arXiv subjects

Takeo Fujiwara

Publications and source records attributed to Takeo Fujiwara.

At least 19 recordsLinked to original sources

arXiv2009

Domain boundary formation in helical multishell gold nanowire

Helical multishell gold nanowire is studied theoretically for the formation mechanism of helical domain boundary. Nanowires with the wire length of more than 10 nm are relaxed by quantum mechanical molecular dynamics simulation with tight-binding form Hamiltonian. In results, non-helical nanowires are transformed into helical ones with the formation of atom pair defects at domain boundary, where the defective atom pair is moved from an inner shell. Analysis of local electronic structure shows a competitive feature of the energy gain of reconstruction on wire surface and the energy loss of the defect formation. A simple energy scaling theory gives a general discussion of domain boundary formation.

cond-mat.mtrl-sci↗

arXiv2008

Electronic structure and effects of dynamical electron correlation in ferromagnetic bcc-Fe, fcc-Ni and antiferromagnetic NiO

LDA+DMFT method in the framework of the iterative perturbation theory (IPT) with full LDA Hamiltonian without mapping onto the effective Wannier orbitals. We then apply this LDA+DMFT method to ferromagnetic bcc-Fe and fcc-Ni as a test of transition metal, and to antiferromagnetic NiO as an example of transition metal oxide. In Fe and Ni, the width of occupied 3d bands is narrower than those in LDA and Ni 6eV satellite appears. In NiO, the resultant electronic structure is of charge-transfer insulator type and the band gap is 4.3eV. These results are in good agreement with the experimental XPS. The configuration mixing and dynamical correlation effects play a crucial role in these results.

cond-mat.str-el↗

arXiv2008

Ill-Contact Effects of d-Orbital Channels in Nanometer-Scale Conductor

Electronic current in a nanometer-size rod is theoretically investigated by an eigen-channel decomposition method in nonequilibrium Green's function formalism. Physical properties, such as the local density of electrons and local current, are decomposed into contributions of eigen-channels. We observe that the evanescent modes and nonlinear conductance are enhanced in d-orbital systems, and the structure of the transmission function, local current density, and penetration depth are discussed. The two effects of the ill-contact at electrodes in d-orbital systems, evanescent modes and the nonlinearity of conductance, are regarded as originating in the peak structure of the transmission function of eigen-channels in the energy region between chemical potentials of left and right lead wires.

cond-mat.mes-hall↗

arXiv2008

Development of simulation package 'ELSES' for extra-large-scale electronic-structure calculation

An early-stage version of simulation package ' ELSES' (Extra-Large-Scale Electronic-Structure calculation) is developed for electronic structure and dynamics of large systems, particularly, nm-scale or 10nm-scale systems (www.elses.jp). Input and output files are written in the Extensible Markup Language (XML) style for general users. Related pre-/post-simulation tools are also available. Practical work flow and example are described. A test calculation of GaAs bulk system is shown to demonstrate that the present code can handle systems with more than one atom species. Several future aspects are also discussed.

cond-mat.mtrl-sci↗

arXiv2008

Shifted COCG method and its application to double orbital extended Hubbard model

We explains the shifted COCG method which can solve a series of the linear equations generated by numbers of scaler shifts, without time consuming matrix-vector operations, except at the only one reference energy. This is a family of the CG method and sharing the robustness and the capability of the accuracy estimation. Then shifted COCG is quite useful to calculate the Green's function of the many-electron Hamiltonian which have very large dimension. We applied it to the double orbital extended Hubbard model with twelve electrons on the periodic sqrt(8) x sqrt(8) site system, the dimension of the Hamiltonian equals to 64,128,064, and found the ground state is insulator. We also explained the crucial points of the shifted COCG algorithm for reducing the amount of required memory.

cond-mat.str-el↗

arXiv2008

Theory of large-scale matrix computation and applications to electronic structure calculation

We review our recently developed methods for large-scale electronic structure calculations, both in one-electron theory and many-electron theory. The method are based on the density matrix representation, together with the Wannier state representation and the Krylov subspace method, in one-electron theory of a-few-tens nm scale systems. The hybrid method of quantum mechanical molecular dynamical simulation is explained.The Krylov subspace method, the CG (conjugate gradient) method and the shifted-COCG (conjugate orthogonal conjugate gradient) method, can be applied to the investigation of the ground state and the excitation spectra in many-electron theory. The mathematical foundation of the Krylov subspace method for large-scale matrix computation is focused and the key technique of the shifted-COCG method, e.g. the collinear residual and seed switching, is explained. A wide variety of applications of these extended novel algorithm is also explained. These are the fracture formation and propagation, liquid carbon and formation process of gold nanowires, together with the application to the extend Hubbard model.

cond-mat.mtrl-sci↗

arXiv2007

Development of simulation package for atomic processes of ultra-large-scale system based on electronic structure theory

An early-stage version of simulation package is developed for electronic structure calculation and dynamics of atom process in large-scale systems, particularly, nm-scale or 10nm-scale systems. We adopted the Extensible Markup Language (XML)-style in the input and the output of our simulation code, and developed some modeling and analysis tools for dynamical simulations of atomic processes. GaAs bulk system was calculated to demonstrate that the present code can handle systems with more than one atom specie.

cond-mat.mtrl-sci↗

arXiv2007

Two-Stage Formation Model and Helicity of Gold Nanowires

A model for formation of helical multishell gold nanowires is proposed and is confirmed with the quantum mechanical molecular dynamics simulations. The model can explain the magic number of the helical gold nanowires in the multishell structure. The reconstruction from ideal non-helical to realistic helical nanowires consists of two stages: dissociations of atoms on the outermost shell from atoms on the inner shell and slip deformations of atom rows generating (111)-like structure on the outermost shell. The elementary processes are governed by competition between energy loss and gain by s- and d-electrons together with the width of the d-band. The possibility for the helical nanowires of platinum, silver and copper is discussed.

cond-mat.mtrl-sci↗

arXiv2006

Large-scale electronic structure theory for simulating nanostructure process

Fundamental theories and practical methods for large-scale electronic structure calculations are given, in which the computational cost is proportional to the system size. Accuracy controlling methods for microscopic freedoms are focused on two practical solver methods, Krylov-subspace method and generalized-Wannier-state method. A general theory called the 'multi-solver' scheme is also formulated, as a hybrid between different solver methods. Practical examples are carried out in several insulating and metallic systems with 10^3-10^5 atoms. All the theories provide general guiding principles of constructing an optimal calculation for simulating nanostructure processes, since a nanostructured system consists of several competitive regions, such as bulk and surface regions, and the simulation is designed to reproduce the competition with an optimal computational cost.

cond-mat.mtrl-sci↗

arXiv2006

Draw out Carbon Nanotube from Liquid Carbon

Carbon nanotube (CNT) is expected for much more important and broader applications in the future, because of its amazing electrical and mechanical properties. However, today, the prospect is detained by the fact that the growth of CNTs cannot be well controlled. In particular, controlling the chirality of CNTs seems formidable to any existing growth method. In addition, a systematic method for a designed interconnected network has not been established yet, which is focused particularly in nano-electronics field. Up to now, there is a strong need for novel synthetic method that is controllable for chirality and architecture and applicable to mass production. Recently, the experimental evidences were reported for the growth of pure carbon-arc-product nanotubes from a liquid precursor. Here we show the simulation results of elongated growth process of CNT by drawing out a well-formed nanotube from liquid carbon and discuss the mechanism and suitable synthesis conditions. The process is also simulated for creation of a Y junction from two isolated CNTs as first step to CNT network. We propose this novel synthetic method as a promising one for catalyst-free mechanical design of interconnected CNT network and CNT cloning.

cond-mat.mtrl-sci↗

arXiv2005

Nanoscale structures formed in silicon cleavage studied with large-scale electronic structure alculations; surface reconstruction, step and bending

The 10-nm-scale structure in silicon cleavage is studied by the quantum mechanical calculations for large-scale electronic structure. The cleavage process on the order of 10 ps shows surface reconstruction and step formation. These processes are studied by analyzing electronic freedom and compared with STM experiments. The discussion presents the stability mechanism of the experimentally observed mode, the $(111)$-$(2 x 1)$ mode, beyond the traditional approach with surface energy. Moreover, in several results, the cleavage path is bent into the experimentally observed planes, owing to the relative stability among different cleavage modes. Finally, several common aspects between cleavage and other phenomena are discussed from the viewpoints of the nonequilibrium process and the 10-nm-scale structure.

cond-mat.mtrl-sci↗

arXiv2005

The electronic structure around As antisite near (110) surface of GaAs

The electronic structure around a single As antisite in GaAs is investigated in bulk and near the surface both in the stable and the metastable atomic configurations. The most characteristic electronic structures of As antisite is the existence of the localized p-orbitals extending from the As antisite. The major component of the highest occupied state on As antisite in the stable configuration is s-orbital connecting with neighboring As atoms with nodes whereas that in the metastable configuration is p-orbital connecting without nodes. Localized p-orbitals on the surrounding As atoms around the As antisite exist in every configuration of As antisite. Such features are retained except the case of the As antisite located just in the surface layer in which the midgap level is smeared into the conduction band and no localized states exist near the top of the valence band. Scanning tunneling microscopic images of defects observed in low-temperature grown GaAs, possibly assigned as As antisite, the origin of the metastability, and the peculiarity of the defects in the surface layer are discussed.

cond-mat.mtrl-sci↗

arXiv2004

k-dependent spectrum and optical conductivity near metal-insulator transition in multi-orbital Hubbard bands

We apply the dynamical mean field theory (DMFT) in the iterative perturbation theory(IPT) to doubly degenerate eg bands and triply degenerate tg bands on a simple cubic lattice and calculate the spectrum and optical conductivity in arbitrary electron occupation. The spectrum simultaneously shows the effects of multiplet structure and DMFT together with the electron ionization and affinity levels of different electron occupations, coherent peaks at the Fermi energy in the metallic phase and a gap at an integer filling of electrons for sufficiently large Coulomb U. We also calculate the critical value of the Coulomb U for degenerate orbitals.

cond-mat.str-el↗

arXiv2004

Krylov Subspace Method for Molecular Dynamics Simulation based on Large-Scale Electronic Structure Theory

For large scale electronic structure calculation, the Krylov subspace method is introduced to calculate the one-body density matrix instead of the eigenstates of given Hamiltonian. This method provides an efficient way to extract the essential character of the Hamiltonian within a limited number of basis set. Its validation is confirmed by the convergence property of the density matrix within the subspace. The following quantities are calculated; energy, force, density of states, and energy spectrum. Molecular dynamics simulation of Si(001) surface reconstruction is examined as an example, and the results reproduce the mechanism of asymmetric surface dimer.

cond-mat.mtrl-sci↗

arXiv2003

Dynamical brittle fractures of nanocrystalline silicon using large-scale electronic structure calculations

A hybrid scheme between large-scale electronic structure calculations is developed and applied to nanocrystalline silicon with more than 10$^5$ atoms. Dynamical fracture processes are simulated under external loads in the [001] direction. We shows that the fracture propagates anisotropically on the (001) plane and reconstructed surfaces appear with asymmetric dimers. Step structures are formed in larger systems, which is understood as the beginning of a crossover between nanoscale and macroscale samples.

cond-mat.mtrl-sci↗

arXiv2003

Million-atom molecular dynamics simulation by order-N electronic structure theory and parallel computation

Parallelism of tight-binding molecular dynamics simulations is presented by means of the order-N electronic structure theory with the Wannier states, recently developed (J. Phys. Soc. Jpn. 69,3773 (2000)). An application is tested for silicon nanocrystals of more than millions atoms with the transferable tight-binding Hamiltonian. The efficiency of parallelism is perfect, 98.8 %, and the method is the most suitable to parallel computation. The elapse time for a system of $2\times 10^6$ atoms is 3.0 minutes by a computer system of 64 processors of SGI Origin 3800. The calculated results are in good agreement with the results of the exact diagonalization, with an error of 2 % for the lattice constant and errors less than 10 % for elastic constants.

cond-mat.mtrl-sci↗

arXiv2003

Electronic Structure of Transition Metals Fe, Ni and Cu in the GW Approximation

The quasiparticle band structures of 3d transition metals, ferromagnetic Fe, Ni and paramagnetic Cu, are calculated by the GW approximation. The width of occupied 3d valence band, which is overestimated in the LSDA, is in good agreement with experimental observation. However the exchange splitting and satellite in spectra are not reproduced and it is required to go beyond the GW approximation. The effects of static screening and dynamical correlation are discussed in detail in comparison with the results of the static COHSEX approximation. The dynamical screening effects are important for band width narrowing.

cond-mat.mtrl-sci↗

arXiv2002

Electronic structure of the MO oxides (M=Mg, Ca, Ti, V) in the GW approximation

The quasiparticle band structures of nonmagnetic monoxides, MO (M=Mg, Ca, Ti, and V), are calculated by the GW approximation. The band gap and the width of occupied oxygen 2p states in insulating MgO and CaO agree with experimental observation. In metallic TiO and VO, conduction bands originated from metal 3d states become narrower. Then the partial densities of transition metal e_g and t_2g states show an enhanced dip between the two. The effects of static screening and dynamical correlation are discussed in detail in comparison with the results of the Hartree-Fock approximation and the static Coulomb hole plus screened exchange approximation. The d-d Coulomb interaction is shown to be very much reduced by on-site and off-site d-electron screening in TiO and VO. The dielectric function and the energy loss spectrum are also presented and discussed in detail.

cond-mat.mtrl-sci↗