arXiv subjects

Taisuke Ozaki

Publications and source records attributed to Taisuke Ozaki.

At least 37 records · Page 2Linked to original sources

arXiv2021

Methods for constructing parameter-dependent flat band lattices

We present two methods for constructing a flat band (FB) system having a flat energy dispersion over the entire Brillouin zone within tight-binding model, where the resulting Hamiltonian may not be easily obtained by existing methods based on a bipartite graph and line graph techniques. In the first method, we derive a set of conditions equivalent to the appearance of FBs for a given graph structure. This method allows parameter to be tuned so that systems with a small number of sites per unit cell has a FB. In the second method, we show that FB systems can be obtained by removing or adding sites to an existing FB system under specific rules. In particular, the site addition method enables us to construct multiple FB systems stemming from a single FB system. The FB system obtained by the second method has the characteristics that the component ratios in the FB eigenstate are partially common to the original system. We illustrate how lattices having a FB can be constructed by applying the latter method starting from an existing lattice such as a kagome lattice, demonstrating that a wide variety of lattices can possess a FB in the band structure.

cond-mat.mtrl-sci↗

arXiv2020

Scaling law for the Rashba-type spin splitting in quantum well films

We use laser-based spin- and angle-resolved photoemission spectroscopy (laser-SARPES) with high-resolution, and experimentally determine, for the first time, the Rashba-parameters of quantum well states (QWSs) systematically changing with the film thickness and the quantum numbers, through the observation of the Ag films grown on an Au(111) substrate. The data are very well reproduced by the theoretical calculations based on the density functional theory. Most importantly, we find a scaling law for the Rashba parameter ($α_{\rm R}$) that the magnitude of $α_{\rm R}$ is scaled by the charge density at the interface and the spin-orbit coupling ratio between the film and the substrate, and it is expressed by a single straight line regardless of the film thickness and the quantum numbers. The new finding not only is crucial to understand the Rashba effect in QWSs but also gives a foundation of film growth engineering to fine-tune the spin splitting in 2D heterostructure systems.

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↗

arXiv2020

First-principles study on the stability and electronic structure of monolayer GaSe with trigonal-antiprismatic structure

The structural stability and electronic states of GaSe monolayer with trigonal-antiprismatic (AP) structure, which is a recently discovered new polymorph, were studied by first-principles calculations. The AP phase GaSe monolayer was found stable, and the differences in energy and lattice constant were small when compared to those calculated for a GaSe monolayer with conventional trigonal-prismatic (P) structure which was found to be the ground state. Moreover, it was revealed that the relative stability of P phase and AP phase GaSe monolayers reverses under tensile strain. These calculation results provide insight into the formation mechanism of AP phase GaSe monolayers in epitaxially-grown GaSe thin films.

cond-mat.mtrl-sci↗

arXiv2020

Single-site orthogonalization for first-principles computations of exchange coupling constants

For accurate first-principles computations of exchange coupling constants $J_{ij}$ by the Liechtenstein method with localized basis sets, we developed a scheme using the single-site orthogonalization (SO). In contrast to the non-orthogonal (NO) scheme, where the basis set is used to compute $J_{ij}$ without modification, and the Löwdin orthogonalization (LO) scheme, the SO scheme exhibits much less dependence of $J_{ij}$ on the choice of the basis set. The SO scheme achieves convergence of $J_{ij}$ for bcc Fe, hcp Co, and fcc Ni with an increase in the number of the basis set, while the NO and LO schemes result in the fluctuation depending on the basis set. This improvement by the SO scheme is attributed to the removal of orbital overlaps with avoiding ill-defined single-site effective potentials. We further improve the SO scheme by introducing appropriate spin population, so that the SO with spin-population scaling (SOS) scheme can provide converged Curie temperatures for transition metals. Moreover, negative values of $J_{ij}$ for dhcp Nd and rhombohedral Sm obtained by the SOS scheme can coincide with the experimentally-found magnetic order that cannot be reproduced by positive sets of $J_{ij}$.

cond-mat.mtrl-sci↗

arXiv2020

Emergence of Nearly Flat Bands through an Embedded Kagome Lattice in an Epitaxial Two-dimensional Ge Layer on ZrB2(0001)

Ge atoms segregating on zirconium diboride thin films grown on Ge(111) were found to crystallize into a two-dimensional bitriangular structure which was recently predicted to be a flat band material. Angle-resolved photoemission experiments together with theoretical calculations verified the existence of a nearly flat band in spite of non-negligible in-plane long-range hopping and interactions with the substrate. This provides the first experimental evidence that a flat band can emerge from the electronic coupling between atoms and not from the geometry of the atomic structure.

cond-mat.str-el↗

arXiv2020

Benchmark of density functional theory for superconductors in elemental materials

Systematic benchmark calculations for elemental bulks are presented to validate the accuracy of density functional theory for superconductors. We developed a method to treat the spin-orbit interaction (SOI) together with the spin fluctuation (SF) and examine their effect on the superconducting transition temperature. We found the following results from the benchmark calculations: (1) The calculations, including SOI and SF, reproduce the experimental superconducting transition temperature ($T_c$) quantitatively. (2) The effect by SOI is small excepting a few elements such as Pb, Tl, and Re. (3) SF reduces $T_c$s, especially for the transition metals, while this reduction is too weak to reproduce the $T_c$s of Zn and Cd. (4) We reproduced the absence of superconductivity for alkaline (earth) and noble metals. These calculations confirm that our method can be applied to a wide range of materials and implies a direction for the further improvement of the methodology.

cond-mat.supr-con↗

arXiv2019

Efficient algorithm based on Liechtenstein method for computing exchange coupling constants using localized basis set

For large-scale computation of the exchange coupling constants $J_{ij}$, we reconstruct the Liechtenstein formula for localized orbital representation and simplify the energy integrations by adopting the finite pole approximation of the Fermi function proposed by Ozaki [Phys. Rev. B 75, 035123 (2007)]. We calculate the exchange coupling constant $J_{\mathrm{1NN}}$ of the first-nearest-neighbor sites in body-centered-cubic Fe systems of various sizes to estimate the optimal computational parameters that yield appropriate values at the lowest computational cost. It is shown that the number of poles needed for a computational accuracy of 0.05 meV is determined as $\sim$ 60, whereas the number of necessary Matsubara poles needed to obtain similar accuracy, which was determined in previous studies, is on the order of 1000. Finally, we show $J_{ij}$ as a function of atomic distance, and compared it with one derived from Korringa-Kohn-Rostoker Green's function formalism. The distance profile of $J_{ij}$ derived by KKR formalism agrees well with that derived by our study, and this agreement supports the reliability of our newly derived formalism.

cond-mat.mtrl-sci↗

arXiv2019

Hidden mechanism for embedding the flat bands of Lieb, kagome, and checkerboard lattices in other structures

The interplay of hopping parameters that can give rise to flat bands in consequence of quantum interference in electronic, photonic, and other interesting materials has become an extensively studied topic. Most of the recognized structures having flat bands are the lattices that can be understood by the mathematical theory of line graphs, such as the Lieb, kagome, and checkerboard lattices. Here, we demonstrate that the structures that can realize the same kind of flat bands given by those well-known lattices hosting exotic quantum phases are more flexible. The flat bands belonging to the recognized structures can be ideally embedded into the new structures that cannot be considered as the original ones in terms of a unitary transformation. The uncovered mechanism enriches the understanding of physics behind the localized quantum states and broadens the choice of materials that can be used for designing electronic and photonic devices from the zero band dispersion.

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↗

arXiv2019

OpenMX Viewer: A web-based crystalline and molecular graphical user interface program

The OpenMX Viewer (Open source package for Material eXplorer Viewer) is a web-based graphical user interface (GUI) program for visualization and analysis of crystalline and molecular structures and 3D grid data in the Gaussian cube format such as electron density and molecular orbitals. The web-based GUI program enables us to quickly visualize crystalline and molecular structures by dragging and dropping XYZ, CIF, or OpenMX input/output files, and analyze static/dynamic structural properties conveniently in a web browser. Several basic functionalities such as analysis of Mulliken charges, molecular dynamics, geometry optimization and band structure are included. In addition, based on marching cubes, marching tetrahedra and surface nets algorithms with Affine transformation, 3D isosurface techniques are supported to visualize electron density and molecular/crystalline orbitals in the cube format with superposition of a crystalline or molecular structure. Furthermore, the Band Structure Viewer is implemented for showing a band structure in a web browser. By accessing the website of the OpenMX Viewer, the latest OpenMX Viewer is always available for users to visualize various structures and analyze their properties without installations, upgrades, updates, registration, sign-in and terminal commands.

cs.HC↗

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↗

arXiv2018

Peculiar bonding associated with atomic doping and hidden honeycombs in borophene

Engineering atomic-scale structures allows great manipulation of physical properties and chemical processes for advanced technology. We show that the B atoms deployed at the centers of honeycombs in boron sheets, borophene, behave as nearly perfect electron donors for filling the graphitic $σ$ bonding states without forming additional in-plane bonds by first-principles calculations. The dilute electron density distribution owing to the weak bonding surrounding the center atoms provides easier atomic-scale engineering and is highly tunable via in-plane strain, promising for practical applications, such as modulating the extraordinarily high thermal conductance that exceeds the reported value in graphene. The hidden honeycomb bonding structure suggests an unusual energy sequence of core electrons that has been verified by our high-resolution core-level photoelectron spectroscopy measurements. With the experimental and theoretical evidence, we demonstrate that borophene exhibits a peculiar bonding structure and is distinctive among two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2017

Reliability and applicability of magnetic force linear response theory: Numerical parameters, predictability, and orbital resolution

We investigated the reliability and applicability of so-called magnetic force linear response method to calculate spin-spin interaction strengths from first-principles. We examined the dependence on the numerical parameters including the number of basis orbitals and their cutoff radii within non-orthogonal LCPAO (linear combination of pseudo-atomic orbitals) formalism. It is shown that the parameter dependence and the ambiguity caused by these choices are small enough in comparison to the other computation approach and experiments. Further, we tried to pursue the possible extension of this technique to a wider range of applications. We showed that magnetic force theorem can provide the reasonable estimation especially for the case of strongly localized moments even when the ground state configuration is unknown or the total energy value is not accessible. The formalism is extended to carry the orbital resolution from which the matrix form of the magnetic coupling constant is calculated. From the applications to Fe-based superconductors including LaFeAsO, NaFeAs, BaFe$_2$As$_2$ and FeTe, the distinctive characteristics of orbital-resolved interactions are clearly noticed in between single-stripe pnictides and double-stripe chalcogenides.

cond-mat.str-el↗

arXiv2017

Single-particle excitation of core states in epitaxial silicene

Recent studies of core-level X-ray photoelectron spectroscopy (XPS) spectra of silicene on ZrB$_2$(0001) were found to be inconsistent with the density of states (DOS) of a planar-like structure that has been proposed as the ground state by density functional theory (DFT). To resolve the discrepancy, a reexamination of the XPS spectra and direct theoretical access of accurate single-particle excitation energies are desired. By analyzing the XPS data using symmetric Voigt functions, different binding energies and its sequence of Si $2p$ orbitals can be assigned from previously reported ones where asymmetric pseudo-Voigt functions are adopted. Theoretically, we have adopted an approach developed very recently, which follows the sophisticated $Δ$ self-consistent field ($Δ$SCF) methods, to study the single-particle excitation of core states. In the calculations, each single-particle energy and the renormalized core-hole charge density are calculated straightforwardly via two SCF calculations. By comparing the results, the theoretical core-level absolute binding energies including the splitting due to spin-orbit coupling are in good agreement with the observed high-resolution XPS spectra. The good agreement not only resolves the puzzling discrepancy between experiment and theory (DOS) but also advocates the success of DFT in describing many-body interactions of electrons at the surface.

cond-mat.mtrl-sci↗