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Masahiro Fukuda

Publications and source records attributed to Masahiro Fukuda.

18 recordsLinked to original sources
arXiv2026

Electron chirality and hydrodynamic helicity: Analysis in the atomic limit

Electron chirality has been proposed as a microscopic quantity that characterizes electronic handedness, yet its underlying control parameter has not been clearly identified. Furthermore, its applicability is limited to systems with spin-orbit coupling, which motivates the need for alternative measures of chirality. In this work, we explore two complementary measures of chirality: electron chirality and hydrodynamic helicity. By analyzing a minimal atomic model under chiral crystal fields, we clarify how the interplay among crystal fields, spin-orbit coupling, and electron correlation gives rise to non-zero values of chirality measures. Although electron chirality increases with both spin-orbit coupling and chiral crystal field strength, the dependence on these two factors is highly non-trivial. Particularly, when the chiral crystal field is varied continuously and the energy levels approach quasidegenerate points, the electron chirality is insensitive to spin-orbit coupling, resulting in a remarkable enhancement of chirality. In contrast, the hydrodynamic helicity, defined as a two-body pseudoscalar quantity, remains non-zero even without spin-orbit coupling, originating from electron-electron interactions. Perturbative analysis reveals distinct symmetry selection rules governing the two quantities. Our results provide fundamental insight into the origin of chiralities in electronic systems.

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↗
arXiv2025

In-orbit Performance of the Soft X-ray Imaging Telescope Xtend aboard XRISM

We present a summary of the in-orbit performance of the soft X-ray imaging telescope Xtend onboard the XRISM mission, based on in-flight observation data, including first-light celestial objects, calibration sources, and results from the cross-calibration campaign with other currently-operating X-ray observatories. XRISM/Xtend has a large field of view of $38.5'\times38.5'$, covering an energy range of 0.4--13 keV, as demonstrated by the first-light observation of the galaxy cluster Abell 2319. It also features an energy resolution of 170--180 eV at 6 keV, which meets the mission requirement and enables to resolve He-like and H-like Fe K$α$ lines. Throughout the observation during the performance verification phase, we confirm that two issues identified in SXI onboard the previous Hitomi mission -- light leakage and crosstalk events -- are addressed and suppressed in the case of Xtend. A joint cross-calibration observation of the bright quasar 3C273 results in an effective area measured to be $\sim420$ cm$^{2}$@1.5 keV and $\sim310$ cm$^{2}$@6.0 keV, which matches values obtained in ground tests. We also continuously monitor the health of Xtend by analyzing overclocking data, calibration source spectra, and day-Earth observations: the readout noise is stable and low, and contamination is negligible even one year after launch. A low background level compared to other major X-ray instruments onboard satellites, combined with the largest grasp ($Ω_{\rm eff}\sim60$ ${\rm cm^2~degree^2}$) of Xtend, will not only support Resolve analysis, but also enable significant scientific results on its own. This includes near future follow-up observations and transient searches in the context of time-domain and multi-messenger astrophysics.

astro-ph.IM↗
arXiv2025

Initial operations of the Soft X-ray Imager onboard XRISM

XRISM (X-Ray Imaging and Spectroscopy Mission) is an astronomical satellite with the capability of high-resolution spectroscopy with the X-ray microcalorimeter, Resolve, and wide field-of-view imaging with the CCD camera, Xtend. Xtend consists of the mirror assembly (XMA: X-ray Mirror Assembly) and detector (SXI: Soft X-ray Imager). The SXI is composed of CCDs, analog and digital electronics, and a mechanical cooler. After the successful launch on September 6th, 2023 (UT) and subsequent critical operations, the mission instruments were turned on and set up. The CCDs have been kept at the designed operating temperature of $-110^\circ$C after the electronics and cooling system were successfully set up. During the initial operation phase, which continued for more than a month after the critical operations, we verified the observation procedure, stability of the cooling system, all the observation options with different imaging areas and/or timing resolutions, and time-tagged and automated operations including those for South Atlantic Anomaly passages. We optimized the operation procedure and observation parameters including the cooler settings, imaging areas for the small window modes, and event selection algorithm. We summarize our policy and procedure of the initial operations for the SXI. We also report on a couple of issues we faced during the initial operations and lessons learned from them.

astro-ph.IM↗
arXiv2025

Soft X-ray Imager of the Xtend system onboard XRISM

The Soft X-ray Imager (SXI) is the X-ray charge-coupled device (CCD) camera for the soft X-ray imaging telescope Xtend installed on the X-ray Imaging and Spectroscopy Mission (XRISM), which was adopted as a recovery mission for the Hitomi X-ray satellite and was successfully launched on 2023 September 7 (JST). In order to maximize the science output of XRISM, we set the requirements for Xtend and find that the CCD set employed in the Hitomi/SXI or similar, i.e., a $2 \times 2$ array of back-illuminated CCDs with a $200~μ$m-thick depletion layer, would be practically best among available choices, when used in combination with the X-ray mirror assembly. We design the XRISM/SXI, based on the Hitomi/SXI, to have a wide field of view of $38' \times 38'$ in the $0.4-13$ keV energy range. We incorporated several significant improvements from the Hitomi/SXI into the CCD chip design to enhance the optical-light blocking capability and to increase the cosmic-ray tolerance, reducing the degradation of charge-transfer efficiency in orbit. By the time of the launch of XRISM, the imaging and spectroscopic capabilities of the SXI has been extensively studied in on-ground experiments with the full flight-model configuration or equivalent setups and confirmed to meet the requirements. The optical blocking capability, the cooling and temperature control performance, and the transmissivity and quantum efficiency to incident X-rays of the CCDs are also all confirmed to meet the requirements. Thus, we successfully complete the pre-flight development of the SXI for XRISM.

astro-ph.IM↗
arXiv2024

Status of Xtend telescope onboard X-Ray Imaging and Spectroscopy Mission (XRISM)

Xtend is one of the two telescopes onboard the X-ray imaging and spectroscopy mission (XRISM), which was launched on September 7th, 2023. Xtend comprises the Soft X-ray Imager (SXI), an X-ray CCD camera, and the X-ray Mirror Assembly (XMA), a thin-foil-nested conically approximated Wolter-I optics. A large field of view of $38^{\prime}\times38^{\prime}$ over the energy range from 0.4 to 13 keV is realized by the combination of the SXI and XMA with a focal length of 5.6 m. The SXI employs four P-channel, back-illuminated type CCDs with a thick depletion layer of 200 $μ$m. The four CCD chips are arranged in a 2$\times$2 grid and cooled down to $-110$ $^{\circ}$C with a single-stage Stirling cooler. Before the launch of XRISM, we conducted a month-long spacecraft thermal vacuum test. The performance verification of the SXI was successfully carried out in a course of multiple thermal cycles of the spacecraft. About a month after the launch of XRISM, the SXI was carefully activated and the soundness of its functionality was checked by a step-by-step process. Commissioning observations followed the initial operation. We here present pre- and post-launch results verifying the Xtend performance. All the in-orbit performances are consistent with those measured on ground and satisfy the mission requirement. Extensive calibration studies are ongoing.

astro-ph.IM↗
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

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

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↗
arXiv2020

Densest binary sphere packings and phase diagram : revisited

We revisit the densest binary sphere packings (DBSP) under the periodic boundary conditions and present an updated phase diagram, including newly found 12 putative densest structures over the $x - α$ plane, where $x$ is the relative concentration and $α$ is the radius ratio of the small and large spheres. To efficiently explore the DBSP, we develop an unbiased random search approach based on both the piling up method to generate initial structures in an unbiased way and the iterative balance method to optimize the volume of a unit cell while keeping the overlap of hard spheres minimized. With those two methods, we have discovered 12 putative DBSP and thereby the phase diagram is updated, while our results are consistent with those of the previous study [Hopkins et al., Phys. Rev. E 85, 021130 (2012)] with a small correction for the case of 12 or fewer spheres in the unit cell. The 5 of the new 12 densest packings are discovered in the small radius range of $0.42 \le α\le 0.50$ where several structures are competitive to each other with respect to packing fraction. Through the exhaustive search, diverse dense packings are discovered and accordingly we find that packing structures achieve high packing fractions by introducing distortion and/or combining a few local dense structural units. Furthermore, we investigate the correspondence of the DBSP with crystals based on the space group. The result shows that many structural units in real crystals, e.g., $\mathrm{LaH_{10}}$ and $\mathrm{SrGe_{2-δ}}$ being high-pressure phases, can be understood as DBSP. The correspondence implies that the densest sphere packings can be used effectively as structural prototypes for searching complex crystal structures, especially for high-pressure phases.

cond-mat.mtrl-sci↗
arXiv2020

Unfolding optical transition weights of impurity materials for first-principles LCAO electronic structure calculations

A method to analyze optical transitions is developed by combining the Kubo-Greenwood formula with the unfolding method to construct an unfolded electronic band structure with optical transition weights, which allows us to investigate how optical transitions are perturbed by imperfections such as impurity, vacancy, and structural distortions. Based on the Kubo-Greenwood formula, we first calculate frequency-dependent optical conductivity based on the first-principles electronic structure calculations using the linear combinations of atomic orbitals. Benefiting from the atomic orbital basis sets, the frequency-dependent optical conductivity can be traced back to their individual components before summations over all of $k$ points and bands. As a result, optical transition weights of the material can be put on the unfolded electronic band structure to show contributions at different $k$ points and bands. This method is especially useful to study the effects of broken symmetry in the optical transitions due to presence of impurities in the materials. As a demonstration, decomposed optical transition weights of a monolayer Si-doped graphene are shown in the electronic band structure.

cond-mat.mtrl-sci↗
arXiv2019

Structure map of AB$_2$ type 2D materials by high-throughput DFT calculations

By high-throughput calculations based on the density functional theory, we construct a structure map for AB$_2$ type monolayers of 3844 compounds which are all the combinations of 62 elements selected from the periodic table. The structure map and its web version (www.openmx-square.org/2d-ab2/) provide comprehensive structural trends of the 3844 compounds in two dimensional (2D) structures, and predict correctly structures of most of existing 2D compounds such as transition metal dichalcogenides and MXenes having 1T or 1H type structures. We also summarize all the families of 1T/1H type AB$_2$ monolayers for each combination of groups in the periodic table on the basis of our structure map, and propose new types of structures such as a memory structure, which may be a candidate material for data storage applications with an extremely high areal density. In addition, planar and distorted planar structures and other geometrically characteristic structures are found through the high-throughput calculations. These characteristic structures might give new viewpoints and directions to search unknown 2D materials. Our structure map and database will promote efforts towards synthesizing undiscovered 2D materials experimentally and investigating properties of the new structures theoretically.

cond-mat.mtrl-sci↗
arXiv2018

Efficient O($N$) divide-conquer method with localized natural orbitals

An efficient O($N$) divide-conquer (DC) method based on localized natural orbitals (LNOs) is presented for large-scale density functional theories (DFT) calculations of gapped and metallic systems. The LNOs are non-iteratively calculated by a low-rank approximation via a local eigendecomposition of a projection operator for the occupied space. Introducing LNOs to represent the long range region of a truncated cluster reduces the computational cost of the DC method while keeping computational accuracy. A series of benchmark calculations and high parallel efficiency in a multilevel parallelization clearly demonstrate that the O($N$) method enables us to perform large-scale simulations for a wide variety of materials including metals with sufficient accuracy in accordance with development of massively parallel computers.

physics.comp-ph↗
arXiv2018

Tight-binding calculations of optical matrix elements for conductivity using non-orthogonal atomic orbitals: Anomalous Hall conductivity in bcc Fe

We present a general formula for the tight-binding representation of momentum matrix elements needed for calculating the conductivity based on the Kubo-Greenwood formula using atomic orbitals, which are in general not orthogonal to other orbitals at different sites. In particular, the position matrix element is demonstrated to be important for delivering the exact momentum matrix element. This general formula, applicable to both orthonormal and non-orthonormal bases, solely needs the information of the position matrix elements and the ingredients that have already contained in the tight-binding representation. We then study the anomalous Hall conductivity in the standard example, ferromagnetic bcc Fe, by a first-principles tight-binding Hamiltonian. By assuming the commutation relation $\hat{\vec{p}} = (i m_e/\hbar) [\hat{H},\hat{\vec{r}}]$, the obtained frequency-dependent Hall conductivity is found to be in good agreement with existing theoretical and experimental results. Better agreement with experiments can be reached by introducing a reasonable bandwidth renormalization, evidencing the strong correlation among 3$d$ orbitals in bcc Fe. Since a tight-binding Hamiltonian can be straightforwardly obtained after finishing a first-principles calculation using atomic basis functions that are generated before the self-consistent calculation, the derived formula is particularly useful for those first-principles calculations.

cond-mat.mtrl-sci↗
arXiv2018

Realization of intrinsically broken Dirac cones in graphene via the momentum-resolved electronic band structure

A way to represent the band structure that distinguishes between energy-momentum and energy-crystal momentum relationships is proposed upon the band-unfolding concept. This momentum-resolved band structure offers better understanding of the physical processes requiring the information of wave functions in momentum space and provides a good description of angle-resolved photoelectron spectroscopy (ARPES) spectra together with a still informative band structure. Following this approach, we demonstrate that Dirac cones in graphene are intrinsically broken in momentum space and can be described by a conceptual unit cell smaller than the primitive unit cell. This hidden degree of freedom can be measured by ARPES experiments as missing weight that is retrievable by probing the chirality and Berry phases by linearly and circularly polarized light. Having the energy-momentum relationship, we provide alternative understanding of the retrieved momentum intensity, that is, the retrieved momentum intensity is assisted with the properties of final states, not from the Dirac cones directly. The revealed broken Dirac cones and momenta supplied by the lattice give interesting ingredients for designing advanced nanodevices.

cond-mat.mtrl-sci↗
arXiv2014

Study of Simulation Method of Time Evolution of Atomic and Molecular Systems by Quantum Electrodynamics

We discuss a method to follow step-by-step time evolution of atomic and molecular systems based on QED (Quantum Electrodynamics). Our strategy includes expanding the electron field operator by localized wavepackets to define creation and annihilation operators and following the time evolution using the equations of motion of the field operator in the Heisenberg picture. We first derive a time evolution equation for the excitation operator, the product of two creation or annihilation operators, which is necessary for constructing operators of physical quantities such as the electronic charge density operator. We then describe our approximation methods to obtain time differential equations of the electronic density matrix, which is defined as the expectation value of the excitation operator. By solving the equations numerically, we show "electron-positron oscillations", the fluctuations originated from virtual electron-positron pair creations and annihilations, appear in the charge density of a hydrogen atom and molecule. We also show that the period of the electron-positron oscillations becomes shorter by including the self-energy process, in which the electron emits a photon and then absorbs it again, and it can be interpreted as the increase in the electron mass due to the self-energy.

physics.atom-ph↗
arXiv2012

Study of Simulation Method of Time Evolution in Rigged QED

We discuss how we formulate time evolution of physical quantities in the framework of the Rigged QED (Quantum Electrodynamics). The Rigged QED is a theory which has been proposed to treat dynamics of electrons, photons and atomic nuclei in atomic and molecular systems in a quantum field theoretic way. To solve the dynamics in the Rigged QED, we need different techniques from those developed for the conventional QED. As a first step toward this issue, we propose a procedure to expand the Dirac field operator, which represents electrons, by the electron annihilation/creation operators and solutions of the Dirac equation for electrons in nuclear potential. Similarly, the Schrodinger field operators, which represent atomic nuclei, are expanded by nucleus annihilation/creation operators. Then we derive time evolution equations for these annihilation and creation operators and discuss how time evolution of the operators for physical quantities can be calculated. In the end, we propose a method to approximate the evolution equations of the operators by the evolution equations for the density matrices of electrons and atomic nuclei. Under this approximation, we carry out numerical simulation of the time evolution of electron charge density of a hydrogen atom.

physics.atom-ph↗