arXiv subjects

Takeo Hoshi

Publications and source records attributed to Takeo Hoshi.

46 records · Page 3Linked to original sources

arXiv2007

Accuracy control in ultra-large-scale electronic structure calculation

Numerical aspects are investigated in ultra-large-scale electronic structure calculation. Accuracy control methods in process (molecular-dynamics) calculation are focused. Flexible control methods are proposed so as to control variational freedoms, automatically at each time step, within the framework of generalized Wannier state theory. The method is demonstrated in silicon cleavage simulation with 10^2-10^5 atoms. The idea is of general importance among process calculations and is also used in Krylov subspace theory, another large-scale-calculation theory.

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↗

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

Large-scale electronic structure calculation and its application

Several methodologies are developed for large-scale atomistic simulations with fully quantum mechanical description of electron systems. The important methodological concepts are (i) generalized Wannier state, (ii) Krylov subspace and (iii) hybrid scheme within quantum mechanics. Test calculations are done with upto 10$^6$ atoms using a standard workstation. As a practical nanoscale calculation, the dynamical fracture of nanocrystalline silicon was simulated.

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↗

arXiv2000

Theory of composite-band Wannier states and order-N electronic-structure calculations

From the order-N electronic-structure formulation, a Hamiltonian is derived, of which lowest eigen state is the generalized or composite-band Wannier state. This Hamiltonian maps the locality of the Wannier state to that of a virtual impurity state and to a perturbation from a bonding orbital. These theories are demonstrated in the diamond-structure solids, where the Wannier states are constructed by a practical order-N algorithm with the Hamiltonian. The results give a prototypical picture of the Wannier states in covalent-bonded systems.

cond-mat.mtrl-sci↗