arXiv subjects

Mitsuharu Uemoto

Publications and source records attributed to Mitsuharu Uemoto.

16 recordsLinked to original sources

arXiv2026

First-Principles Prediction of Nonlinear Optical Response in TiO$_2$ for High-Power Dielectric Mirror Applications

Dielectric multilayer mirrors are essential components in optical experiments using high-power lasers, where titanium dioxide (TiO$_2$) is widely employed as a high-refractive-index dielectric material. In this study, we investigate the nonlinear optical response of TiO$_2$ under intense ultrashort laser pulses using real-time first-principles electron-dynamics simulations based on time-dependent density functional theory (TDDFT). We reveal intensity-dependent absorption driven by multiphoton excitation and optically excited free carriers, and simulate the resulting electron-light coupled dynamics in TiO$_2$ nanofilms using a multiscale Maxwell-TDDFT framework. Direct evaluation of the reflected and transmitted fields shows reduced reflectance at high intensities ($I \sim 10^{13}$ W/cm$^2$), demonstrating a pronounced nonlinear optical response. Furthermore, the optical response properties are compared among various stable and metastable crystalline phases of TiO$_2$-rutile, anatase, brookite, TiO$_2$-II, and TiO$_2$-B-as well as an amorphous supercell model. These results provide microscopic insight into intensity-dependent optical degradation in TiO$_2$-based dielectric optical components exposed to intense femtosecond laser fields.

physics.optics↗

arXiv2026

Oxygen-induced Fe surface segregation at the $L1_0$-FePd(001)/graphene heterointerface for spintronics devices: a first-principles study

We theoretically investigate the atomic-scale structure of the heterointerface formed between the (001) surface of the $L1_0$-ordered iron palladium (FePd) intermetallic alloy and graphene (Gr), namely, $L1_0$-FePd(001)/Gr, which serves as an essential component in spintronic devices. Using density functional theory (DFT) calculations, we demonstrate that the topmost surface layer consisting of Pd (Pd-terminated surface) is energetically more stable than that consisting of Fe in vacuum, and that Pd-terminated surfaces are unfavorable for graphene adsorption. In contrast, under an oxygen atmosphere, the strong Fe--O bonding stabilizes Fe-terminated surfaces. The predicted Fe--O bonds on the oxidized surface are consistent with our X-ray photoelectron spectroscopy (XPS) measurements. These results reproduce the mechanism responsible for the graphene coverage observed in recent experiments. Similar oxygen-induced Fe surface segregation has been studied in heterogeneous catalysis on FePt and FePd alloys. In this work, we exploit this mechanism as a termination-engineering strategy to fabricate high-quality 2D-material/alloy heterointerfaces for nanoscale device applications.

cond-mat.mtrl-sci↗

arXiv2025

Density functional theory study of effect of NO annealing on electronic structure and carrier-scattering property of 4H-SiC(0001)/SiO$_2$ interface

The effect of the nitrided layer introduced by NO annealing on the electronic structure and carrier scattering property of the 4H-SiC(0001)/SiO$_2$ interface is investigated by the density functional theory calculation using the interface models where the areal N atom density corresponds to that in practical devices. The areal N atom density is one third of the areal C atom density in practical devices. It is found that the nitrided layer screens the unfavorable Coulomb interaction of O atoms in the SiO$_2$. However, the electrons flowing under the nitrided layer are significantly scattered by the fluctuation of potential due to the low areal N atom density. These results imply that the areal N atom density should be increased so that the fluctuation of potential is suppressed.

cond-mat.mtrl-sci↗

arXiv2025

Theoretical investigation of interface atomic structure of graphene on NiFe alloy substrate

Two processes have been proposed to fabricate graphene/NiFe alloy interfaces for tunneling magnetoresistance devices. One is the transfer of graphene and the other is the evaporation of alloys onto graphene. The formation energy of a NiFe alloy substrate and the adsorption energy of graphene on the NiFe alloy substrate are investigated by a density functional theory calculations to reveal the difference in the atomic structure of the interface between the two processes. It is found that Ni-rich surfaces are preferable for the bare substrate, whereas Fe surfaces are stable for the graphene adsorbed on the substrate. This result indicates that the composition ratio of the surface layer depends on the interface fabrication process.

cond-mat.mtrl-sci↗

arXiv2023

First-principle study of spin transport property in $L1_0$-FePd(001)/graphene heterojunction

In our previous work, we synthesized a metal/2D material heterointerface consisting of $L1_0$-ordered iron-palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/$m$-Gr/FePd(001), where $m$ represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150-200%) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in $m$ not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite $π$-band-like behavior. Additionally, we investigate the spin-transfer torque-induced magnetization switching behavior of our \color{blue} junction structures \color{black} using micromagnetic simulations. Furthermore, we examine the impact of lateral displacements (``sliding'') at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO.

cond-mat.mtrl-sci↗

arXiv2023

Valley filters using graphene blister defects from first principles

Valleytronics, which makes use of the two valleys in graphenes, attracts considerable attention and a valley filter is expected to be the central component in valleytronics. We propose the application of the graphene valley filter using blister defects to the investigation of the valley-dependent transport properties of the Stone--Wales and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition from the $\mathbf{K}$ valley to the $\mathbf{K}^\prime$ valleys is completely suppressed in some defects. Using a large bipartite honeycomb cell including several carbon atoms in a cell and replacing atomic orbitals with molecular orbitals in the tight-binding model, we demonstrate analytically and numerically that the symmetry between the A and B sites of the bipartite honeycomb cell contributes to the suppression of the intervalley transition. In addition, the universal rule for the atomic structures of the blisters suppressing the intervalley transition is derived. Furthermore, by introducing additional carbon atoms to graphenes to form blister defects, we can split the energies of the states at which resonant scattering occurs on the $\mathrm{K}$ and $\mathrm{K}^\prime$ channel electrons. Because of this split, the fully valley-polarized current will be achieved by the local application of a gate voltage.

cond-mat.mes-hall↗

arXiv2023

Density functional theory study on effect of NO annealing for SiC(0001) surface with atomic-scale steps

Density functional theory calculations for the electronic structures of the 4H-SiC(0001)/SiO$_2$ interface with atomic-scale steps are carried out to investigate the effect of NO annealing. The characteristic behavior of the conduction band edge states of SiC is strongly affected over a wide area of the interface by the Coulomb interaction of the O atoms in the SiO$_2$ region as well as the step structure of the interface, resulting in the discontinuity of the inversion layers at the step edges under the gate bias. The spatially discontinued band only allows the very limited conduction paths in the inversion layer, leading to the significantly decreased mobile carrier density. It is found that the Coulomb interaction of the O atoms is screened and the inversion layers become continuous when the nitrided layers are inserted at the interface by NO annealing. This result is in good agreement with experimental findings that the improvement of the performance of SiC metal-oxide-semiconductor field-effect-transistors by NO annealing is attributed to an increase in the mobile electron density rather than an increase in the mobility of electrons in the inversion layer.

cond-mat.mtrl-sci↗

arXiv2022

Density functional study of twisted graphene $L1_0$-FePd heterogeneous interface

Graphene on $L1_0$-FePd(001), which has been experimentally studied in recent years, is a heterogeneous interface with a significant lattice symmetry mismatch between the honeycomb structure of graphene and tetragonal alloy surface. In this work, we report on the density functional study of its atomic-scale configurations, electronic and magnetic properties, and adsorption mechanism, which have not been well understood in previous experimental studies. We propose various atomic-scale models, including simple nontwisted and low-strain twisted interfaces, and analyze their energetical stability by performing structural optimizations using the van der Waals interactions of both DFT-D2 and optB86b-vdW functionals. The binding energy of the most stable structure reached $E_\mathrm{B}=-0.22$~eV/atom for DFT-D2 ($E_\mathrm{B}=-0.19$~eV/atom for optB86b-vdW). The calculated FePd-graphene spacing distance was approximately 2~Å, which successfully reproduced the experimental value. We also find out characteristic behaviors: the modulation of $π$-bands, the suppression of the site-dependence of adsorption energy, and the rise of \color{blue} moiré-like \color{black} corrugated buckling. In addition, our atomic structure is expected to help build low-cost computational models for investigating the physical properties of $L1_0$ alloys/two-dimensional interfaces.

cond-mat.mtrl-sci↗

arXiv2022

First-principles method for nonlinear light propagation at oblique incidence

We have developed a computational method to describe the nonlinear light propagation of an intense and ultrashort pulse at oblique incidence on a flat surface. In the method, coupled equations of macroscopic light propagation and microscopic electron dynamics are simultaneously solved using a multiscale modeling. The microscopic electronic motion is described by first-principles time-dependent density functional theory. The macroscopic Maxwell equations that describe oblique light propagation are transformed into one-dimensional wave equations. As an illustration of the method, light propagation at oblique incidence on a silicon thin film is presented.

physics.optics↗

arXiv2022

Density functional theory calculations for investigation of atomic structures of 4H-SiC/SiO$_2$ interface after NO annealing

We propose the atomic structures of the 4H-SiC/SiO$_2$ interface for the $a$, $m$, C, and Si faces after NO annealing. Our proposed structures preferentially form at the topmost layers of the SiC side of the interface, which agrees with the experimental finding of secondary-ion mass spectrometry, that is, the N atoms accumulate at the interface. In addition, the areal N-atom density is on the order of 10$^{14}$ atom/cm$^2$ for each plane, which is also consistent with the experimental result. Moreover, the electronic structure of the interface after NO annealing, in which the CO bonds are removed and the nitride layer only at the interface is inserted, is free from gap states, although some interface models before NO annealing include the gap states arising from the CO bonds near the valence band edge of the bandgap. Our results imply that NO annealing can contribute to the reduction in the density of interface defects by forming the nitride layer.

cond-mat.mtrl-sci↗

arXiv2022

Theoretical investigation of vacancy related defects at 4H-SiC(000$\bar{1}$)/SiO$_2$ interface after wet oxidation

The stability and formation mechanism of the defects relevant to silicon and carbon vacancies at the 4H-SiC(000$\bar{1}$)/SiO$_2$ interface after wet oxidation are investigated by first-principles calculation based on the density functional theory. The difference in the total energy of the defects agrees with the experimental results concerning the dencity of defects. We found that the characteristic behaviors of the generation of defects are explained by the positions of vacancies and antisites in the SiC(000$\bar{1}$) substrate and that the formation of silicon and carbon vacancies is relevant to the generation mechanism of defects. The generation of silicon and carbon vacancies is attributed to the termination of dangling bonds by H atoms introduced by wet oxidation, resulting in generation of carbon-antisite--carbon-vacancy and divacancies defects in wet oxidation.

cond-mat.mtrl-sci↗

arXiv2021

First-principles study on structure and anisotropy of high N-atom density layer in 4H-SiC

A nitridation annealing process is well employed to reduce interface trap states that degrade the channel mobility of 4H-SiC/SiO${}_2$ metal-oxide-semiconductor field-effect transistor. In recent experiments, the existence of high N-atom density layers at the annealed interface is reported and their concentrations are known to be anisotropic in the crystal planes. Until now, the role of atomic structure and the electronic states surrounding the N atoms incorporated by the nitridation annealing process on the origin of anisotropy is not well understood. In this work, we propose a simplified atomic-scale model structure of 4H-SiC with the a high N-atom density layer ($\sim 10^{15}~\mathrm{atom}/\mathrm{cm}^2$), which is of the order of the experimental observation. We use bulk 4H-SiC as host crystal and consider several sets of the atomic configurations of the N-atom incorporated structure at the quasi cubic-($k$-) and hexagonal-($h$-)sites on $a$-, $m$-, and Si-(C-)planes. Based on the density functional theory calculations, we investigate the influence of the energy stability on the distribution directions. Although our bulk model is simplified compared to the realistic interface structures, we confirm significant difference among models and observe that the incorporation of N atoms on the $a$-face is stable. Furthermore, from the analysis of the electronic states, we suggest that this anisotropy of the formation energy originates from the change of the coordinating number due to the difference in geometric configurations of the N-atom incorporated structures.

cond-mat.mtrl-sci↗

arXiv2021

First-principles study of ultrafast and nonlinear optical properties of graphite thin films

We theoretically investigate ultrafast and nonlinear optical properties of graphite thin films based on first-principles time-dependent density functional theory. We first calculate electron dynamics in a unit cell of graphite under a strong pulsed electric field and explore the transient optical properties of graphite. It is shown that the optical response of graphite shows a sudden change from conducting to insulating phase at a certain intensity range of the applied electric field. It also appears as a saturable absorption, the saturation in the energy transfer from the electric field to electrons. We next investigate a light propagation in graphite thin films by solving coupled dynamics of the electrons and the electromagnetic fields simultaneously. It is observed that the saturable absorption manifests in the propagation with small attenuation in the spatial region where the electric field amplitude is about $4 \sim 7 \times 10^{-2}$ V/Angstrom.

physics.optics↗

arXiv2020

Attosecond state-resolved carrier motion in quantum materials probed by soft X-ray XANES

Recent developments in attosecond technology led to tabletop X-ray spectroscopy in the soft X-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near edge spectroscopy (XANES) to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 x 10^21 cm^-3 and that the light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities towards an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, quantum materials like graphene, TMDCs, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics such as whether the electrons or lattice trigger phase transitions.

cond-mat.mtrl-sci↗

arXiv2018

Nonlinear polarization evolution using time-dependent density functional theory

We propose a theoretical and computational approach to investigate temporal behavior of a nonlinear polarization in perturbative regime induced by an intense and ultrashort pulsed electric field. First-principles time-dependent density functional theory is employed to describe the electron dynamics. Temporal evolution of third-order nonlinear polarization is extracted from a few calculations of electron dynamics induced by pulsed electric fields with the same time profile but different amplitudes. We discuss characteristic features of the nonlinear polarization evolution as well as an extraction of nonlinear susceptibilities and time delays by fitting the polarization. We also carry out a decomposition of temporal and spatial changes of the electron density in power series with respect to the field amplitude. It helps to get insight into the origin of the nonlinear polarization in atomic scale.

physics.optics↗

arXiv2018

SALMON: Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience

SALMON (Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience, http://salmon-tddft.jp) is a software package for the simulation of electron dynamics and optical properties of molecules, nanostructures, and crystalline solids based on first-principles time-dependent density functional theory. The core part of the software is the real-time, real-space calculation of the electron dynamics induced in molecules and solids by an external electric field solving the time-dependent Kohn-Sham equation. Using a weak instantaneous perturbing field, linear response properties such as polarizabilities and photoabsorptions in isolated systems and dielectric functions in periodic systems are determined. Using an optical laser pulse, the ultrafast electronic response that may be highly nonlinear in the field strength is investigated in time domain. The propagation of the laser pulse in bulk solids and thin films can also be included in the simulation via coupling the electron dynamics in many microscopic unit cells using Maxwell's equations describing the time evolution of the electromagnetic fields. The code is efficiently parallelized so that it may describe the electron dynamics in large systems including up to a few thousand atoms. The present paper provides an overview of the capabilities of the software package showing several sample calculations.

physics.comp-ph↗