arXiv subjects

Tamio Oguchi

Publications and source records attributed to Tamio Oguchi.

At least 19 recordsLinked to original sources

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

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↗

arXiv2024

Topological Semimetal KAlGe with Novel Electronic Instability

Compounds with the anti-PbFCl structure exhibit a variety of electronic instabilities and intriguing physical properties. NaAlSi and NaAlGe are similar topological nodal-line semimetals, but they have distinct properties. NaAlSi is a superconductor at 6.8 K, whereas NaAlGe is an insulator with a pseudogap of approximately 100 K. Using the potassium-indium flux method, we succeeded in synthesizing a single crystal of KAlGe, a new anti-PbFCl compound. First principles electronic structure calculations reveal that KAlGe is isoelectronic with NaAlSi and NaAlGe. KAlGe undergoes a metal-to-metal transition at 89 K and exhibits no superconductivity above 1.8 K. The low temperature phase has significantly lower carrier density and extremely high mobility, similar to Dirac electron systems. Furthermore, X-ray diffraction experiments show a structural change that breaks the fourfold symmetry during the phase transition. Electron-phonon interactions may be responsible for superconductivity in NaAlSi, whereas excitonic electron-hole interactions are thought to play an important role in KAlGe and possibly NaAlGe. Our findings demonstrate that fascinating physics lies within the compound family.

cond-mat.str-el↗

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↗

arXiv2024

Accelerating crystal structure search through active learning with neural networks for rapid relaxations

Global optimization of crystal compositions is a significant yet computationally intensive method to identify stable structures within chemical space. The specific physical properties linked to a three-dimensional atomic arrangement make this an essential task in the development of new materials. We present a method that efficiently uses active learning of neural network force fields for structure relaxation, minimizing the required number of steps in the process. This is achieved by neural network force fields equipped with uncertainty estimation, which iteratively guide a pool of randomly generated candidates towards their respective local minima. Using this approach, we are able to effectively identify the most promising candidates for further evaluation using density functional theory (DFT). Our method not only reliably reduces computational costs by up to two orders of magnitude across the benchmark systems Si16 , Na8Cl8 , Ga8As8 and Al4O6 , but also excels in finding the most stable minimum for the unseen, more complex systems Si46 and Al16O24 . Moreover, we demonstrate at the example of Si16 that our method can find multiple relevant local minima while only adding minor computational effort.

physics.comp-ph↗

arXiv2023

Fermi surface and light quasi particles in hourglass nodal chain metal \b{eta}-ReO2

Quantum oscillations in magnetic torque and electrical resistivity were measured to investigate the electronic structure of \b{eta}-ReO2, a candidate hourglass nodal chain metal (Dirac loop chain metal). All the de Haas-van Alphen oscillation branches measured at 30 mK in magnetic fields of up to 17.5 T were consistent with first-principles calculations predicting four Fermi surfaces (FSs). The small-electron FS of the four FSs exhibited a very small cyclotron mass, 0.059 times that of the free electrons, which is likely to be related to the linear dispersion of the energy band. The consistency between the quantum oscillation results and band calculations indicates the presence of the hourglass nodal chain predicted for \b{eta}-ReO2 in the vicinity of the Fermi energy.

cond-mat.str-el↗

arXiv2022

Pseudogap Formation in the Nodal-Line Semimetal NaAlGe

NaAlSi and NaAlGe are isostructural and isoelectronic semimetals with topological nodal lines close to the Fermi level. Despite having virtually identical electronic structures, NaAlSi exhibits superconductivity below Tc = 6.8 K, whereas NaAlGe does not. We investigate NaAlGe by measuring its electrical resistivity, Hall effect, magnetic susceptibility, and heat capacity using single crystals. It is revealed that NaAlGe is not a simple semimetal but rather has an unusual ground state with a small pseudogap of approximately 100 K close to the Fermi level. We argue that the formation of the pseudogap in NaAlGe is due to an unexpected Fermi surface instability, such as an excitonic instability, as opposed to the electron-phonon instability that leads to the formation of the superconducting gap in NaAlSi.

cond-mat.mtrl-sci↗

arXiv2022

Electronic Origin of Phase Stability in Mg-Zn-Y Alloys with a Long-Period Stacking Order

The origin of the phase stability of 18$R$ Mg-Zn-Y alloys with a long-period stacking order (LPSO) is studied using first-principles calculations. We calculate the heat of formation as a function of the number of Zn vacancies to discuss the role of Zn atoms. The calculated convex hull indicates that the Zn atoms in the LPSO alloys are stable even if they number about half of the Y atoms. The bonding state with Zn $p$ orbitals leads to the stability of the LPSO structure because the partial density of states of Mg nearest to the solute cluster forms a valley structure.

cond-mat.mtrl-sci↗

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

Effect of magnetocrystalline anisotropy on magnetocaloric properties of AlFe$_{2}$B$_{2}$ compound

It is well known that the temperature dependence of the effective magnetocrystalline anisotropy energy obeys the $l(l+1)/2$ power law of magnetization in the Callen-Callen theory. Therefore, according to the Callen-Callen theory, the magnetocrystalline anisotropy energy is assumed to be zero at the critical temperature where the magnetization is approximately zero. This study estimates the temperature dependence of the magnetocrystalline anisotropy energy by integrating the magnetization versus magnetic field ($M$--$H$) curves, and found that the magnetocrystalline anisotropy is still finite even above the Curie temperature in the uniaxial anisotropy, whereas this does not appear in the cubic anisotropy case. The origin is the fast reduction of the anisotropy field, which is the magnetic field required to saturate the magnetization along the hard axis, in the case of cubic anisotropy. Therefore, the magnetization anisotropy and anisotropic magnetic susceptibility, those are the key factors of magnetic anisotropy, could not be established in the case of cubic anisotropy. In addition, the effect of magnetocrystalline anisotropy on magnetocaloric properties, as the difference between the entropy change curves of AlFe$_{2}$B$_{2}$ appears above the Curie temperature, which is in good agreement with a previous experimental study. This is proof of magnetic anisotropy at slightly above Curie temperature.

cond-mat.mtrl-sci↗

arXiv2021

Anisotropic magnetocaloric effect of CrI$_{3}$: A theoretical study

CrI$_{3}$ is considered to be a promising candidate for spintronic devices and data storage. We derived the Heisenberg Hamiltonian for CrI$_{3}$ from density functional calculations using the Liechtenstein formula. Moreover, the Monte--Carlo simulations with the Sucksmith--Thompson method were performed to analyze the effect of magnetic anisotropy energy on the thermodynamic properties. Our method successfully reproduced the negative sign of isothermal magnetic entropy changes when a magnetic field was applied along the hard plane. We found that the temperature dependence of the magnetocrystalline anisotropy energy is not negligible at temperatures slightly above the Curie temperature. We clarified that the origin of this phenomenon is attributed to anisotropic magnetic susceptibility and magnetization anisotropy. The difference between the entropy change of the easy axis and the hard plane is proportional to the temperature dependence of the magnetic anisotropy energy, implying that the anisotropic entropy term is the main source of the temperature dependence of the free energy difference when magnetizing in a specific direction other than the easy axis. We also investigated the magnetic susceptibility that can be used for the characterization of the negative sign of the entropy change in the case of a hard plane. The competition of magnetocrystalline anisotropy energy and external magnetic field at low temperature and low magnetic field region causes a high magnetic susceptibility as the fluctuation of magnetization. Meanwhile, the anisotropy energy is suppressed at a sufficient magnetic field applied along the hard axis, the magnetization is fully rotated to the direction of the external magnetic field.

cond-mat.mtrl-sci↗

arXiv2021

Extremely large magnetoresistance in the hourglass Dirac loop chain metal β-ReO$_{2}$

The transport and thermodynamic properties of $β$-ReO$_{2}$ crystallizing in a nonsymmorphic structure were studied using high-quality single crystals. An extremely large magnetoresistance (XMR) reaching 22,000 $\%$ in a transverse magnetic field of 10 T at 2 K was observed. However, distinguished from other topological semimetals with low carrier densities that show XMR, $β$-ReO$_{2}$ has a high electron carrier density of 1 $\times$ $10^{22}$ cm$^{-3}$ as determined by Hall measurements and large Fermi surfaces in the electronic structure. In addition, a small Fermi surface with a small effective mass was evidenced by de Haas-van Alphen oscillation measurements. The previous band structure calculations [S. S. Wang, et al., Nat. Commun. 8, 1844 (2017)] showed that two kinds of loops made of Dirac points of hourglass-shaped dispersions exist and are connected to each other by a point to form a string of alternating loops, called the Dirac loop chain (DLC), which are protected by the multiple glide symmetries. Our first-principles calculations revealed the complex Fermi surfaces with the smallest one corresponding to the observed small Fermi surface, which is just located near the DLC. The XMR of $β$-ReO$_{2}$ is attributed to the small Fermi surface and thus is likely caused by the DLC.

cond-mat.str-el↗

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

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↗

arXiv2019

DFT-Based Engineering of Dirac Surface Energy in Topological-Insulator Multilayers

Aiming at the future spintronics device applications of the spin-polarized surface states in three-dimensional topological insulator, a highly insulating bulk state and a tunable Dirac cone surface state are required. Here we employ a slab model having hetero-structural Bi2Se3-related quintuple layers and perform first-principles simulations. Our computational results show that the Dirac-point energy can be optimally tuned by selecting an appropriate pair of materials so that the work function at the surface quintuple layer is slightly different from that at the inner quintuple layers. The ideal surface state is obtained in Bi2Te3/(Bi2Te2Se)4/Bi2Te3 slab, in which the Fermi lines show the significant warping effect and both the in-plane and the out-of-plane components of the spin polarization emerge.

cond-mat.mtrl-sci↗