arXiv subjects

Hideki Matsuoka

Publications and source records attributed to Hideki Matsuoka.

13 recordsLinked to original sources

arXiv2026

Carrier-tunable RKKY magnetism in a crystalline magnet

In itinerant magnets governed by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the exchange coupling depends on both the moment-moment distance $r$ and the Fermi wavevector $k_{\mathrm{F}}$, yet in bulk synthesis the two are tightly coupled: a change in composition typically alters both. Here, we use a thin-film approach to tune these two variables independently in a single crystalline host. Using molecular-beam epitaxy (MBE), we stabilize either $2\times2\mathrm{R}0^\circ$ Cr$_{1/4}$NbSe$_2$ or $\sqrt{3}\times\sqrt{3}\mathrm{R}30^\circ$ Cr$_{1/3}$NbSe$_2$ within the same NbSe$_2$ host through separate growth windows. Controlled post-growth annealing performed across a series of temperatures then modifies the carrier density while leaving the Cr superstructure intact below a structural-transition threshold. The two as-grown phases are distinct in electronic structure, magnetic ground state, and transport. Along this annealing series, the Hall response evolves systematically while the magnetic response changes in a structurally insensitive manner, with ferromagnetic order emerging within the same $\sqrt{3}\times\sqrt{3}\mathrm{R}30^\circ$ structural class only above a critical annealing temperature, experimentally disentangling carrier density and moment geometry. The Hall magnitude and sign evolution point to a low-carrier-density system in which $k_{\mathrm{F}}$ is susceptible to modest external tuning. Cr-NbSe$_2$ thus realizes carrier-sensitive RKKY magnetism in a single crystalline host, within an MBE-plus-annealing approach extensible across the intercalated transition-metal dichalcogenide family.

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

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

Electric-field control of two-dimensional ferromagnetic properties by chiral ionic gating

Chiral molecular systems offer unique pathways to control spin and magnetism beyond conventional symmetry operations. Here, we demonstrate that chiral ionic liquids enable electric-field modulation of two-dimensional (2D) ferromagnetism in FeSi(111) thin films via electric double-layer transistor (EDLT) gating. FeSi hosts chemically-stable, surface-confined ferromagnetism without bulk moments, making the interfacial spins highly responsive to chiral-ion adsorption. Using both achiral and chiral ionic liquids, we systematically compare electrochemical and electrostatic gating effects. While both gating modes modulate magnetic properties such as anomalous Hall conductivity and coercive field, only chiral ionic gating biases the ratio of up- and down-magnetized domains in a handedness-dependent manner, evidencing chirality-induced symmetry breaking. This work establishes chiral ion gating as a novel strategy for controlling magnetic order and opens new directions for chiral spintronics.

cond-mat.mes-hall↗

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

Misfit layered superconductor (PbSe)1.14(NbSe2)3 with possible layer-selective FFLO state

Two-dimensional (2D) superconductors are known for their novel emergent phenomena, however, lack of experimental probes beyond resistivity has hindered further exploration of diverse superconducting states. Bulk 2D superconductors, with superconducting layers separated by non-superconducting layers, offer a unique opportunity to break this limit. Here, we synthesized a single crystal of misfit layered compound (PbSe)1.14(NbSe2)3, composed of alternately stacked tri-layer NbSe2 and non-superconducting block layers with incompatible unit cells. Due to its unique structure, 2D Ising superconductivity is maintained even in a bulk form. Resistivity and tunnel diode oscillator measurements reveal two distinct superconducting phases in magnetic field vs. temperature phase diagram. Combined with the theoretical analysis, the high-magnetic-field phase is identified as a layer-selective Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, where Ising and finite-q superconductivity are mixed due to the tri-layer structure. Bulk 2D superconductors with misfit structure offer a novel opportunity for understanding of 2D superconductivity through bulk measurements and interlayer engineering.

cond-mat.supr-con↗

arXiv2023

Electron-doped magnetic Weyl semimetal LixCo3Sn2S2 by bulk-gating

Manipulating carrier density through gate effects, both in electrostatic charge storage and electrochemical intercalation mode, offers powerful control over material properties, although commonly restricted to ultra-thin films or van der Waals materials. Here we demonstrate the application of gate-driven carrier modulation in the microdevice of magnetic Weyl semimetal Co3Sn2S2, fabricated from a bulk single crystal via focused ion beam (FIB). We discovered a Li-intercalated phase LixCo3Sn2S2 featuring electron doping exceeding 5*1021 cm-3, resulting in the Fermi energy shift of 200 meV. The carrier density dependent anomalous Hall conductivity shows fair agreement with density functional theory (DFT) calculation, which also predicts intercalated Li+ ion stabilization within the anion layer while maintaining the kagome-lattice intact. This likely explains the observed rigid band behavior and constant Curie temperature, contrasting with magnetic site substitution experiments. Our findings suggest ionic gating on FIB devices broadens the scope of gate-tuning in quantum materials.

cond-mat.mtrl-sci↗

arXiv2023

Gate-tunable ferromagnetism in a van der Waals magnetic semimetal

Magnetic semimetals form an attractive class of materials because of the non-trivial contributions of itinerant electrons to magnetism. Due to their relatively low-carrier-density nature, a doping level of those materials could be largely tuned by a gating technique. Here we demonstrate gate-tunable ferromagnetism in an emergent van der Waals magnetic semimetal Cr3Te4 based on an ion-gating technique. Upon doping electrons into the system, the Curie temperature (TC) sharply increases, approaching near to room temperature, then decreases to some extent. Interestingly, this non-monotonous variation of TC accompanies the switching of the magnetic anisotropy. Furthermore, such evolutions of TC and anisotropy occur synchronously with the sigh changes of the ordinary and anomalous Hall effects. Those results clearly elucidate that the magnetism in Cr3Te4 should be governed by its semimetallic band nature, where the band crossing points play a crucial role both for the magneto-transport properties and magnetism itself.

cond-mat.mtrl-sci↗

arXiv2022

Signature of topological band crossing in ferromagnetic Cr1/3NbSe2 epitaxial thin film

In intercalated transition metal dichalcogenides (I-TMDC), transition metal intercalation introduces magnetic phases which in some cases induce topological band crossing. However, evidence of the topological properties remains elusive in such materials. Here we employ angle-resolved photoemission spectroscopy to reveal the band structure of epitaxially grown ferromagnetic Cr1/3NbSe2. Experimental evidence of the Weyl crossing shows Cr1/3NbSe2 to be a topological ferromagnet. This work highlights I-TMDC as platform towards the interplay of magnetic and topological physics in low-dimensional systems.

cond-mat.mtrl-sci↗

arXiv2022

Spontaneous spin-valley polarization in NbSe2 at a van der Waals interface

A proximity effect at a van der Waals (vdW) interface enables creation of an emergent quantum electronic ground state. Here we demonstrate that an originally-superconducting two-dimensional (2D) NbSe2 forms a ferromagnetic ground state with spontaneous spin polarization at a vdW interface with a 2D ferromagnet V5Se8. We investigated the anomalous Hall effect (AHE) of the NbSe2/V5Se8 magnetic vdW heterostructures, and found that the sign of the AHE was reversed as the number of the V5Se8 layer was thinned down to the monolayer limit. Interestingly, the AHE signal of those samples was enhanced with the in-plane magnetic fields, suggesting an additional contribution to the AHE signal other than magnetization. This unusual behavior is well reproduced by band structure calculations, where the emergence of the Berry curvature along the spin-degenerate nodal lines in 2D NbSe2 by the in-plane magnetization plays a key role, unveiling a unique interplay between magnetism and Zeeman-type spin-orbit interaction in a non-centrosymmetric 2D quantum material.

cond-mat.mes-hall↗

arXiv2019

Intrinsic 2D Ferromagnetism in V5Se8 Epitaxial Thin Films

The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy (MBE) exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is 'originally antiferromagnetic (AFM)'. Moreover, thickness-dependence measurements reveal that this newly-developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

cond-mat.mtrl-sci↗

arXiv2018

Signatures of charge-order correlations in transport properties of electron-doped cuprate superconductors

The high-temperature superconductivity in copper oxides emerges under strong influence of spin correlations in doped Mott insulators. Recent discoveries of charge-order (CO) correlations in Y-based hole-doped cuprates as well as in electron-doped cuprates suggest that charge correlations should also play an important role on the electronic states of cuprates, although those correlations have been so far detected mainly by x-ray scattering measurements. Here we show signatures of CO correlations in transport properties of electron-doped cuprates as anomalous enhancement of the metal-to-insulator crossover temperature (Tmin) appears in the limited doping range near the onset of superconductivity, while it decreases exactly when superconductivity sets in. We explain this non-monotonous peak-like behavior of Tmin in terms of the evolution of the electronic states through development of CO correlations and appearance of the hole pockets in the folded Fermi surface, which impact on transport properties consecutively at different locations in the momentum space.

cond-mat.str-el↗

arXiv2017

Layer-by-layer epitaxial growth of scalable WSe2 on sapphire by molecular-beam epitaxy

Molecular-beam epitaxy (MBE) provides a simple but powerful way to synthesize large-area high-quality thin films and heterostructures of a wide variety of materials including accomplished group III-V and II-VI semiconductors as well as newly-developing oxides and chalcogenides, leading to major discoveries in condensed-matter physics. For two-dimensional (2D) materials, however, main fabrication routes have been mechanical exfoliation and chemical-vapor deposition by making good use of weak van der Waals bonding nature between neighboring layers, and MBE growth of 2D materials, in particular on insulating substrates for transport measurements, has been limited despite its fundamental importance for future advanced research. Here we report layer-by-layer epitaxial growth of scalable transition-metal dichalocogenide (TMDC) thin films on insulating substrates by MBE, and demonstrate ambipolar transistor operation. The proposed growth protocol is broadly applicable to other TMDC, providing a key milestone toward fabrication of van der Waals heterostructures with various 2D materials for novel properties and functionalities.

cond-mat.mtrl-sci↗