arXiv subjects

Shunsuke Kitou

Publications and source records attributed to Shunsuke Kitou.

At least 19 recordsLinked to original sources

arXiv2026

Observation of g-wave altermagnetic multipole

Over the past few years, altermagnets have emerged as a new class of collinear magnets with broken time-reversal symmetry, offering novel opportunities for spintronics beyond conventional magnets. Rather than from net magnetization, as in ferromagnets, the unconventional time-reversal symmetry breaking of altermagnets originates from antiferroic magnetic dipoles locked to higher-order multipoles. Here we report the direct visualization of a $g$-wave altermagnetic multipole in the canonical altermagnet CrSb. Combining high-energy synchrotron X-ray diffraction with valence electron density (VED) analysis, we uncover a pronounced directional anisotropy of the VED distribution alternating between Cr sublattices. This evidences the antiferroic order of electric hexadecapoles predicted in $g$-wave altermagnets. Its coexistence with antiferroic magnetic dipoles induces ferroic magnetic multipoles, as probed by polarized neutron diffraction. We further identify a microscopic model of altermagnetism that directly relates the $g$-wave multipole and the $g$-wave spin splitting. Through direct observation and quantification of multipoles, this study provides a real-space fingerprint of altermagnetism and establishes a general probe of hidden multipole order in quantum materials.

cond-mat.str-el↗

arXiv2026

Correlated topological-polarization surface states in the narrow-gap insulator FeSb2

Strong electron correlations and band topology each generate rich quantum phases, but conflicting elemental requirements have largely kept them apart. Topological polarization offers a route to unite them, producing polar surface states from bonding charge without spin-orbit coupling and thereby extending band topology to correlated 3d transition-metal compounds. Here we demonstrate that epitaxial thin films of the narrow-gap insulator FeSb2 host metallic polar surface states of topological-polarization origin, governed by the strong correlations of the bulk. Nonreciprocal surface transport emerges only below the onset temperature of a correlation-driven reconstruction of the bulk Fe 3d orbital occupation, providing direct evidence of bulk-edge correspondence in a correlated topological system. Moreover, electrostatic gating drives this correlated surface across a quantum phase transition into a ferromagnetic or possibly altermagnetic state. Our results establish topological polarization as a design principle for correlated topological phases in a broad range of materials.

cond-mat.str-el↗

arXiv2026

Single-crystal structural phase diagram of stoichiometric bilayer nickelate La3Ni2O7 under hydrostatic pressure

The bilayer nickelate La3Ni2O7 has attracted intense interest following the discovery of high-temperature superconductivity under pressure, representing the first nickelate superconductor realized in bulk form. However, the crystal structure of the superconducting phase remains under active discussion, complicating efforts to establish its microscopic origin. Here we resolve these structural controversies by establishing a definitive pressure-temperature phase diagram, including the superconducting region of stoichiometric La3Ni2O7 single crystals under hydrostatic conditions using helium as the pressure-transmitting medium. At ambient pressure, La3Ni2O7 adopts a polar orthorhombic Am2m structure characterized by charge order between inequivalent Ni sites and NiO6 octahedral tilting. Upon compression, the system undergoes a direct transition from the charge-ordered Am2m phase to the tetragonal I4/mmm phase near 10 GPa, coinciding with the onset of bulk superconductivity. These results establish the intrinsic structural evolution of La3Ni2O7 and provide a structural framework for microscopic theories of nickelate superconductivity.

cond-mat.supr-con↗

arXiv2026

Density-wave phases, anisotropic transport, and Planckian dissipation in single crystals of the superconductor La3Ni2O7

Pressure-induced superconductivity in bilayer nickelates provides a platform for investigating intertwined roles of charge/spin orders and electric transport in unconventional superconductivity. However, important quantitative information on the transport, such as the absolute value of the resistivity, the anisotropy, and the scattering rate of carriers, remains insufficient due to the lack of accurate measurements using large single crystals. Here we establish a high-precision pressure-temperature phase diagram of high-quality La3Ni2O7 single crystals, by measuring the in-plane and out-of-plane resistivities. We resolve two distinct anomalies associated with density-wave formation with contrasting pressure dependences. The pressure-induced structural transition enhances not only the resistivity values for both directions, but also its anisotropy at low temperatures, demonstrating a pronounced effect of density-wave order on the charge dynamics. Superconductivity with zero-resistance emerges near the boundary where the density-wave phases are fully suppressed, and above Tc, the resistivity exhibits a temperature-linear dependence over a wide temperature range while the scattering rate falls within a regime of the Planckian limit. Our results show that pressure dramatically changes the anisotropic charge transport via modifying density-wave orders, and eventually produces a pronounced strange-metal state with strong scatterings, from which superconductivity develops. This establishes robust density-wave correlations and Planckian dissipation as remarkable features of La3Ni2O7.

cond-mat.supr-con↗

arXiv2026

Dominant in-plane anomalous Hall effect in a monoclinic room-temperature ferromagnet

Ferromagnetic metals are characterized by enhanced dissipationless transverse transport responses via the anomalous Hall effect, offering a route towards magnetic sensing and spintronic readout functionalities. In most ferromagnets, the anomalous Hall current is constrained to lie in the plane perpendicular to the magnetization (or applied magnetic field). Recently, it has been recognized that selected symmetries can also permit a Hall response in a traditionally forbidden configuration, where the Hall current lies in the same plane as the magnetization, realizing an in-plane anomalous Hall effect. Reported realizations of this effect, however, are typically much weaker than the conventional Hall response in the same material. Here, through engineering specific crystallographic mirror symmetry-breaking, we realize a strongly enhanced in-plane anomalous Hall response in monoclinic Cr3Te4 with room-temperature ferromagnetism. Remarkably, the in-plane anomalous Hall signal exceeds the out-of-plane response by a factor of five, with which we demonstrate a unique in-plane field and current sensing functionality. Combined with density functional theory calculations, our results establish low-crystalline-symmetry ferromagnets with near-Fermi-level Weyl points as a practical platform for symmetry-engineered Hall responses, and point to a route towards room-temperature, geometry-flexible sensing devices.

cond-mat.mtrl-sci↗

arXiv2026

Magnetoelastic mapping of the high-field phase diagram in the topological cubic helimagnet SrFeO$_{3}$

The cubic perovskite SrFeO$_{3}$ is a prototypical centrosymmetric itinerant magnet that hosts a quadruple-${\mathbf Q}$ hedgehog-antihedgehog lattice and exhibits a complex magnetic-field-temperature phase diagram. Yet, the microscopic mechanism underlying the emergence of its versatile multiple-${\mathbf Q}$ phases remains unresolved. Here, we reveal the field-orientation dependence of the magnetic phase diagram and establish an effective spin Hamiltonian for SrFeO$_{3}$ that incorporates a cubic single-ion anisotropy together with bilinear and biquadratic interactions in momentum space, which originate from the spin-charge coupling. In addition, we observe magnetoelastic signatures of a redistribution of the ligand-hole density upon entering the forced ferromagnetic phase. These findings emphasize the pivotal importance of electronic itinerancy arising from the formation of a ligand-hole band in stabilizing multiple-${\mathbf Q}$ phases.

cond-mat.str-el↗

arXiv2026

Experimental quantification of electronic symmetry breaking through orbital hybridization phase

Symmetry classification of crystal structures has been central to predicting physical properties of materials. While such structural classification identifies which physical responses are symmetry-allowed, the magnitudes of these responses are governed by the degree of symmetry breaking in the electronic state. However, a well-defined quantitative descriptor for the electronic symmetry breaking has been established only in limited cases such as electric polarization and magnetization. No analogous descriptor exists for most other types, including chirality. Here, we propose an experimental framework for quantifying electronic symmetry breaking from the anisotropy of valence electron density distribution. We show that the orbital hybridization phases governing this anisotropy can be uniquely determined under site symmetry constraints. Applying this framework to structurally chiral transition-metal silicides, we determine hybridization phases from their valence electron densities observed by synchrotron X-ray diffraction. From the obtained complex hybridization, we quantify an electronic chirality $\chi$ and theoretically demonstrate that it is directly proportional to circular dichroism, establishing $\chi$ as a predictive descriptor of chiral responses. This approach is systematically applicable to various point groups, offering a general route to quantifying electronic symmetry breaking and predicting associated physical properties.

cond-mat.str-el↗

arXiv2026

Real-space determination of orbital states driving successive phase transitions in FeV2O4

Direct experimental access to orbital states in strongly correlated materials remains a major challenge, despite their central role in driving coupled structural and magnetic phase transitions. In systems where electronic correlations, electron-lattice coupling, and relativistic spin-orbit interactions compete on comparable energy scales, even first-principles calculations often yield multiple metastable solutions, hindering the unambiguous identification of the ground state. Here, we demonstrate that the orbital states of the spinel oxide FeV2O4, which possesses active orbital degrees of freedom on both Fe and V ions, are uniquely resolved by combining valence electron density (VED) analysis based on state-of-the-art synchrotron x-ray diffraction with spin-polarized density-functional-theory calculations. Our results reveal that temperature-dependent rearrangements of orbital occupations drive successive structural transitions that accompany collinear and noncoplanar ferrimagnetic orders, establishing a direct correspondence between orbital anisotropy and spin structure. More broadly, this work shows that experimentally determined VED provides a decisive real-space constraint on competing theoretical solutions, offering a powerful and broadly applicable framework for elucidating the microscopic mechanisms of complex phase transitions in strongly correlated electron systems.

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

Magnetoelectric Control of Toroidal Moment in Ferroaxial Crystal PbMn$_{2}$Ni$_{6}$Te$_{3}$O$_{18}$

Ferroic multipole orders break some symmetry and often activate unique physical properties, such as nonreciprocal directional dichroism and linear magnetoelectric effects. Here, we demonstrate the control of magnetic toroidal orientation in PbMn$_{2}$Ni$_{6}$Te$_{3}$O$_{18}$ through the application of electric and magnetic fields in two distinct configurations. Through directional dichroism, we successfully visualize magnetic ferrotoroidic domains, establishing the intrinsic coupling among magnetic toroidal moment, crystallographic ferroaxial moment, and magnetoelectric monopoles. Our findings not only present an effective pathway for controlling magnetic toroidal moment but also provide a novel approach for investigating ferroaxial ordering.

cond-mat.str-el↗

arXiv2026

Polar, checkerboard charge order in bilayer nickelate La3Ni2O7

Competing charge and spin orders are central to uncovering the nature of unconventional superconductivity. Here we utilize synchrotron X-ray diffraction on a high-quality single crystal to reveal the charge order of La$_3$Ni$_2$O$_7$ at ambient pressure, which competes with the high-temperature superconducting phase under pressure. Enabled by the high synchrotron photon flux and a large dynamic range, we resolve faint reflections -- nearly four orders of magnitude weaker than the main Bragg reflections -- that were overlooked in prior diffraction studies. This observation evidences a broken glide-mirror symmetry, leading to a polar crystal structure, rather than the widely used centrosymmetric structure model. The polarity is induced by checkerboard charge order on nickel sites in combination with octahedral tilting, reminiscent of bilayer manganese oxides. Our results provide a foundation for understanding phase competition and the mechanism of pressure-induced superconductivity in bilayer nickelates.

cond-mat.supr-con↗

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 moir\'e 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↗

arXiv2026

Molecular Orbital Degeneracy Lifting in a Tetrahedral Cluster System NbSeI

The lifting of degenerate electronic states, in which multiple electronic states share the same energy, is a fundamental issue in the physics of crystalline solids. In real materials, this problem has been extensively studied in transition metal compounds, where various quantum phenomena arise from the spin and orbital degeneracy of the d electrons on individual transition-metal atoms. In contrast, materials containing high-symmetry clusters composed of multiple transition-metal atoms are expected to exhibit more emergent phenomena due to the entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct mechanisms of orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose average crystal structure is predicted to host a flat-band metal. Below 106 K, NbSeI is found to be a nonmagnetic molecular orbital-ordered insulator. Above this temperature, the average structure becomes face-centered cubic without any superlattice, while the orbital degeneracy remains lifted by significant local distortions of Nb4 tetrahedra, which may be associated with a molecular orbital-liquid or orbital-frozen state. This noncooperative Jahn-Teller distortion stabilizes a nonmagnetic insulating state above 106 K, in stark contrast to the flat-band metal predicted from the average structure.

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

A rutile-based homologous series Na(PtO$_2$)$_{2\it{n}+1}$ discovered by computationally assisted high-pressure synthesis

Layered transition metal oxides typified by the Ruddlesden-Popper phase have been extensively studied for its applications in high-temperature superconductivity, catalysis, and battery technologies. Despite the remarkable structural diversity and catalytic functionality of platinum oxides, the exploration of layered polymorphs has remained significantly constrained mainly due to the high inertness of platinum. Here, we discover a new homologous series of layered ternary oxides, Na(PtO$_2$)$_{2\it{n}+1}$, by a combination of highly oxidizing high-pressure methods and density functional theory (DFT) calculations. This series features unprecedented layered structural motifs, rutile-based PtO$_6$ octahedra and one-dimensional PtO$_4$ square-planar columns, which enables systematic control of dimensionality. Furthermore, we demonstrate a computationally-assisted identification of isomeric and putative members of this homologous series as confirmed by controlled synthesis and quantitative analysis of diffuse scattering data. This approach provides an effective platform for the exhaustive exploration of metastable transition metal oxides with rich structural variations.

cond-mat.mtrl-sci↗

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↗