arXiv subjects

Kunihiko Yamauchi

Publications and source records attributed to Kunihiko Yamauchi.

At least 19 recordsLinked to original sources

arXiv2026

Substitution effects in RuO$_2$ single crystals

RuO$_2$ has emerged as one of the leading candidates for investigating altermagnetism. Recent quantum oscillation and angle-resolved photoemission spectroscopy measurements found the absence of altermagnetism or antiferromagnetism in pure RuO$_2$ crystals. The continuing debate over intrinsic magnetic order in RuO$_2$ may reflect its proximity to an altermagnetic instability. In this work, we grow single crystals of Ru$_{0.9}$V$_{0.1}$O$_2$ and investigate their structural, transport, and magnetic properties. X-ray photoelectron spectroscopy reveals an average V oxidation state near $+4$. The paramagnetic susceptibility remains nearly unchanged up to room temperature, with no evidence of magnetic ordering. Thus, the 10\% V-substitution in RuO$_2$ does not induce altermagnetism. Electronic structure calculations for the V-substituted systems using two methods suggest that a higher level of V-substitution leads to a significant change in the density of states. These findings underscore the potential of nonmagnetic substitution in RuO$_2$ as an attractive candidate for probing altermagnetic transitions and their experimental signatures.

cond-mat.mtrl-sci↗

arXiv2026

Two Microscopic Mechanisms of Piezomagnetism in CoF$_2$ from First-Principles Calculations

Rutile-structured CoF$_2$ has long been recognized as a prototypical piezomagnetic material. Recently, it has attracted renewed interest as an altermagnet, exhibiting spin-split electronic bands even in the absence of spin-orbit coupling. Although the piezomagnetic response of CoF$_2$ has been extensively discussed from the viewpoint of magnetic symmetry, its microscopic origin has remained elusive. First-principles calculations reveal two distinct microscopic mechanisms of piezomagnetism in CoF$_2$. Under $xy$ shear strain, the local volumes of the CoF$_6$ octahedra surrounding the two Co sites become different, leading to unequal magnetic moments on the two sublattices and hence a net magnetization. In contrast, under $yz$ shear strain, the piezomagnetic response originates from spin canting induced by the Dzyaloshinskii--Moriya interaction through spin-orbit coupling. The presence of two distinct microscopic mechanisms may be a general feature of piezomagnetic antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2026

Interplay of Magnetic Order, Structural Stability, and Orbital Ordering in BaFe2X3 (X = S and Se)

BaFe2X3 (X= S and Se) are quasi-one-dimensional Mott insulators with a ladder structure that exhibit Stripe- and Block-type antiferromagnetic order, respectively. The ladder arrangement of Fe atoms and strong electron correlations give rise to rich magnetic and orbital phenomena, including pressure-induced superconductivity and orbital-selective electronic states. Both compounds also show resistivity anomalies above the Neel temperature, suggesting that orbital degrees of freedom play an important role in their electronic properties. To clarify the interplay among magnetic order, structural stability, orbital ordering, and transport properties, we performed resistivity measurements and first-principles calculations for BaFe2S3 and BaFe2Se3, systematically comparing candidate magnetic and crystal structures. We find that the magnetic configuration strongly influences the stable crystal structure, orbital ordering, and transport anisotropy, providing a microscopic understanding of the contrasting electronic properties of the two compounds.

cond-mat.str-el↗

arXiv2026

Fermi-liquid behavior and characteristic temperature-dependent susceptibility in clean RuO$_2$ crystal

The magnetic nature of the altermagnet candidate RuO$_2$ remains under debate. It has been recently shown from quantum oscillations and angle-resolved photoemission spectroscopy (ARPES) that the high-quality RuO$_2$ bulk single crystal is a paramagnetic metal. Here we report the specific heat and magnetic susceptibility in ultra-clean RuO$_2$ single crystals with residual resistivity ratio up to 1200. The magnetic susceptibility increases with temperature and is phenomenologically fitted with an inclusion of $T\textrm{ln}(T/T_0)$ over a wide temperature range up to 400 K. In contrast, the energy dependence of the density of states and thermal activation of quasiparticles lead to a decrease with temperature. Such characteristic temperature dependence, similar to that observed in other $d$-electron metals, is attributable to an enhanced orbital contribution arising from lattice-expansion-induced changes in the band structure. The electronic specific heat, the magnetic susceptibility, and the $T^2$ coefficient in resistivity point to a weakly-correlated 3D Fermi-liquid state with a modest electron correlation, as supported by the Wilson and Kadowaki-Woods ratios.

cond-mat.mtrl-sci↗

arXiv2026

Phonon-driven tuning of exchange interactions in Y3Fe5O12

Yttrium iron garnet (Y3Fe5O12) is a prototypical ferrimagnetic insulator widely used in spin-wave and magnonic devices owing to its extremely low magnetic damping and long magnon propagation length, and recent experiments suggest that lattice vibrations can influence magnetic properties, motivating a microscopic understanding of how phonons modify exchange interactions. In this work, phonon-driven tuning of exchange interactions in Y3Fe5O12 is investigated from a mode-resolved perspective based on first-principles calculations. We focus on how optical phonons modify the dominant superexchange pathways and how lattice distortions affect the Fe-O-Fe bond geometry that governs the exchange interaction. To this end, phonon modes are computed from density functional theory, and the exchange interactions are evaluated from a Wannier-based tight-binding model and mapped onto a spin Hamiltonian, while displaced structures along individual infrared-active modes are used to quantify their impact on the magnetic interactions.

cond-mat.mtrl-sci↗

arXiv2026

Quantum anomalous Hall effect in monolayer transition-metal trihalides

We present systematic first-principles results for the electronic and magnetic properties of two-dimensional transition-metal trihalide monolayers MX3 (M = V, Cr, Mn, Fe, Ni, Pd; X = F, Cl, Br, I), focusing on their potential to host the quantum anomalous Hall effect. In particular, MnF3 and PdF3 exhibit a spin-polarized Dirac cone at the K point, spin-orbit coupling opens a sizable gap with a nonzero Chern number. Nanoribbon slab calculations reveal gap-crossing chiral edge states, establishing the nontrivial topological character. Beyond these case studies, our systematic screening clarifies general trends across the MX3 family and provides insight into how electronic configuration and spin-orbit coupling cooperate to produce magnetic and topological phases in two-dimensional magnets.

cond-mat.mtrl-sci↗

arXiv2024

Universal Role of Combined Symmetry for the Protection of the Dirac Cone in Antiferromagnetic Topological Insulators

Antiferromagnetic topological insulators (AF TIs) are predicted to exhibit exotic physical properties such as gigantic optical and topological magnetoelectric responses. While a key to achieving such phenomena relies on how to break the symmetry protecting the Dirac-cone surface state (SS) and acquire the mass of Dirac fermions, the mechanism has yet to be clarified. To address this issue, we carried out micro-focused angle-resolved photoemission spectroscopy for GdBi hosting the type-II AF order, and uncovered the stripe-type 2$\times$1 reconstruction of the Fermi surface associated with the AF band folding. Intriguingly, in contrast to NdBi with the type-I AF order displaying the surface-selective Dirac-fermion mass, GdBi shows massless behavior irrespective of AF domains due to the robust topological protection. These results strongly suggest a crucial role of the ThetaTD (time-reversal and translational) symmetry to create the Dirac-fermion mass in AF TIs.

cond-mat.mes-hall↗

arXiv2024

Dislocation-induced flexoelectricity in SrTiO$_3$ nanostructure from first principles

Flexoelectricity refers to a linear coupling between the electric polarization and the strain gradient, such as bending or asymmetric compression. This effect is enhanced in nano-scale structures, where grain boundaries or dislocation cores induce the strain gradient. In this study, we theoretically investigate the flexoelectric polarization induced by misfit dislocations in a thin film. A nano-scale dislocation structure is modeled in a periodic SrTiO$_3$ supercell, and then the structure is optimized by using neural-network-potential and first-principles approaches. We point out that a pyramidal TiO$_5$ coordination forms near the dislocation cores, which in turn dominantly causes the sizable flexoelectric polarization.

cond-mat.mtrl-sci↗

arXiv2023

Goodenough-Kanamori-Anderson rules in 2D magnet: A chemical trend in MCl2 with M=V, Mn, and Ni

Density-functional-theory calculations were performed to investigate the magnetism in a series of triangular-lattice monolayer MCl2 (M=V, Mn, and Ni). The magnetic stability manifests a distinct chemical trend; VCl2 and MnCl2 show the antiferromagnetic ground states and NiCl2 shows the ferromagnetic ground state. The microscopic mechanism behind the magnetic interaction is explained by the so-called Goodenough-Kanamori-Anderson rules and by the virtual-hopping process through the hopping integrals between the 3d-orbital maximally localized Wannier functions. Our result highlights the role of the direct exchange interaction and the superexchange interaction in the magnetic stabilization in two-dimensional magnets.

cond-mat.mtrl-sci↗

arXiv2023

Ab initio prediction of anomalous Hall effect in antiferromagnetic CaCrO$_3$

While the anomalous Hall effect takes place typically in ferromagnets with finite magnetization, large anomalous Hall conductivity in noncollinear antiferromagnetic systems has been recently observed and attracted much attention. In this study, we predict the anomalous Hall effect in perovskite CaCrO$_3$ as a representative of 'collinear' antiferromagnetic materials. Our result shows that the C-type antiferromagnetic ordering generates the sizable anomalous Hall conductivity. Based on symmetry analyses, we show that the antiferromagnetic order parameter belongs to the same irreducible representation as the ferromagnetic order parameter in the nonsymmorphic space group, allowing the non-vanishing Berry curvatures in k space. By performing first-principles density-functional theory calculations, we find that the Berry-curvature 'hot spots' lie along the gapped nodal lines where spin-orbit coupling induces the spin splitting of Cr-3d bands near the Fermi energy and enhances the anomalous Hall effect in CaCrO$_3$.

cond-mat.str-el↗

arXiv2022

Hydrogen-Induced Metal-Insulator Transition Accompanied by Inter-Layer Charge Ordering in SmNiO$_3$

The microscopic mechanism of the hydrogen-induced metal-insulator transition in SmNiO$_3$ is clarified by means of density-functional theory with the Hubbard U correction. While 100% of hydrogen doping per Ni atom has been supposed to be responsible for the metal-insulator transition, we found that 50% of hydrogen doping results in an outstandingly stable atomic structure showing the insulating property. The stable crystal structure shows the peculiar layered pattern of charge disproportionation of Ni$^{2+}$ and Ni$^{3+}$ valences together with the strong Jahn-Teller distortion that causes the eg orbital state splitting and opens the band gap.

cond-mat.str-el↗

arXiv2022

Microscopic origin of magnetism in monolayer $3d$ transition metal dihalides

Motivated by the recent wealth of exotic magnetic phases emerging in two-dimensional frustrated lattices, we investigate the origin of possible magnetism in the monolayer family of triangular lattice materials $MX_2$ ($M$={V, Mn, Ni}, $X$={Cl, Br, I}). We first show that consideration of general properties such as filling and hybridization enables to formulate trends for the most relevant magnetic interaction parameters. In particular, we observe that the effects of spin-orbit coupling (SOC) can be effectively tuned through the ligand elements as the considered 3$d$ transition metal ions do not strongly contribute to the anisotropic component of the inter-site exchange interaction. Consequently, we find that the corresponding SOC matrix-elements differ significantly from the atomic limit. In a next step and by using two complementary approaches based on first principles, we extract realistic effective spin models and find that in the case of heavy ligand elements, SOC effects manifest in anisotropic exchange and single-ion anisotropy only for specific fillings.

cond-mat.str-el↗

arXiv2021

First-Principles Study on Cathode Properties of Li2MTiO4 and Na2MTiO4 (M = V, Cr, Mn, Fe, Co, Ni)

The cathode properties of Na2MTiO4 (M: transition-metal element) are investigated by means of density-functional-theory calculations. The stability between the layered structure and the disordered structure are focused in comparison with the Li2MTiO4 prototypical case. It is found that the layered structure is more stable than the disordered structure in Na2MTiO4 while those structure shows the similar stability in Li2MTiO4. In layered-structure Na2MTiO4, the formation enthalpies at the intermediate compounds during charge/discharge reactions are significantly low, leading to the unstable voltage-capacity profiles. A machine-learning analysis reveals that the total-energy difference between these structures can be described by a simple function of ionic radii.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic properties of bilayer VI3: Role of trigonal crystal field and electric-field tuning

The magnetic properties of two-dimensional VI3 bilayer are the focus of our first-principles analysis, highlighting the role of trigonal crystal-field effects and carried out in comparison with the CrI3 prototypical case, where the effects are absent. In VI3 bilayers, the empty a1g state - consistent with the observed trigonal distortion - is found to play a crucial role in both stabilizing the insulating state and in determining the inter-layer magnetic interaction. Indeed, an analysis based on maximally localized Wannier functions allows to evaluate the interlayer exchange interactions in two different VI3 stackings (labelled AB and AB'), to interpret the results in terms of virtual-hopping mechanism, and to highlight the strongest hopping channels underlying the magnetic interlayer coupling. Upon application of electric fields perpendicular to the slab, we find that the magnetic ground-state in the AB' stacking can be switched from antiferromagnetic to ferromagnetic, suggesting VI3 bilayer as an appealing candidate for electric-field-driven miniaturized spintronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Modulation of Dirac electrons in epitaxial Bi2Se3 ultrathin films on van-der-Waals ferromagnet Cr2Si2Te6

We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi2Se3 was epitaxially grown on a van-der-Waals ferromagnet Cr2Si2Te6 (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in 6 quintuple-layer (QL) Bi2Se3 on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in 2QL counterpart. Intriguingly, this gap is much larger than those for Bi2Se3 films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small characteristic length and energy scale of the magnetic proximity effect. The present results suggest a crucial role of interfacial coupling for modulating Dirac electrons in topological-insulator hybrids.

cond-mat.mtrl-sci↗

arXiv2020

Unusual temperature evolution of band structure of Bi(111) studied by angle-resolved photoemission spectroscopy and density functional theory

We have performed angle-resolved photoemission spectroscopy of Bi(111) thin films grown on Si(111), and investigated the evolution of band structure with temperature. We revealed an unexpectedly large temperature variation of the energy dispersion for the Rashba-split surface state and the quantum-well states, as seen in the highly momentum-dependent energy shift as large as 0.1 eV. A comparison of the band dispersion between experiment and first-principles band-structure calculations suggests that the interlayer spacing at the topmost Bi bilayer expands upon temperature increase. The present study provides a new pathway for investigating the interplay between lattice and electronic states through the temperature dependence of band structure.

cond-mat.mes-hall↗

arXiv2020

Origin of magnetovolume effect in a cobaltite

The layered perovskite PrBaCo2O5.5+x demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we report with compelling evidences that the continuous-like MVE in PrBaCo2O5.5+x is intrinsically of discontinuous character, originating from an magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic metallic small-volume (FMSV) phase. Furthermore, the magnetoelectric effect (ME) shows high sensitivity to multiple external stimuli such as temperature, carrier doping, hydrostatic pressure, magnetic field etc. In contrast to the well-known ME such as colossal magnetoresistance and multiferroic effect which involve symmetry breaking of crystal structure, the ME in the cobaltite is purely isostructural. Our discovery provides a new pathway to realizing the ME as well as the NTE, which may find applications in new techniques.

cond-mat.str-el↗

arXiv2020

First-principles Study on Piezoelectricity and Spontaneous Polarization in Bi(Fe,Co)O3

Solid solution BiFe1-xCoxO3 shows anti-ferromagnetic order and pyroelectric order, simultaneously. It has been known that BiFe1-xCoxO3 exhibits a structural phase transition between monoclinic and tetragonal phases as x increases. This kinds of transition is often called morphotoropic phase boundary, which is well known to take place in a representative piezoelectric oxide, PbZr1-xTixO3. In order to theoretically understand the piezoelectric property in BiFe1-xCoxO3, we performed ab-initio electronic-structure calculations and studied the structural stability, the magnetic property, and the electronic polarization by means of super-cell approach. It turns out that the large electric polarization and the particular pyramidal coordination suppress the response of the electric polarization under strain. A way to enhance the piezoelectric effect in BiFe1-xCoxO3 is proposed.

cond-mat.mtrl-sci↗