arXiv subjects

Taisuke Ozaki

Publications and source records attributed to Taisuke Ozaki.

54 records · Page 3Linked to original sources

arXiv2016

Core level binding energies in solids from first-principles

A general method is presented to calculate absolute binding energies of core levels in metals and insulators, based on a penalty functional and an exact Coulomb cutoff method in a framework of the density functional theory. The spurious interaction of core holes between supercells is avoided by the exact Coulomb cutoff method, while the variational penalty functional enables us to treat multiplet splittings due to chemical shift, spin-orbit coupling, and exchange interaction on equal footing, both of which are not accessible by previous methods. It is demonstrated that the absolute binding energies of core levels for both metals and insulators are calculated by the proposed method in a mean absolute (percentage) error of 0.4 eV (0.16~\%) for eight cases compared to experimental values measured with X-ray photoemission spectroscopy within a generalized gradient approximation to the exchange-correlation functional.

cond-mat.mtrl-sci↗

arXiv2015

Performance Tuning of a Parallel 3-D FFT Package OpenFFT

The fast Fourier transform (FFT) is a primitive kernel in numerous fields of science and engineering. OpenFFT is an open-source parallel package for 3-D FFTs, built on a communication-optimal domain decomposition method for achieving minimal volume of communication. In this paper, we analyze and tune the performance of OpenFFT, paying a particular attention to tuning of communication that dominates the run time of large-scale calculations. We first analyze its performance on different machines for an understanding of the behaviors of the package and machines. Based on the performance analysis, we develop six communication methods for performing communication with the aim of covering varied calculation scales on a variety of computational platforms. OpenFFT is then augmented with an auto-tuning of communication to select the best method in run time depending on their performance. Numerical results demonstrate that the optimized OpenFFT is able to deliver relatively good performance in comparison with other state-of-the-art packages at different computational scales on a number of parallel machines.

cs.MS↗

arXiv2015

First-principles study on cubic pyrochlore iridates Y2Ir2O7 and Pr2Ir2O7

Fully relativistic first-principles electronic structure calculations based on a noncollinear local spin density approximation (LSDA) are performed for pyrochlore iridates Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$. The all-in, all-out antiferromagnetic (AF) order is stablized by the on-site Coulomb repulsion $U>U_c$ in the LSDA+$U$ scheme, with $U_c\sim1.1$~eV and 1.3~eV for Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$, respectively. AF semimetals with and without Weyl points and then a topologically trivial AF insulator successively appear with further increasing $U$. For $U=1.3$~eV, Y$_2$Ir$_2$O$_7$ is a topologically trivial narrow-gap AF insulator having an ordered local magnetic moment $\sim0.5μ_B$/Ir, while Pr$_2$Ir$_2$O$_7$ is barely a paramagnetic semimetal with electron and hole concentrations of $0.016$/Ir, in overall agreements with experiments. With decreasing oxygen position parameter $x$ describing the trigonal compression of IrO$_6$ octahedra, Pr$_2$Ir$_2$O$_7$ is driven through a non-Fermi-liquid semimetal having only an isolated Fermi point of $Γ_8^+$, showing a quadratic band touching, to a $Z_2$ topological insulator.

cond-mat.str-el↗

arXiv2014

Avoiding critical-point phonon instabilities in two-dimensional materials: The origin of the stripe formation in epitaxial silicene

The origin of the large-scale stripe pattern of epitaxial silicene on the ZrB$_2$(0001) surface observed by scanning tunneling microscope experiments is revealed by first-principles calculations. Without stripes, the ($\sqrt{3}\times\sqrt{3}$)-reconstructed, one-atom-thick Si layer is found to exhibit a "zero-frequency" phonon instability at the $M$ point. In order to avoid a divergent response, the relevant phonon mode triggers the spontaneous formation of a new phase with a particular stripe pattern offering a way to lower both the atomic surface density and the total energy of silicene on the particular substrate. The observed mechanism is a way for the system to handle epitaxial strain and may therefore be more common in two-dimensional epitaxial materials exhibiting a small lattice mismatch with the substrate.

cond-mat.mtrl-sci↗

arXiv2014

Band structure of silicene on the zirconium diboride (0001) thin film surface - convergence of experiment and calculations in the one-Si-atom Brillouin zone

So far, it represents a challenging task to reproduce angle-resolved photoelectron (ARPES) spectra of epitaxial silicene by first-principles calculations. Here, we report on the resolution of the previously controversial issue related to the structural configuration of silicene on the ZrB$_2$(0001) surface and its band structure. In particular, by representing the band structure in a large Brillouin zone associated with a single Si atom, it is found that the imaginary part of the one-particle Green's function follows the spectral weight observed in ARPES spectra. By additionally varying the in-plane lattice constant, the results of density functional theory calculations and ARPES data obtained in a wide energy range converge into the "planar-like" phase and provide the orbital character of electronic states in the vicinity of the Fermi level. It is anticipated that the choice of a smaller commensurate unit cell for the representation of the electronic structure will be useful for the study of epitaxial two-dimensional materials on various substrates in general.

cond-mat.mtrl-sci↗

arXiv2013

Diverse forms of $σ$ bonding in two-dimensional Si allotropes: Nematic orbitals in the $MoS_2$ structure

The interplay of $sp^2$- and $sp^3$-type bonding defines silicon allotropes in two- and three-dimensional forms. A novel two-dimensional phase bearing structural resembleance to a single MoS$_2$ layer is found to possess a lower total energy than low-buckled silicene and to be stable in terms of its phonon dispersion relations. A new set of cigar-shaped, nematic orbitals originating from the Si $sp^2$ orbitals realizes bonding with a 6-fold coordination of the inner Si atoms of the layer. The identification of these nematic orbitals advocates diverse Si bonding configurations different from those of C atoms.

cond-mat.mtrl-sci↗

arXiv2013

Competing magnetism in $π$ electrons in graphene with a single carbon vacancy

One intriguing finding in graphene is the vacancy-induced magnetism that highlights the interesting interaction between local magnetic moments and conduction electrons. Within density functional theory, the current understanding of the ground state is that a Stoner instability gives rise to ferromagnetism of $π$ electrons aligned with the localized moment of a $σ$ dangling bond and the induced $π$ magnetic moments vanish at low vacancy concentrations. However, the observed Kondo effect suggests that $π$ electrons around the vacancy should antiferromagnetically couple to the local moment and carry non-vanishing moments. Here we propose that a phase possessing both significant out-of-plane displacements and $π$ bands with antiferromagnetic coupling to the localized $σ$ moment is the ground state. With the features we provide, it is possible for spin-resolved STM, STS, and ARPES measurements to verify the newly proposed phase.

cond-mat.mtrl-sci↗

arXiv2013

First-principles study on competing phases of silicene: Effect of substrate and strain

The stability and electronic structure of competing silicene phases under in-plane compressive stress, either free-standing or on the ZrB$_2$(0001) surface, has been studied by first-principles calculations. A particular ($\sqrt{3}\times\sqrt{3}$)-reconstructed structural modification was found to be stable on the ZrB$_2$(0001) surface under epitaxial conditions. In contrast to the planar and buckled forms of free-standing silicene, in this "planar-like" phase, all but one of the Si atoms per hexagon reside in a single plane. While without substrate, for a wide range of strain, this phase is energetically less favorable than the buckled one, it is calculated to represent the ground state on the ZrB$_2$(0001) surface. The atomic positions are found to be determined by the interactions with the nearest neighbor Zr atoms competing with Si-Si bonding interactions provided by the constraint of the honeycomb lattice.

cond-mat.mtrl-sci↗

arXiv2013

Unfolding method for the first-principles LCAO electronic structure calculations

Unfolding the band structure of a supercell to a normal cell enables us to investigate how symmetry breakers such as surfaces and impurities perturb the band structure of the normal cell. We generalize the unfolding method, originally developed based on Wannier functions, to the linear combination of atomic orbitals (LCAO) method, and present a general formula to calculate the unfolded spectral weight. The LCAO basis set is ideal for the unfolding method because of the invariance that basis functions allocated to each atomic species are invariant regardless of existence of surface and impurity. The unfolded spectral weight is well defined by the property of the LCAO basis functions. In exchange for the property, the non-orthogonality of the LCAO basis functions has to be taken into account. We show how the non-orthogonality can be properly incorporated in the general formula. As an illustration of the method, we calculate the dispersive quantized spectral weight of ZrB2 slab and show strong spectral broadening in the out-of-plane direction, demonstrating the usefulness of the unfolding method.

cond-mat.mtrl-sci↗

arXiv2013

A decomposition method with minimum communication amount for parallelization of multi-dimensional FFTs

The fast Fourier transform (FFT) is undoubtedly an essential primitive that has been applied in various fields of science and engineering. In this paper, we present a decomposition method for parallelization of multi-dimensional FFTs with smallest communication amount for all ranges of the number of processes compared to previously proposed methods. This is achieved by two distinguishing features: adaptive decomposition and transpose order awareness. In the proposed method, the FFT data are decomposed based on a row-wise basis that maps the multi-dimensional data into one-dimensional data, and translates the corresponding coordinates from multi-dimensions into one-dimension so that the resultant one-dimensional data can be divided and allocated equally to the processes. As a result, differently from previous works that have the dimensions of decomposition pre-defined, our method can adaptively decompose the FFT data on the lowest possible dimensions depending on the number of processes. In addition, this row-wise decomposition provides plenty of alternatives in data transpose, and different transpose order results in different amount of communication. We identify the best transpose orders with smallest communication amounts for the 3-D, 4-D, and 5-D FFTs by analyzing all possible cases. Given both communication efficiency and scalability, our method is promising in development of highly efficient parallel packages for the FFT.

math.NA↗

arXiv2012

A three-dimensional domain decomposition method for large-scale DFT electronic structure calculations

With tens of petaflops supercomputers already in operation and exaflops machines expected to appear within the next 10 years, efficient parallel computational methods are required to take advantage of such extreme-scale machines. In this paper, we present a three-dimensional domain decomposition scheme for enabling large-scale electronic calculations based on density functional theory (DFT) on massively parallel computers. It is composed of two methods: (i) atom decomposition method and (ii) grid decomposition method. In the former, we develop a modified recursive bisection method based on inertia tensor moment to reorder the atoms along a principal axis so that atoms that are close in real space are also close on the axis to ensure data locality. The atoms are then divided into sub-domains depending on their projections onto the principal axis in a balanced way among the processes. In the latter, we define four data structures for the partitioning of grids that are carefully constructed to make data locality consistent with that of the clustered atoms for minimizing data communications between the processes. We also propose a decomposition method for solving the Poisson equation using three-dimensional FFT in Hartree potential calculation, which is shown to be better than a previously proposed parallelization method based on a two-dimensional decomposition in terms of communication efficiency. For evaluation, we perform benchmark calculations with our open-source DFT code, OpenMX, paying particular attention to the O(N) Krylov subspace method. The results show that our scheme exhibits good strong and weak scaling properties, with the parallel efficiency at 131,072 cores being 67.7% compared to the baseline of 16,384 cores with 131,072 diamond atoms on the K computer.

cond-mat.mtrl-sci↗

arXiv2010

Numerically exact O($N^{7/3}$) method for large-scale electronic structure calculations

An efficient low-order scaling method is presented for large-scale electronic structure calculations based on the density functional theory using localized basis functions, which directly computes selected elements of the density matrix by a contour integration of the Green function evaluated with a nested dissection approach for resultant sparse matrices. The computational effort of the method scales as O($N(\log_2N)^2$), O($N^{2}$), and O($N^{7/3}$) for one, two, and three dimensional systems, respectively, where $N$ is the number of basis functions. Unlike O($N$) methods developed so far the approach is a numerically exact alternative to conventional O($N^{3}$) diagonalization schemes in spite of the low-order scaling, and can be applicable to not only insulating but also metallic systems in a single framework. It is also demonstrated that the nested algorithm and the well separated data structure are suitable for the massively parallel computation, which enables us to extend the applicability of density functional calculations for large-scale systems together with the low-order scaling.

cond-mat.str-el↗

arXiv2009

Efficient implementation of the nonequilibrium Green function method for electronic transport calculations

An efficient implementation of the nonequilibrium Green function (NEGF) method combined with the density functional theory (DFT) using localized pseudo-atomic orbitals (PAOs) is presented for electronic transport calculations of a system connected with two leads under a finite bias voltage. In the implementation, accurate and efficient methods are developed especially for evaluation of the density matrix and treatment of boundaries between the scattering region and the leads. Equilibrium and nonequilibrium contributions in the density matrix are evaluated with very high precision by a contour integration with a continued fraction representation of the Fermi-Dirac function and by a simple quadratureon the real axis with a small imaginary part, respectively. The Hartree potential is computed efficiently by a combination of the two dimensional fast Fourier transform (FFT) and a finite difference method, and the charge density near the boundaries is constructed with a careful treatment to avoid the spurious scattering at the boundaries. The efficiency of the implementation is demonstrated by rapid convergence properties of the density matrix. In addition, as an illustration, our method is applied for zigzag graphene nanoribbons, a Fe/MgO/Fe tunneling junction, and a LaMnO$_3/$SrMnO$_3$ superlattice, demonstrating its applicability to a wide variety of systems.

cond-mat.mes-hall↗

arXiv2009

Spin-Orbit Integrated Ground State and Magnetic Anisotropy in Sr$_2$IrO$_4$

We present a microscopic model for the anisotropic exchange interactions in Sr$_{2}$IrO$_{4}$. A direct construction of Wannier functions from first-principles calculations proves the $j_{\mathrm{eff}}$=1/2 character of the spin-orbit integrated states at the Fermi level. An effective $j_{\mathrm{eff}}$-spin Hamiltonian explains the observed weak ferromagnetism and anisotropy of antiferromagnetically ordered magnetic state, which arise naturally from the $j_{\mathrm{eff}}$=1/2 state with a rotation of IrO$_{6}$ octahedra. It is suggested that Sr$_{2}$IrO$_{4}$ is a unique class of materials with effective exchange interactions in the spin-orbital Hilbert space.

cond-mat.str-el↗

arXiv2009

Dual Spin Filter Effect in a Zigzag Graphene Nanoribbon

By first principle calculations, a dual spin filter effect under finite bias voltages is demonstrated in an antiferromagnetic junction of symmetric zigzag graphene nanoribbon (ZGNR). Unlike conventional spin filter devices using half metallic materials, the up- and down-spin electrons are unidirectionally filtered in the counter direction of the bias voltage, making the junction a dual spin filter. On the contrary, asymmetric ZGNRs do not exhibit such a spin filter effect. By analyzing Wannier functions and a tight-binding model, we clarify that an interplay between the spin polarized band structure of $π$ and $π^*$ states near the Fermi level and decoupling of the interband hopping of the two states, arising from the symmetry of the wave functions, plays a crucial role in the effect.

cond-mat.mes-hall↗

arXiv2009

Maximally localized Wannier function within linear combination of pseudo-atomic orbital method: Implementation and applications to transition-metal-benzene complex

Construction of maximally localized Wannier functions (MLWFs) has been implemented within the linear combination of pseudo-atomic orbital (LCPAO) method. Detailed analysis using MLWFs is applied to three closely related materials, single benzene (Bz) molecule, organometallic Vanadium-Bz infinite chain, and V$_2$Bz$_{3}$ sandwich cluster. Two important results come out from the present analysis: 1) for the infinite chain, the validity of the basic assumption in the mechanism of Kanamori and Terakura for the ferromagnetic (FM) state stability is confirmed; 2) for V$_2$Bz$_3$, an important role played by the difference in the orbital energy between the edge Bzs and the middle Bz is newly revealed: the on-site energy of p$δ$ states of edge Bzs is higher than that of middle Bz, which further reduces the FM stability of V$_2$Bz$_3$.

cond-mat.mtrl-sci↗

arXiv2007

Theoretical analysis of magnetic coupling in sandwich clusters V_n(C_6H_6)_{n+1}

The mechanism of ferromagnetism stability in sandwich clusters V$_n$(C$_6$H$_6$)$_{n+1}$ has been studied by first-principles calculation and model analysis. It is found that each of the three types of bonds between V and benzene (Bz) plays different roles. V 3d$_{z^2}$ orbital, extending along the molecular axis, is weakly hybridized with Bz's HOMO-1 orbital to form the $σ$-bond. It is quite localized and singly occupied, which contributes 1$μ_B$ to the magnetic moment but little to the magnetic coupling of neighboring V magnetic moments. The in-plane d$_{x^2-y^2}$, d$_{xy}$ orbitals are hybridized with the LUMO of Bz and constitute the $δ$-bond. This hybridization is medium and crucial to the magnetic coupling though the $δ$ states have no net contribution to the total magnetic moment. d$_{xz}$, d$_{yz}$ and HOMO of Bz form a quite strong $π$-bond to hold the molecular structure but they are inactive in magnetism because their energy levels are far away from the Fermi level. Based on the results of first-principles calculation, we point out that the ferromagnetism stability is closely related with the mechanism proposed by Kanamori and Terakura [J. Kanamori and K. Terakura, J. Phys. Soc. Jpn. 70, 1433 (2001)]. However, the presence of edge Bz's in the cluster introduces an important modification. A simple model is constructed to explain the essence of the physical picture.

cond-mat.mtrl-sci↗

arXiv2005

Linear scaling Krylov subspace method for large scale {\it ab initio} electronic structure calculations of metals

An efficient and robust linear scaling method is presented for large scale {\it ab initio} electronic structure calculations of a wide variety of materials including metals. The detailed short range and the effective long range contributions to the electronic structure are taken into account by solving an embedded cluster defined in a Krylov subspace, which provides rapid convergence for not only insulators but also metals. As an illustration of the method, we present a large scale calculation based on density functional theory for a palladium cluster with a single iron impurity.

cond-mat.other↗