arXiv subjects

Taisuke Ozaki

Publications and source records attributed to Taisuke Ozaki.

At least 19 recordsLinked to original sources

arXiv2026

Application of Regional Chemical Potential Analysis to Si Adsorption on the Diamond (001) Surface

Adsorption of carbon dimers and Si atoms on the reconstructed diamond (001) surface is inves- tigated using density functional theory and regional chemical potential (RCP) analysis. We first demonstrate that the RCP distribution provides a real-space description of the bonding rearrange- ments responsible for the site-selective growth of experimentally observed carbon-dimer ribbons. We then examine the adsorption of single and multiple Si atoms. The calculated adsorption ener- gies show that a single Si atom preferentially bridges a surface carbon dimer and that subsequently adsorbed Si atoms favor neighboring dimer sites through Si-Si bond formation. The RCP analysis identifies electron-donating regions at the ends of finite Si chains, providing an intuitive explanation for their preferential one-dimensional growth and a physically motivated strategy for selecting candi- date adsorption structures. At higher Si coverages, geometry optimizations yield Si stripe and planar square-lattice structures on the diamond surface. Surface phase analysis indicates that an increase in the effective Si chemical potential favors structures with progressively higher Si coverages, from the Si stripe phase to the Si square-lattice phase. The calculated band structures reveal a progressive reduction of the surface band gap with increasing Si coverage. In the Si square-lattice structure, several bands cross the Fermi level, and the conducting states along the two in-plane directions have distinct Si and C character because the underlying diamond (001) substrate lacks fourfold rotational symmetry. These results establish RCP analysis as a useful approach for interpreting surface covalent bonding and guiding the exploration of adsorption-driven surface structures.

cond-mat.mtrl-sci↗

arXiv2026

Linear and nonlinear optical responses in the chiral multifold semimetal BeAu: A quantum-geometric perspective

Chiral topological semimetals provide a natural platform for exploring how multifold band topology and quantum geometry manifest in optical and photovoltaic responses. BeAu is a chiral multifold semimetal hosting band crossings at $Γ$, $M$, and $R$ with Chern numbers $C_Γ=-4$, $C_M=-2$, and $C_R=+4$, respectively. In this work, we study the linear optical conductivity and second-order dc photocurrent responses of BeAu using fully relativistic first-principles calculations. The calculated interband linear optical conductivity, Re $σ_{xx}(ω)$, is quantitatively reproduced by $(e^2/\hbar)ωg_{xx}(ω)$, showing that its spectral features are governed by the photon energy factor and the variation of the photon energy-resolved quantum-metric spectral weight. The linear shift current conductivity is closely related to the symplectic connection, whereas the circular injection current susceptibility is governed by the transition-resolved product of Berry curvature and the interband group velocity difference. At the Fermi level, the linear shift current conductivity reaches approximately -810 $μ$A/V$^2$ at a photon energy of 0.05 eV. Aligning the chemical potential with the multifold crossings strongly reshapes both responses, producing the largest linear shift current conductivity peak for $μ=μ_R$ and pronounced changes in the magnitude and sign of the circular injection current susceptibility. The circular photogalvanic trace is strongly photon-energy and chemical-potential dependent and does not exhibit a broad quantized plateau, indicating competing multiband transitions. Our results establish a unified quantum-geometric description of the linear and nonlinear optical responses of BeAu and identify it as a promising platform for optoelectronic phenomena governed by multifold band topology and quantum geometry.

cond-mat.mtrl-sci↗

arXiv2026

GPU Acceleration of Collinear and Noncollinear DFT Using a Numerical Atomic Orbital-Based DFT Code

We implement GPU acceleration of collinear and noncollinear density functional theory (DFT) calculations in the numerical atomic orbitals (NAOs) code OpenMX by offloading matrix multiplications and eigenvalue solves (plus selected auxiliary steps) to cuBLAS/cuSOLVER and OpenACC. Benchmarks on the Pegasus supercomputer (per node: a 48-core Intel Xeon Platinum 8468 CPU and one NVIDIA H100 GPU) compare GPU-accelerated and CPU-only runs under identical settings. For a 512-atom collinear case on two nodes (two GPUs total), the GPU-accelerated calculation achieves a 2.02 times speedup over a CPU-only run on two nodes (96 CPU cores total); for a 384-atom noncollinear case on two nodes (two GPUs total), the speedup is 2.60 times over two CPU-only nodes (96 cores). These results demonstrate practical GPU-accelerated DFT in an NAO-based code for both collinear and noncollinear calculations.

physics.comp-ph↗

arXiv2025

Exceptional Alkaline Methanol Electrooxidation on Bi-modified Pt3M Intermetallics: Kinetic Origins and an OH Binding Energy Descriptor

The exploration of advanced CO-free catalysts and clarifying the ambiguous kinetic origins and governing factors would undoubtedly open up opportunities to overcome the sluggish kinetics of methanol electrooxidation and promote the development of direct methanol fuel cells. Herein, we constructed a family of Bi-modified Pt3M intermetallic catalysts (Bi-Pt3M/C, M=Cr, Mn, Co, Zn, In, Ga, and Sn) that follow CO-free dominated pathway and exhibit exceptional catalytic activity. More significantly, leveraging this platform, we have identified the pivotal factor governing the reaction kinetics in CO-free pathway, namely OH binding energy (OHBE). This arises because the rate-determining step (RDS) encompasses both C-H bond activation and water dissociation, whose respective barriers can be reflected by the OHBE. Accordingly, OHBE can act as an activity descriptor. Specifically, Bi-Pt3In/C stands out from other Bi-Pt3M/C and delivers the unprecedented mass activity of 36.7 A mgPt-1 at peak potential, far exceeding state-of-the-art Pt-based catalysts reported to date. Taking Bi-Pt3In/C as a proof of concept, we clearly elucidate the origin of enhanced MOR activity by combining theoretical calculations, kinetic isotope effects, and formaldehyde electrooxidation. Moreover, there exhibits a volcano-type trend between OHBE and the activity of Bi-Pt3M/C. Beyond the discovery of ultrahigh-performance catalysts, these findings provide a detailed mechanistic picture of RDS and offer an innovative design principle for advanced catalysts.

physics.chem-ph↗

arXiv2025

Density functional theory for core-level X-ray absorption

We establish a rigorous density functional theory (DFT) framework for core-level X-ray absorption spectroscopy (XAS) by formulating a constrained search for core-excited states based on the Gunnarsson-Lundqvist theorem. Within this framework, the explicit-core Delta SCF scheme enables shift-free absolute edge alignment and a consistent treatment of L/M edges with spin-orbit-resolved projectors. In addition, by exploiting dipole selection rules, we recast the evaluation of the dipole matrix elements, which otherwise requires many independent Slater determinant calculations, into a compact single determinant form. This reduces the computational scaling from $\mathcal{O}(N^4)$ to $\mathcal{O}(N^3)$, where $N$ is the number of electrons, without introducing additional approximations. Across representative C, B, O, and Li K-edge benchmarks in molecules and solids, the method reproduces line shapes, polarization anisotropies, and absolute onsets without empirical shifts, providing a robust and scalable route to quantitatively reliable XAS simulations within DFT.

cond-mat.mtrl-sci↗

arXiv2025

Regional chemical potential analysis for material surfaces

We propose a local regional chemical potential (RCP) analysis method based on an energy window scheme to quantitatively estimate the selectivity of atomic and molecular adsorption on surfaces, as well as the strength of chemical bonding forces between a probe tip and a surface in atomic force microscopy (AFM) measurements. In particular, focusing on the local picture of covalent bonding, we use a simple H$_2$ molecular model to demonstrate a clear relationship between chemical bonding forces and the local RCP. Moreover, density functional theory calculations on molecular systems and diamond C(001) surfaces reveal that the local RCP at the surfaces successfully visualizes electron-donating regions such as dangling bonds and double bonds. These results suggest that the local RCP can serve as an effective measure to analyze high-resolution non-contact or near-contact AFM images enhanced by chemical bonding forces.

cond-mat.mtrl-sci↗

arXiv2024

Mathematical Crystal Chemistry

Efficient heuristics have predicted many functional materials such as high-temperature superconducting hydrides, while inorganic structural chemistry explains why and how the crystal structures are stabilized. Here we develop the paired mathematical programming formalism for searching and systematizing the structural prototypes of crystals. The first is the minimization of the volume of the unit cell under the constraints of only the minimum and maximum distances between pairs of atoms. We show the capabilities of linear relaxations of inequality constraints to optimize structures by the steepest-descent method, which is computationally very efficient. The second is the discrete optimization to assign five kinds of geometrical constraints including chemical bonds for pairs of atoms. Under the constraints, the two object functions, formulated as mathematical programming, are alternately optimized to realize the given coordination numbers of atoms. This approach successfully generates a wide variety of crystal structures of oxides such as spinel, pyrochlore-$α$, and $\mathrm{K}_2 \mathrm{NiF}_4$ structures.

cond-mat.mtrl-sci↗

arXiv2024

Surface structure of the 3x3-Si phase on Al(111), studied by the multiple usages of positron diffraction and core-level photoemission spectroscopy

The structure of an Al(111)3x3-Si surface was examined by combining data from positron diffraction and core-level photoemission spectroscopy. Analysis of the diffraction rocking curves indicated that the overlayer had a flat honeycomb lattice structure. Simulations of Si core-level spectra calculated via the first-principles indicated that one of the Si atoms in the unit cell was replaced by an Al atom. The surface superstructure was thus a two-dimensional layer of Al-embedded silicene on Al(111).

cond-mat.mtrl-sci↗

arXiv2024

Unveiling High Selectivity Origin of Pt-Bi Catalysts for Alkaline Methanol Electrooxidation via CO-free pathway

A long-standing puzzle for methanol electrooxidation is how to achieve a CO-free pathway and accurately understand the origin of electrocatalytic selectivity. Herein, we unequivocally demonstrate that the Bi-modified Pt/C follows a CO-free dominated pathway during alkaline methanol electrooxidation, and unveil the formaldehyde (HCHO) intermediate as a critical factor influencing pathway selectivity. These findings are substantiated by kinetic isotope effects, formate Faradaic efficiency, in situ spectroscopy, ab initio molecular dynamic simulations, and density functional theory calculations. Bi modification significantly increases the HCHO dehydrogenation barrier, which facilitates its desorption and subsequent conversion to the H2COOH- anion at the alkaline interface, intrinsically avoiding CO formation. More specifically, the formation of ensemble sites featuring V-shaped Bi-Pt-Bi configuration inhibits the cleavage of C-H bond, and the weak OH binding energy at Bi adatoms effectively prevents blockage of oxygenated species, allowing such ensemble sites to fulfill their functional role. Our study opens up a novel dimension for designing advanced CO-free catalysts.

physics.chem-ph↗

arXiv2024

Theory of Rashba splitting in quantum-well states

We present a theory pertaining to the asymptotic behavior of Rashba energy splitting in a quantum-well state (QWS). First, unlike previous studies, we derive $\textbf{k}$-linear Rashba term from a first-principles Hamiltonian in a physically convincing manner. The $\textbf{k}$-dependent in-plane intrinsic magnetic-field term originates from the spin--orbit interaction and hybridized $s$-$p_z$ orbital, whereas a steep nucleus potential realizes the linearity for the $\textbf{k}$ of the effective magnetic field. Next, we analyze the Rashba effect of a QWS using a one-dimensional tight-binding model developed based on the bottom-up approach that is aforementioned. The Rashba-splitting behavior of this system is captured from the density at the interface. The density can be expressed analytically as a function of the monolayer number and well depth. Finally, we apply our formula to the QWS of a few-monolayers Ag on an Au(111) surface to validate the theory based on a realistic system. Our tight-binding analysis qualitatively fits the first-principles result using only two fitting parameters and predicts the optimal condition for achieving a large Rashba splitting.

cond-mat.mes-hall↗

arXiv2024

Atomic observation on diamond (001) surfaces with non-contact atomic force microscopy

To achieve atomic-level characterization of the diamond (001) surface, persistent efforts have been made over the past few decades. The motivation behind the pursuit extends beyond investigating surface defects and adsorbates; it also involves unraveling the mystery of the smooth growth of diamond. However, the inherently low conductivity and the short C-C bonds render atomic resolution imaging exceptionally challenging. Here, we successfully overcame these challenges by employing non-contact atomic force microscopy with reactive Si tips. Atomic resolution imaging was achieved even at room temperature. With density-functional-theory calculations, we clarified that the critical factors for atomic resolution are in the formation of tilted C-Si bonds between scanning probes and surfaces, along with reordering of the surface C-C dimers. Implications of the findings extend beyond the realm of surface characterization. The present atomic-resolution microscopies drive future advancements in diamond technologies by providing avenues for identifying dopants and constructing artificial nanostructures.

cond-mat.mtrl-sci↗

arXiv2023

Closest Wannier functions to a given set of localized orbitals

A non-iterative method is presented to calculate the closest Wannier functions (CWFs) to a given set of localized guiding functions, such as atomic orbitals, hybrid atomic orbitals, and molecular orbitals, based on minimization of a distance measure function. It is shown that the minimization is directly achieved by a polar decomposition of a projection matrix via singular value decomposition, making iterative calculations and complications arising from the choice of the gauge irrelevant. The disentanglement of bands is inherently addressed by introducing a smoothly varying window function and a greater number of Bloch functions, even for isolated bands. In addition to atomic and hybrid atomic orbitals, we introduce embedded molecular orbitals in molecules and bulks as the guiding functions, and demonstrate that the Wannier interpolated bands accurately reproduce the targeted conventional bands of a wide variety of systems including Si, Cu, the TTF-TCNQ molecular crystal, and a topological insulator of Bi$_2$Se$_3$. We further show the usefulness of the proposed method in calculating effective atomic charges. These numerical results not only establish our proposed method as an efficient alternative for calculating WFs, but also suggest that the concept of CWFs can serve as a foundation for developing novel methods to analyze electronic structures and calculate physical properties.

cond-mat.mtrl-sci↗

arXiv2023

Electronic band structure change with structural transition of buckled Au$_2$X monolayers induced by strain

This study investigates the strain-induced structural transitions of $η\leftrightarrow θ$ and the changes in electronic band structures of Au$_2$X (X=S, Se, Te, Si, Ge) and Au$_4$SSe. We focus on Au$_2$S monolayers, which can form multiple meta-stable monolayers theoretically, including $η$-Au$_2$S, a buckled penta-monolayer composed of a square Au lattice and S adatoms. The $θ$-Au$_2$S is regarded as a distorted structure of $η$-Au$_2$S. Based on density functional theory (DFT) calculations using a generalized gradient approximation, the conduction and the valence bands of $θ$-Au$_2$S intersect at the $Γ$ point, leading to linear dispersion, whereas $η$-Au$_2$S has a band gap of 1.02 eV. The conduction band minimum depends on the specific Au-Au bond distance, while the valence band maximum depends on both Au-S and Au-Au interactions. The band gap undergoes significant changes during the $η\leftrightarrow θ$ phase transition of Au$_2$S induced by applying tensile or compressive in-plane biaxial strain to the lattice. Moreover, substituting S atoms with other elements alters the electronic band structures, resulting in a variety of physical properties without disrupting the fundamental Au lattice network. Therefore, the family of Au$_2$X monolayers holds potential as materials for atomic scale network devices.

cond-mat.mtrl-sci↗

arXiv2022

Atomically-thin metallic Si and Ge allotropes with high Fermi velocities

Silicon and germanium are the well-known materials used to manufacture electronic devices for the integrated circuits but they themselves are not considered as promising options for interconnecting the devices due to their semiconducting nature. We have discovered that both Si and Ge atoms can form unexpected metallic monolayer structures which are more stable than the extensively studied semimetallic silicene and germanene, respectively. More importantly, the newly discovered two-dimensional allotropes of Si and Ge have Fermi velocities superior to the Dirac fermions in graphene, indicating that the metal wires needed in the silicon-based integrated circuits can be made of Si atom itself without incompatibility, allowing for all-silicon-based integrated circuits.

cond-mat.mtrl-sci↗

arXiv2022

Prediction of quaternary hydrides based on densest ternary sphere packings

We exhaustively search quaternary metal hydrides based on the (13-2-1) and (13-3-1) structures that are the two of the putative densest ternary sphere packings in cubic systems [R. Koshoji \textit{et al.}, Phys. Rev. E \textbf{104}, 024101 (2021)]. The 73304 candidate hydrides are generated by substituting the small spheres with hydrogen atoms, the medium, large, and fourth spheres with metallic atoms. Especially, the substitution of the small spheres with hydrogen atoms gives the unconventional hydrogen sublattices. We screen unstable hydrides in the candidates through geometrical optimizations, constant pressure molecular dynamics simulations, and phonon calculations under hydrostatic pressure of $10$ GPa, and identify 23 hydrides with static and dynamic stability, including $\mathrm{H}_{12} \mathrm{Sc} \mathrm{Y}_{2} \mathrm{La}$ and $\mathrm{H}_{12} \mathrm{Ti} \mathrm{Ni}_{3} \mathrm{Ba}$. We expect that the 23 candidates of hydrides screened by the exhaustive search for 73304 hydrides provide a guideline to narrow down the search space for trials in the experimental synthesis of quaternary metal hydrides.

cond-mat.mtrl-sci↗

arXiv2022

Diverse densest ternary sphere packings

The exploration of the densest sphere packings is a fundamental problem in mathematics and a wide variety of sciences including materials science. We present our exhaustive computational exploration of the densest ternary sphere packings (DTSPs) for 451 radius ratios and 436 compositions on top of our previous study [Koshoji and Ozaki, Phys. Rev. E 104, 024101 (2021)]. The unbiased exploration discovers diverse 22 putative DTSPs, and thereby 60 putative DTSPs are identified in total including the 38 DTSPs discussed by the previous study. Some of the discovered DTSPs are well-ordered, for example, the medium spheres in the (9-7-3) structure are placed in a straight line with comprising the unit cell, and the DTSP has the $Pm \bar{3}m$ symmetry if the structural distortion is corrected. At a considerable number of radius ratios, the highest packing fractions are achieved by the phase separations consisting of only the FCC and/or the putative densest binary sphere packings (DBSPs) for all compositions, and the tendency is getting evident as the small and medium spheres are getting larger. The result seems to indicate directly that the local structures in the DBSPs may be denser than those consisting of three kinds of spheres. However, the unit cell of undiscovered DTSPs might only be much larger than in this study due to the complexity of the ternary local structures. Finally, we discuss the correspondence of the DTSPs with real crystals based on the space group. Our study suggests that the diverse structures of DTSPs can be effectively used as structural prototypes for searching ternary, quaternary, and quinary crystal structures.

cond-mat.soft↗

arXiv2021

The hidden competing phase revealed by first-principles calculations of phonon instability in the nearly optimally doped cuprate La$_{1.875}$Sr$_{0.125}$CuO$_4$

The representative cuprate, La$_{2-x}$M$_x$CuO$_4$, with M = Sr and $x = 1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in La$_{2-x}$Sr$_x$CuO$_4$, is found to be the ground state, where the structural phase transitions, HTT$\rightarrow$LTO$\rightarrow$LTLO, can be understood via phonon instability. While the La-O composition is identified to be responsible for the phonon softening, the superconducting CuO$_2$ layer is dynamically stable. The LTLO phase, which can exhibit a $\sim$20 meV splitting in the density of states, is proposed to have an intimate relationship with the observed pseudogap and the charge density wave giving the stripe. We argue that at low temperatures, the superconducting LTO La$_{1.875}$Sr$_{0.125}$CuO$_4$ competes with the phonon-preferred LTLO phase by spontaneously forming the Cooper pairs, resulting in suppressing the stripe. Therefore, the revealed LTLO phase is indispensable for understanding La$_{2-x}$Sr$_x$CuO$_4$.

cond-mat.supr-con↗

arXiv2021

Densest ternary sphere packings

We present our exhaustive exploration of the densest ternary sphere packings (DTSPs) for 45 radius ratios and 237 kinds of compositions, which is a packing problem of three kinds of hard spheres with different radii, under periodic boundary conditions by a random structure searching method. To efficiently explore DTSPs we further develop the searching method based on the piling-up and iterative balance methods [Koshoji et al., Phys. Rev. E 103, 023307 (2021)]. The unbiased exploration identifies diverse 38 putative DTSPs appearing on phase diagrams in which 37 DTSPs of them are discovered in the study. The structural trend of DTSPs changes depending especially on the radius of small spheres. In case that the radius of small spheres is relatively small, structures of many DTSPs can be understood as derivatives of densest binary sphere packings (DBSPs), while characteristic structures specific to the ternary system emerge as the radius of small spheres becomes larger. In addition to DTSPs, we reveal a lot of semi-DTSPs (SDTSPs) which are obtained by excluding DBSPs in the calculation of phase diagram, and investigate the correspondence of DTSPs and SDTSPs with real crystals based on the space group, showing a considerable correspondence of SDTSPs having high symmetries with real crystals including $\mathrm{Cu}_2 \mathrm{GaSr}$ and $\mathrm{ThCr}_2 \mathrm{Si}_2$ structures. Our study suggests that the diverse structures of DBSPs, DTSPs, and SDTSPs can be effectively used as structural prototypes for searching complex crystal structures.

physics.comp-ph↗