4,128 research outputs found
A hierarchical research by large-scale and ab initio electronic structure theories -- Si and Ge cleavage and stepped (111)-2x1 surfaces --
The ab initio calculation with the density functional theory and plane-wave
bases is carried out for stepped Si(111)-2x1 surfaces that were predicted in a
cleavage simulation by the large-scale (order-N) electronic structure theory
(T. Hoshi, Y. Iguchi and T. Fujiwara, Phys. Rev. B72 (2005) 075323). The
present ab initio calculation confirms the predicted stepped structure and its
bias-dependent STM image. Moreover, two (meta)stable step-edge structures are
found and compared. The investigation is carried out also for Ge(111)-2x1
surfaces, so as to construct a common understanding among elements. The present
study demonstrates the general importance of the hierarchical research between
large-scale and ab initio electronic structure theories.Comment: 5 pages, 4 figures, to appear in Physica
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.Comment: 8 pages, 3 figures. To appear in J.Phys. Condens. Matter. A preprint
PDF file in better graphics is available at
http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm
Magnetization of a half-quantum vortex in a spinor Bose-Einstein condensate
Magnetization dynamics of a half-quantum vortex in a spin-1 Bose-Einstein
condensate with a ferromagnetic interaction are investigated by mean-field and
Bogoliubov analyses. The transverse magnetization is shown to break the
axisymmetry and form threefold domains. This phenomenon originates from the
topological structure of the half-quantum vortex and spin conservation.Comment: 6 pages, 3 figure
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 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.Comment: 5 pages, 3 figure
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.Comment: 19 pages, 6 figures. To appear in J. Phys. Cond. Matt. A preprint PDF
file in better graphics is available at
http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm
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.Comment: 7 pages, 3 figures; corrected typos; to be published in Journal of
the Phys. Soc. of Japa
Spin accumulation created electrically in an n-type germanium channel using Schottky tunnel contacts
Using high-quality FeSi/-Ge Schottky-tunnel-barrier contacts, we
study spin accumulation in an -type germanium (-Ge) channel. In the
three- or two-terminal voltage measurements with low bias current conditions at
50 K, Hanle-effect signals are clearly detected only at a forward-biased
contact. These are reliable evidence for electrical detection of the spin
accumulation created in the -Ge channel. The estimated spin lifetime in
-Ge at 50 K is one order of magnitude shorter than those in -Si reported
recently. The magnitude of the spin signals cannot be explained by the commonly
used spin diffusion model. We discuss a possible origin of the difference
between experimental data and theoretical values.Comment: 4 pages, 3 figures, To appear in J. Appl. Phy
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