arXiv subjects

Taka-hisa Arima

Publications and source records attributed to Taka-hisa Arima.

At least 37 records · Page 2Linked to original sources

arXiv2026

Effect of uniaxial stress on helimagnetic phases in the square-lattice itinerant magnet EuAl$_{4}$

We investigate uniaxial-stress effects on the magnetic phase diagram of the square-lattice itinerant magnet EuAl$_{4}$, where strong coupling among spin, lattice, and charge produces a variety of helimagnetic phases, including rhombic and square skyrmion lattices. Combining resistivity and magnetization measurements with neutron scattering, we find that compressive stresses of only several tens of megapascal along [010] enhance antiferromagnetic character and shorten the magnetic modulation period in the lowest-temperature single-Q spiral state, thereby driving the critical temperatures and fields of multiple phases to higher values. First-principles calculations show that increasing orthorhombic lattice distortion deforms the Fermi surface relevant to the magnetism, providing compelling evidence that Fermi-surface nesting plays a crucial role in stabilizing the helical magnetic modulations in EuAl$_{4}$.

cond-mat.str-el↗

arXiv2026

Emergent Cooperative Superstructures via Order-Disorder Kinetics in Molecule-Intercalated NbSe2

The design of quantum states at heterointerfaces has enabled a variety of emergent phenomena. Among them, molecular intercalation superlattices have attracted attention as tunable hybrid materials, formed by inserting organic molecules into van der Waals crystals, where molecular structure and chemistry provide new degrees of freedom. Traditionally, the intercalated molecules have been regarded as inactive spacers, while possible molecular ordering and its impact on the host lattice have remained largely unexplored. Here, we report the discovery of a cooperative superstructure (CSS) phase in molecule intercalated NbSe2, where ordering of the guest molecules induce a concomitant superstructure in the NbSe2 host lattice, characterized by a moiré structure due to incommensurability between the molecular layer and the inorganic lattice. Synchrotron X-ray diffraction reveals the emergence of CSS phase, accompanied by crystal symmetry lowering. Complementary resistivity and thermal-quench measurements show that the transition is governed by unusually slow order-disorder kinetics, so that the CSS phase can be selectively accessed under standard laboratory cooling rates. This kinetic behavior arises from slow molecular dynamics coupled to the host lattice, contrasting with fast charge or magnetic ordering in inorganic solids. Our findings establish molecular ordering as a route for engineering heterointerfaces, enabling thermally programmable superstructures.

cond-mat.mtrl-sci↗

arXiv2025

Skyrmion and Meron Crystals in Intermetallic Gd$_3$Ru$_4$Al$_{12}$: Microscopic Model Insights into Chiral Phases

Topological spin textures in frustrated intermetallics hold great promise for spintronics applications. However, understanding their origin and properties remains a significant challenge due to competing and often long-range interactions mediated by conduction electrons. Here, by combining neutron scattering experiments with theoretical modeling via unprecedented multi-target fits that further incorporate the ferromagnentic resonance data and magnetization curve, we construct a realistic microscopic model for the prototypical intermetallic skyrmion host \text{Gd}$_3$\text{Ru}$_4$\text{Al}$_{12}$. Beyond magnetic frustration, we identify the competition between dipolar interactions and easy-plane single-ion anisotropy as a key ingredient for stabilizing the rich chiral magnetic phases observed in this compound -- including a hexagonal skyrmion crystal and two distinct meron crystals. Remarkably, the meron crystal in lower field is revealed to be commensurate with the underlying lattice, and its unique three-meron-one-antimeron spin texture is verified by the polarized x-ray diffraction data. At elevated temperatures, the short-range spin correlations in \text{Gd}$_3$\text{Ru}$_4$\text{Al}$_{12}$ are well described by a codimension-two spiral spin-liquid. Perturbations from staggered Dzyaloshinskii-Moriya interactions give rise to chiral fluctuations that account for the temperature and field dependence of the anomalous Hall response. Our results highlight the unique power of neutron scattering, especially when combined with complementary experimental techniques, to unravel complex magnetic phase transitions and provide new insights into the rich variety of topological spin textures in frustrated systems.

cond-mat.str-el↗

arXiv2025

Linear tetramer formation in nonmagnetic pyrochlore niobate

We investigate displacive short-range order in pyrochlore Y2Nb2O7, which exhibits a nonmagnetic insulating state despite the presence of formally tetravalent Nb4+ (S = 1/2) ions on the pyrochlore network. Synchrotron x-ray diffraction on a single crystal reveals a characteristic x-ray diffuse scattering (XDS) pattern primarily around q = {0.5, 0.5, 2}. Reverse Monte Carlo (RMC) simulations uncover local Nb displacements along the <111> axes, leading to the formation of linear Nb4 tetramers. Our findings highlight a crucial role of molecular orbital degrees of freedom in stabilizing the nonmagnetic insulating state. This study demonstrates that RMC analysis of XDS provides a powerful approach for elucidating short-range correlations and the underlying mechanisms governing the physical properties of crystalline materials.

cond-mat.str-el↗

arXiv2025

Higher-order epitaxy: A pathway to suppressing structural instability and emergent superconductivity

Molecular beam epitaxy enables the growth of thin film materials with novel properties and functionalities. Typically, the lattice constants of films and substrates are designed to match to minimise disorders and strains. However, significant lattice mismatches can result in higher-order epitaxy, where commensurate growth occurs with a period defined by integer multiples of the lattice constants. Despite its potential, higher-order epitaxy is rarely used to enhance material properties or induce emergent phenomena. Here, we report single-crystalline FeTe films grown via 6:5 commensurate higher-order epitaxy on CdTe(001) substrates. Scanning transmission electron microscopy reveals self-organised periodic interstitials near the interface, arising from higher-order lattice matching. Synchrotron x-ray diffraction shows that the tetragonal-to-monoclinic structural transition in bulk FeTe is strongly suppressed. Remarkably, these films exhibit substrate-selective two-dimensional superconductivity, likely due to suppressed monoclinic distortion. These findings demonstrate the potential of higher-order epitaxy as a tool to control materials and inducing emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Visualization of spin-orbit entangled 4f electrons in crystalline materials

Lanthanide 4f electrons are strongly influenced by spin-orbit coupling, resulting in well-defined J multiplets, which are further split by the crystalline electric field in condensed matter. While the anisotropy of 4f electrons is closely linked to material properties, direct experimental observation of the 4f electron distribution in real space remains a significant challenge. Here, we present an approach for visualizing the anisotropic distribution of lanthanide 4f electrons in pyrochlore oxides by combining high-photon-energy X-ray diffraction and valence electron density (VED) analysis based on the core differential Fourier synthesis (CDFS) method. The observed VED distributions around the lanthanide site reveal the parameters of the ground-state wavefunction, which roughly agree with point-charge calculations for the trigonal crystal electric field under the LS coupling scheme. This CDFS-based VED observation method not only provides insights into the anisotropic nature of 4f electrons but also opens a pathway for studying the 4f states in a wide range of crystalline materials.

cond-mat.str-el↗

arXiv2025

Metallic $p$-wave magnet with commensurate spin helix

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.

cond-mat.str-el↗

arXiv2025

Dipolar and quadrupolar correlations in the $5d^2$ Re-based double perovskites Ba$_2$YReO$_6$ and Ba$_2$ScReO$_6$

Double perovskites containing heavy transition metal ions are an important family of compounds for the study of the interplay between electron correlation and spin-orbit coupling. Here, by combining magnetic susceptibility, heat capacity, and neutron scattering measurements, we investigate the dipolar and quadrupolar correlations in two prototype rhenium-based double perovskite compounds, Ba$_2$YReO$_6$ and Ba$_2$ScReO$_6$. A type-I dipolar antiferromagnetic ground state with a propagation vector $\mathbf{q} = (0, 0, 1)$ is observed in both compounds. At temperatures above the magnetic transitions, a quadrupolar ordered phase is identified. Weak spin excitations, which are gapped at low temperatures and softened in the correlated paramagnetic phase, are explained using a minimal model that considers both the dipolar and quadrupolar interactions. At larger wavevectors, we observe dominant phonon excitations that are well described by density functional calculations.

cond-mat.str-el↗

arXiv2025

Unraveling a chemical-bond-driven root of topology in three-dimensional chiral crystals

Chirality manifests across multiple scales, yielding unique phenomena that break mirror symmetry. In chiral materials, unexpectedly large spin-filtering or photogalvanic effects have been observed even in materials composed of light elements, implying crucial influence of their topological electronic states. However, an underlying framework that links chemical bonding and electronic topology remains elusive, preventing the rational design of quantum chiral properties. Here we identify the chiral bonding network responsible for multifold topological fermions by combining synchrotron X-ray diffraction and first-principles calculations on cubic chiral crystals, CoSi and FeSi. Based on the observations of asymmetric valence electron distributions around the transition metals, together with analyses of their bonding to sevenfold-coordinated silicon atoms, we develop a three-dimensional Su-Schrieffer-Heeger model, showing that inter-site hopping on this chiral network creates multifold fermions with doubled topological invariants. Topological features can be switched by reversing the crystalline chirality or tuning electron filling. Our results highlight that implementing strong spin-orbit coupling is not the sole route to realize robust topological phases at elevated temperatures and offer a practical design principle for exploiting chiral topology. Moreover, this real-space framework naturally extends to other elementary excitations or artificial metamaterials, enabling various quantum functionalities through an intuitive approach to chirality engineering.

cond-mat.mtrl-sci↗

arXiv2025

Successive orthorhombic distortions in kagome metals by molecular orbital formation

The kagome lattice, with its inherent frustration, hosts a plethora of exotic phenomena, including the emergence of $3\mathbf{q}$ charge density wave order. The high rotational symmetry, required to realize such an unconventional charge order, is broken in many kagome materials by orthorhombic distortions at high temperature, the origin of which is much less discussed despite their ubiquity. In this study, synchrotron X-ray diffraction reveals a structural phase transition from a parent hexagonal phase to an orthorhombic ground state, mediated by a critical regime of diffuse scattering in the prototypical kagome metals $R$Ru$_3$Si$_2$ ($R$=rare-earth). Structural analysis uncovers an interlayer dimerization of kagome atoms in the low-temperature phase. Accordingly, a dimer model with one-dimensional disorder on kagome layers successfully reproduces the diffuse scattering. The observations point to molecular orbital formation between kagome $4d_{z^2}$ orbitals as the driving force behind the transition, consistent with \textit{ab initio} calculations. A framework based on electronegativity and atomic radii is proposed to evaluate the stability of the hexagonal phase in kagome metals, guiding the design of highly symmetric materials.

cond-mat.str-el↗

arXiv2025

All-optical control of antiferromagnetic domains via an inverse optical magnetoelectric effect

Antiferromagnets are a promising platform for next-generation spintronics due to their ultrafast spin dynamics and robustness to external fields. All-optical control of antiferromagnetic order is essential to fully exploit their potential in energy-efficient and high-speed spintronic and memory applications. However, optical writing of antiferromagnetic domains remains a fundamental challenge, as conventional magneto-optical techniques rely on net magnetization, which is absent in antiferromagnets. In certain multiferroic antiferromagnets, the magnetic toroidal moment provides an additional degree of freedom through its inherent magnetoelectric coupling. This coupling at higher frequencies results in the optical magnetoelectric effect (OME), which manifests as a directional asymmetry in light propagation and enables optical probing of antiferromagnetic states. Here, we demonstrate all-optical writing of antiferromagnetic domains using the inverse optical magnetoelectric effect (IOME) in ferrotoroidic LiNiPO4. The writing process is nonvolatile, non-thermal, and deterministic, driven solely by reversing the light propagation direction. This directional control arises from a strong coupling between the photon linear momentum and the magnetic toroidal moment, enabling the repeatable switching between time-reversed domains with arbitrary light polarization. Our findings establish IOME as a distinct mechanism for manipulating antiferromagnetic order, opening a new paradigm in opto-magnetism driven by photon momentum.

cond-mat.mtrl-sci↗

arXiv2025

Ferroaxial order of the monolayer ice in martyite

Ice Ih, the most stable phase of water at ambient pressure, is a stacking of the honeycomb network of water molecules H2O. What if one layer of ice is exfoliated and confined to a two-dimensional (2D) sheet? Martyite Zn3(V2O7)(OH)2 2H2O, a mineral with the honeycomb lattice of H2O in the porous framework, is an ideal system for studying such monolayer ice. Due to the geometrical frustration and 2D nature, H2O molecules are dynamically disordered at room temperature. In this study, we reveal disorder-order transitions of H2O in martyite using single-crystal x-ray diffraction (XRD). The XRD results visualize the formation of hydrogen-bonded toroidal H2O hexamers, leading to the ferroaxial order below 200 K. Combined with the molecular dynamics simulations, we discuss the formation process of the H2O hexamers and how they compromise the molecular arrangement towards lower temperatures. Our results unveil the ground state of monolayer ice, a fundamental knowledge to understand the polymorphism of H2O.

cond-mat.mtrl-sci↗

arXiv2025

Current-induced successive structural phase transitions beyond thermal equilibrium in single-crystal VO2

Nonequilibrium systems driven by external energy sources host unexplored physics; yet phase transitions beyond thermal equilibrium remain elusive. Here, we demonstrate that electric current induces structural phase transitions in single-crystal VO2, a prototypical strongly correlated material. At room temperature, synchrotron X-ray diffraction shows that a current density of 6.5 A/cm2 disrupts V-V dimers, driving a monoclinic-to-tetragonal insulator-to-metal transition, independent of Joule heating. Increasing the current to 10 A/cm2 triggers a discontinuous isotropic lattice expansion, stabilizing a novel tetragonal structure that does not exist in thermal equilibrium. Optical microscopy and microscopic Raman spectroscopy reveal dynamic domain motion and metastable phases, reminiscent of dissipative structures. These findings establish direct pathways to access hidden phases and symmetry changes beyond thermal equilibrium, broadening the frontiers of nonequilibrium thermodynamics.

cond-mat.str-el↗

arXiv2025

Electronic Structure of Kramers Nodal-Line Semimetal YAuGe and Anomalous Hall Effect Induced by Magnetic Rare-Earth Substitution

Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. We investigate the electronic structure of candidate KNL semimetal YAuGe by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations as well as by density functional theory (DFT) calculations. DFT has revealed that YAuGe hosts KNLs on the G-A-L-M plane of the Brillouin zone, that are protected by the time reversal and mirror-inversion symmetries. Through ARPES and quantum oscillations we identify signatures of hole bands enclosing the G point, and the observed splitting of quantum oscillation frequency with angle is attributed to spin-orbit-coupling-induced band splitting away from the KNLs. Furthermore, we show that the degeneracy of the nodal lines along the G-A line is lifted by the time-reversal-symmetry breaking when the Y is substituted by magnetic R ions (R = rare earth). This becomes a source of Berry curvature and contributes to the anomalous Hall effect in magnetic RAuGe. These findings establish RAuGe as a new class of KNL semimetals offering significant potential for engineering of anomalous magnetotransport properties via magnetic rare-earth substitution.

cond-mat.mtrl-sci↗

arXiv2025

Reply to "Comment on "Reconsidering the nonlinear emergent inductance: time-varying Joule heating and its impact on the AC electrical response"" by Yokouch et al

This is a response to the comments [arXiv:2407.15682 and Phys.Rev.B 111, 146401 (2025)] by Yokouchi et al. on our paper in Phys.Rev.B 110, 174402 (2024). In this Reply, we note that (i) their arguments lack a discussion of whether the overall characteristics of the observed nonlinear impedance, including its magnitude and unphysical negative inductance interpretation, can be explained by the emergent induction scenario, whereas the Joule heating model can, (ii) they incorrectly refer to the Joule heating model, and (iii) their new data in the Comment are also quantitatively explained by the Joule heating model. These findings suggest that, contrary to the opinion by Yokouchi et al., the overall behavior of the observed impedance is irrelevant to emergent induction.

cond-mat.mtrl-sci↗

arXiv2025

Piezoelectric Transition in a Nonpyroelectric Gyroidal Metal-Organic Framework

Among the thirty-two crystallographic point groups, 432 is the only one that lacks an inversion center but does not exhibit piezoelectricity. A gyroidal structure belongs to point group 432 and shows characteristic physical properties attributed to its distinctive strong isotropic network. Here, we investigate a gyroidal cobalt oxalate metal-organic framework (MOF) with disordered orientations of SO4 tetrahedra. Synchrotron X-ray diffraction experiments using a single crystal reveal a cubic-to-cubic structural phase transition at TS = 120 K. This transition involves a change in the point group from nonpiezoelectric 432 to piezoelectric 23. The symmetry change arises from the ordering of distorted SO4 molecules, leading to a three-dimensional helical arrangement of electric dipole moments. Furthermore, pyroelectric current measurements using polycrystalline pellet samples reveal that electric polarization emerges below TS depending on the magnitude of the pelletizing pressure, demonstrating piezoelectricity. The gyroidal MOF offers an opportunity to explore unique dielectric properties induced by the helical ordering of molecules and structural flexibility.

cond-mat.mtrl-sci↗

arXiv2025

Entropy-assisted, long-period stacking of honeycomb layers in an AlB2-type silicide

Configurational entropy can impact crystallization processes, tipping the scales between structures of nearly equal internal energy. Using alloyed single crystals of Gd2PdSi3 in the AlB2-type structure, we explore the formation of complex layer sequences made from alternating, two-dimensional triangular and honeycomb slabs. A four-period and an eight-period stacking sequence are found to be very close in internal energy, the latter being favored by entropy associated with covering the full configuration space of interlayer bonds. Possible consequences of polytype formation on magnetism in Gd2PdSi3 are discussed.

cond-mat.mtrl-sci↗

arXiv2025

Triangular lattice magnet GdGa$_2$ with short-period spin cycloids and possible skyrmion phases

The two-dimensional triangular lattice (TAL) is a model system of magnetic frustration and competing interactions, where skyrmion spin vortices can be induced by a vertical magnetic field $B$. We target the binary compound GdGa$_2$ with an undistorted TAL of Gd$^{3+}$ Heisenberg moments. At higher temperature ($T > 5$ K, $B = 0$, phase II), we reveal the cycloidal spin textures in GdGa$_2$ via resonant elastic X-ray scattering (REXS). Further, a transition with strong magneto-elastic response occurs when cooling into the zero-field ground state ($T < 5$ K, phase I). We also report the thermodynamic phase boundaries of $B$-induced magnetic A-phases, which are suppressed by an in-plane magnetic field and which have enhanced resistivity due to the partial opening of a charge gap. In analogy to Gd$_2$PdSi$_3$ and GdRu$_2$Si$_2$, these phases may represent a superposition of various cycloids, possibly a Néel skyrmion lattice. Our work lays the basis for further studies of the magnetic phase diagram of GdGa$_2$.

cond-mat.str-el↗