arXiv subjects

Yu-ichiro Matsushita

Publications and source records attributed to Yu-ichiro Matsushita.

71 records · Page 4Linked to original sources

arXiv2018

Comparison of Green's functions for transition metal atoms using self-energy functional theory and coupled-cluster singles and doubles (CCSD)

We demonstrate in the present study that self-consistent calculations based on the self-energy functional theory (SFT) are possible for the electronic structure of realistic systems in the context of quantum chemistry. We describe the procedure of a self-consistent SFT calculation in detail and perform the calculations for isolated $3 d$ transition metal atoms from V to Cu as a preliminary study. We compare the one-particle Green's functions (GFs) obtained in this way and those obtained from the coupled-cluster singles and doubles (CCSD) method. Although the SFT calculation starts from the spin-unpolarized Hartree--Fock (HF) state for each of the target systems, the self-consistency loop correctly leads to degenerate spin-polarized ground states. We examine the spectral functions in detail to find their commonalities and differences among the atoms by paying attention to the characteristics of the two approaches. It is demonstrated via the two approaches that calculations based on the density functional theory (DFT) can fail in predicting the orbital energy spectra for spherically symmetric systems. It is found that the two methods are quite reliable and useful beyond DFT.

cond-mat.mtrl-sci↗

arXiv2018

Quasiparticle energy spectra of isolated atoms from coupled-cluster singles and doubles (CCSD): Comparison with exact CI calculations

In this study, we have calculated single-electron energy spectra via the Green's function based on the coupled-cluster singles and doubles (GFCCSD) method for isolated atoms from H to Ne. In order to check the accuracy of the GFCCSD method, we compared the results with the exact ones calculated from the full-configuration interaction (FCI). Consequently, we have found that the GFCCSD method reproduces not only the correct quasiparticle peaks but also satellite ones by comparing the exact spectra with the 6-31G basis set. It is also found that open-shell atoms such as C atom exhibit Mott gaps at the Fermi level, which the exact density-functional theory (DFT) fails to describe. The GFCCSD successfully reproduces the Mott HOMO-LUMO (highest-occupied molecular orbital and lowest-unoccupied molecular orbital) gaps even quantitatively. We also discussed the origin of satellite peaks as shake-up effects by checking the components of wave function of the satellite peaks. The GFCCSD is a novel cutting edge to investigate the electronic states in detail.

cond-mat.mtrl-sci↗

arXiv2018

Band structures in coupled-cluster singles-and-doubles Green's function (GFCCSD)

We demonstrate that coupled-cluster singles-and-doubles Green's function (GFCCSD) method is a powerful and prominent tool drawing the electronic band structures and the total energies, which many theoretical techniques struggle to reproduce. We have calculated single-electron energy spectra via GFCCSD method for various kinds of systems, ranging from ionic to covalent and van der Waals, for the first time: one-dimensional LiH chain, one-dimensional C chain, and one-dimensional Be chain. We have found that the band gap becomes narrower than in HF due to the correlation effect. We also show that the band structures obtained from GFCCSD method include both quasiparticle and satellite peaks successfully. Besides, taking one-dimensional LiH as an example, we discuss the validity of restricting the active space to suppress the computational cost of GFCCSD method while maintaining the accuracy. We show that the calculated results without bands that do not contribute to the chemical bonds are in good agreement with full-band calculations. With GFCCSD method, we can calculate the total energy and band structures with high precision.

cond-mat.mtrl-sci↗

arXiv2017

Quantum Singwi-Tosi-Land-Sjoelander approach for interacting inhomogeneous systems under electromagnetic fields: Comparison with exact results

For inhomogeneous interacting electronic systems under a time-dependent electromagnetic perturbation, we derive the linear equation for response functions in a quantum mechanical manner. It is a natural extension of the original semi-classical Singwi-Tosi-Land-Sjoelander (STLS) approach for an electron gas. The factorization ansatz for the two-particle distribution is an indispensable ingredient in the STLS approaches for determination of the response function and the pair correlation function. In this study, we choose an analytically solvable interacting two-electron system as the target for which we examine the validity of the approximation. It is demonstrated that the STLS response function reproduces well the exact one for low-energy excitations. The interaction energy contributed from the STLS response function is also discussed.

cond-mat.other↗

arXiv2017

Periodicity-free unfolding method of electronic energy spectra: Application to twisted bilayer graphene

We propose a novel periodicity-free unfolding method of the electronic energy spectra. Our new method solves a serious problem that calculated electronic band structure strongly depends on the choice of the simulation cell, i.e., primitive-cell or supercell. The present method projects the electronic states onto the free-electron states, giving rise to the {\it plane-wave unfolded} spectra. Using the method, the energy spectra can be calculated as a completely independent quantity from the choice of the simulation cell. We have examined the unfolded energy spectra in detail for three models and clarified the validity of our method: One-dimensional interacting two chain model, monolayer graphene, and twisted bilayer graphene. Furthermore, we have discussed that our present method is directly related to the experimental ARPES (Angle-Resolved Photo-Emission Spectroscopy) spectra.

cond-mat.mtrl-sci↗

arXiv2017

First-Principles Prediction of Densities of Amorphous Materials: The Case of Amorphous Silicon

A novel approach to predict the atomic densities of amorphous materials is explored on the basis of Car-Parrinello molecular dynamics (CPMD) in density functional theory. Despite that determination of the atomic density of matter is crucial in understanding its physical properties, no such method has ever been proposed for amorphous materials until now. In our approach, by assuming that the canonical distribution of amorphous materials is Gaussian distribution, we generate multiple amorphous structures with several different volumes by CPMD simulations and average the total energies at each volume. The density is then determined to be the one that minimizes the averaged total energy. In this study, this approach is implemented for amorphous silicon ({$a$-Si}) to demonstrate its validity, and we have determined the density of {$a$-Si} to be 4.1 % lower and its bulk modulus to be 28 GPa smaller than those of the crystal, which are in good agreement with experiments. We have also confirmed that generating samples through classical molecular dynamics simulations produces a comparable result and validates our assumption. The findings suggest that the presented method is applicable to other amorphous systems, including those that lack experimental knowledge.

cond-mat.dis-nn↗

arXiv2017

Unfolding energy spectra of multi-periodicity materials

We propose a new unfolding scheme to analyze energy spectra of complex large-scale systems which are inherently of multi-periodicity. Considering twisted bilayer graphene (tBLG) as an example, we first show that the conventional unfolding scheme in the past using a single primitive-cell representation causes serious problems in analyses of the energy spectra. We then introduce our multi-space representation scheme in the unfolding method and clarify its validity for tBLG. Velocity renormalization of Dirac electrons in tBLG is elucidated in the present unfolding scheme.

cond-mat.mtrl-sci↗

arXiv2017

Analysis of single and composite structural defects in pure amorphous silicon: a first-principles study

The structural and electronic properties of amorphous silicon ($a$-Si) are investigated by first-principles calculations based on the density-functional theory (DFT), focusing on the intrinsic structural defects. By simulated melting and quenching of a crystalline silicon model through the Car-Parrinello molecular dynamics (CPMD), we generate several different $a$-Si samples, in which three-fold ($T_3$), five-fold ($T_5$), and anomalous four-fold ($T_{4a}$) defects are contained. Using the samples, we clarify how the disordered structure of $a$-Si affects the characters of its density of states (DOS). We subsequently study the properties of defect complexes found in the obtained samples, including one that comprises three $T_5$ defects, and we show the conditions for the defect complexes to be energetically stable. Finally, we investigate the hydrogen passivation process of the $T_5$ defects in $a$-Si and show that the hydrogenation of $T_5$ is an exothermic reaction and that the activation energy for a H$_2$ molecule to passivate two $T_5$ sites is calculated to be 1.05 eV.

cond-mat.mtrl-sci↗

arXiv2017

A novel intrinsic interface state controlled by atomic stacking sequence at interfaces of SiC/SiO$_2$

On the basis of ab-initio total-energy electronic-structure calculations, we find that interface localized electron states at the SiC/SiO$_2$ interface emerge in the energy region between 0.3 eV below and 1.2 eV above the bulk conduction-band minimum (CBM) of SiC, being sensitive to the sequence of atomic bilayers in SiC near the interface. These new interface states unrecognized in the past are due to the peculiar characteristics of the CBM states which are distributed along the crystallographic channels. We also find that the electron doping modifies the energetics among the different stacking structures. Implication for performance of electron devices fabricated on different SiC surfaces are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic Mechanism of Carbon Annihilation upon SiC Oxidation due to Phosphorous Treatment: Density-Functional Calculations Combined with Ion Mass Spectrometry

We report first-principles static and dynamic calculations that clarify the microscopic mechanism of carbon annihilation due to phosphorous treatment upon oxidation of silicon carbide (SiC). We identify the most stable form of the phosphorus (P) in the oxide as the four-fold coordinated with the dangling PO unit and find that the unit attracts carbon ejected from the interface, thus operating as a carbon absorber. This finding provides a microscopic reasoning for the first time for the promotion of the oxidation reaction on one hand and the annihilation of the C-related defects at the interface on the other. Secondary ion mass spectrometry measurements are also performed and the obtained carbon profile corroborates the theoretical finding above.

cond-mat.mtrl-sci↗

arXiv2017

Comprehensive study on band-gap variations in $sp^3$-bonded semiconductors: roles of electronic states floating in internal space

We have performed electronic structure calculations to explore the band-gap dependence on polytypes for $sp^3$-bonded semiconducting materials, i.e., SiC, AlN, BN, GaN, Si, and diamond. In this comprehensive study, we have found that band-gap variation depending on polytypes is common in $sp^3$-bonded semiconductors; SiC, AlN, and BN exhibit smallest band gaps in $3C$ structure, whereas diamond does in $2H$ structure. We have also clarified that the microscopic mechanism of the band-gap variations is attributed to peculiar electron states $floating$ in internal channel space at the conduction-band minimum (CBM), and that internal channel length and the electro-static potential in channel affect the energy level of CBM.

cond-mat.mtrl-sci↗

arXiv2016

Mechanisms of initial oxidation of 4H-SiC (0001) and (000$\bar{1}$) surfaces unraveled by first-principles calculations

We have performed electronic state calculations to clarify the initial stage of the oxidation of the Si- and C-faces in 4H-SiC based on the density-functional theory. We investigate how each Si and C atomic site is oxidized on C- and Si-face, and explore most probable reaction pathways, corresponding energy barriers, and possible defects generated during the oxidation. We have found that carbon annihilation process is different between on Si- and on C-face, and this difference causes different defects in interface; In C-face case, (1), carbon atoms are dissociated directly from the substrate as CO molecules. (2), after CO dissociation, 3-fold coordinated oxygen atoms (called Y-lid) are observed at the interface. (3), high density of C-dangling bonds can remain at the interface. In Si-face case, (1), C atoms inevitably form carbon nano clusters (composed of a few atoms) in interface to reduce the number of dangling bonds there. Moreover, we have found that the carbon nano clusters are composed of not only single but also double chemical bonds. (2), We have observed that CO molecules are dissociated from the carbon nano clusters in MD simulations. Furthermore, we have investigated whether H$_2$ and NO molecules react with the defects found in this study.

cond-mat.mtrl-sci↗

arXiv2016

First-principles calculations that clarify energetics and reactions of oxygen adsorption and carbon desorption on 4H-SiC ($11\bar20$) surface

We report static and dynamic first-principles calculations that provide atomistic pictures of the initial stage of the oxidation processes occurring at the ($11\bar20$) surface of 4H-SiC. Our results unveil reaction pathways and their associated free-energy barriers for the adsorption of oxygen and the desorption of carbon atoms. We find that oxygen adsorption shows structural multi-stability and that the surface-bridge sites are the most stable and crucial sites for subsequent oxidation. We find that an approaching O$_2$ molecule is adsorbed, then dissociated and finally migrates toward these surface-bridge sites with a free-energy barrier of 0.7 eV at the ($11\bar20$) surface. We also find that a CO molecule is desorbed from the metastable oxidized structure upon the overcoming of a free-energy barrier of 2.4$\sim$2.6 eV, thus constituting one of the annihilation process of C during the oxidation. The results of the CO molecule desorption on the ($11\bar20$) surface are compared with the ($000\bar1$) surface. A catalytic effect of dangling bonds at the surface, causing a drastic reduction of the CO desorption energy, is found on the ($000\bar1$) surface and the microscopic picture of the effect is ascribed to an electron transfer from the Si to C dangling bonds. The intrinsic ($11\bar20$) surface does not show this catalytic effect, and this is because the surface consists of an equal amount of Si and C dangling bonds and the electron transfer occurs before the desorption.

cond-mat.mtrl-sci↗

arXiv2016

Band-unfolding approach to Moirè-induced band-gap opening and Fermi-level-velocity reduction in twisted bilayer graphene

We report on the energy spectrum of electrons in twisted bilayer graphene (tBLG) obtained by the band-unfolding method in the tight-binding model. We find the band-gap opening at particular points in the reciprocal space, that elucidates the drastic reduction of the Fermi-level velocity with the tiny twisted angles in tBLGs. We find that Moirè pattern caused by the twist of the two graphene layers generates interactions among Dirac cones, otherwise absent, and the resultant cone-cone interactions peculiar to each point in the reciprocal space causes the energy gap and thus reduced the Fermi-level velocity.

cond-mat.mtrl-sci↗

arXiv2015

L1$_0$ stacked binaries as candidates for hard magnets: FePt, MnAl and MnGa

A novel strategy of stacking binary magnets to enhance the magneto crystalline anisotropy is explored. This strategy is used in the search for hard magnets by studying FePt/MnGa and FePt/MnAl stacks. The choice of these binaries is motivated by the fact that they already possess large magneto crystalline anisotropy. Several possible alternative structures for these materials are explored in order to reduce the amount of Pt owing to its high cost.

cond-mat.mtrl-sci↗

arXiv2014

Interstitial Channels that Control Band Gaps and Effective Masses in Tetrahedrally Bonded Semiconductors

We find that electron states at the bottom of the conduction bands of covalent semiconductors are distributed mainly in the interstitial channels and that this floating nature leads to the band-gap variation and the anisotropic effective masses in various polytypes of SiC. We find that the channel length, rather than the hexagonality prevailed in the past, is the decisive factor for the band-gap variation in the polytypes. We also find that the floating nature causes two-dimensional electron and hole systems at the interface of different SiC polytypes and even one-dimensional channels near the inclined SiC surface.

cond-mat.mtrl-sci↗

arXiv2011

Comparative study of hybrid functionals applied to structural and electronic properties of semiconductors and insulators

We present a systematic study that clarifies validity and limitation of current hybrid functionals in density functional theory for structural and electronic properties of various semiconductors and insulators. The three hybrid functionals, PBE0 by Perdew, Ernzerhof, and Becke, HSE by Heyd, Sucseria, and Ernzerhof, and a long-range corrected (LC) functional, are implemented in a well-established plane-wave-basis-set scheme combined with norm-conserving pseudopotentials, thus enabling us to assess applicability of each functional on equal footing to the properties of the materials. The materials we have examined in this paper range from covalent to ionic materials as well as a rare-gas solid whose energy gaps determined by experiments are in the range of 0.6 eV - 14.2 eV: i.e., Ge, Si, BaTiO$_3$, $β$-GaN, diamond, MgO, NaCl, LiCl, Kr, and LiF. We find that the calculated bulk moduli by the hybrid functionals show better agreement with the experiments than the generalized gradient approximation (GGA) provides, whereas the calculated lattice constants by the hybrid functionals and GGA show comparable accuracy. The calculated energy band gaps and the valence-band widths for the ten prototype materials show substantial improvement using the hybrid functional compared with GGA. In particular, it is found that the band gaps of the ionic materials as well as the rare-gas solid are well reproduced by the LC-hybrid functional, whereas those of covalent materials are well described by the HSE functional. We also examine exchange effects due to short-range and long-range components of the Coulomb interaction and propose an optimum recipe to the short-range and long-range separation in treating the exchange energy.

cond-mat.mtrl-sci↗