeycomb lattice

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.

cond-mat.dis-nn↗

arXiv2025

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

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 $\alpha$-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 $\alpha$-RuCl3 and other material candidates; and cover $\alpha$-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 $\alpha$-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 $\alpha$-RuCl3 devices. The experimental techniques and theoretical insights developed for $\alpha$-RuCl3 establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases.

cond-mat.str-el↗

arXiv2025

Thermal Hall transport in Kitaev spin liquids

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.

cond-mat.str-el↗