57 research outputs found
Fully spin-dependent transport of triangular graphene flakes
The magnetic moment and spin-polarized electron transport properties of
triangular graphene flakes surrounded by boron nitride sheets (BNC structures)
are studied by using first-principles calculations based on density functional
theory. Their dependence on the BNC structure is discussed, revealing that
small isolated graphene flakes have large magnetic moment. When the BNC
structure is suspended between graphene electrodes, the spin-polarized charge
density distribution accumulates at the edge of the graphene flakes and no spin
polarization is observed in the graphene electrodes. We also found that the BNC
structure demonstrates perfectly spin-polarized transport properties in the
wide energy window around the Fermi level. Our first-principles results
indicate that the BNC structure provides new possibilities to electrically
control spin
First-principles calculation method for electron transport based on grid Lippmann-Schwinger equation
We develop a first-principles electron-transport simulator based on the
Lippmann--Schwinger (LS) equation within the framework of the real-space
finite-difference scheme. In our fully real-space based LS (grid LS) method,
the ratio expression technique for the scattering wave functions and the
Green's function elements of the reference system is employed to avoid
numerical collapse. Furthermore, we present analytical expressions and/or
prominent calculation procedures for the retarded Green's function, which are
utilized in the grid LS approach. In order to demonstrate the performance of
the grid LS method, we simulate the electron-transport properties of the
semiconductor/oxide interfaces sandwiched between semi-infinite metal
electrodes. The results confirm that the leakage current through the
(001)Si/SiO model becomes much larger when the dangling-bond (DB) state is
induced by a defect in the oxygen layer while that through the (001)Ge/GeO
model is insensitive to the DB state
Efficient calculation of the Green's function in scattering region for electron-transport simulations
We propose a first-principles method of efficiently evaluating
electron-transport properties of very long systems. Implementing the recursive
Green's function method and the shifted conjugate gradient method in the
transport simulator based on real-space finite-difference formalism, we can
suppress the increase in the computational cost, which is generally
proportional to the cube of the system length to a linear order. This enables
us to perform the transport calculations of double-walled carbon
nanotubes~(DWCNTs) with 196,608 atoms. We find that the conductance spectra
exhibit different properties depending on the periodicity of doped impurities
in DWCNTs and they differ from the properties for systems with less than 1,000
atoms.Comment: 13 pages, 5 figures, 1 tabl
Novel time-saving first-principles calculation method for electron-transport properties
We present a time-saving simulator within the framework of the density
functional theory to calculate the transport properties of electrons through
nanostructures suspended between semi-infinite electrodes. By introducing the
Fourier transform and preconditioning conjugate-gradient algorithms into the
simulator, a highly efficient performance can be achieved in determining
scattering wave functions and electron-transport properties of nanostructures
suspended between semi-infinite jellium electrodes. To demonstrate the
performance of the present algorithms, we study the conductance of metallic
nanowires and the origin of the oscillatory behavior in the conductance of an
Ir nanowire. It is confirmed that the - channel of the Ir nanowire
exhibits the transmission oscillation with a period of two-atom length, which
is also dominant in the experimentally obtained conductance trace
First-principles transport calculation method based on real-space finite-difference nonequilibrium Green's function scheme
We demonstrate an efficient nonequilibrium Green's function transport
calculation procedure based on the real-space finite-difference method. The
direct inversion of matrices for obtaining the self-energy terms of electrodes
is computationally demanding in the real-space method because the matrix
dimension corresponds to the number of grid points in the unit cell of
electrodes, which is much larger than that of sites in the tight-binding
approach. The procedure using the ratio matrices of the overbridging
boundary-matching technique [Phys. Rev. B {\bf 67}, 195315 (2003)], which is
related to the wave functions of a couple of grid planes in the matching
regions, greatly reduces the computational effort to calculate self-energy
terms without losing mathematical strictness. In addition, the present
procedure saves computational time to obtain Green's function of the
semi-infinite system required in the Landauer-B\"uttiker formula. Moreover, the
compact expression to relate Green's functions and scattering wave functions,
which provide a real-space picture of the scattering process, is introduced. An
example of the calculated results is given for the transport property of the BN
ring connected to (9,0) carbon nanotubes. The wave function matching at the
interface reveals that the rotational symmetry of wave functions with respect
to the tube axis plays an important role in electron transport. Since the
states coming from and going to electrodes show threefold rotational symmetry,
the states in the vicinity of the Fermi level, whose wave function exhibits
fivefold symmetry, do not contribute to the electron transport through the BN
ring.Comment: 34 page
Theoretical study on tunneling current flowing between STM tip and acetylene-adsorbed Si(001) surface
We present first-principles electron-transport studies on scanning tunneling microscopy (STM) images of an acetylene-adsorbed Si(001) surfaces. In the simulated STM images, bright spots are observed in bare Si atoms and darker spots locates on the absorbed acetylene molecule. Tunneling current flowing between the sample and a STM tip is strongly affected by the π states of bare Si atoms and the contributions of states around the molecule to the current are smaller. This causes the STM images where bare Si atoms look higher than the adsorbed molecule.Nagasaki Symposium on Nano-Dynamics 2008 (NSND2008) 平成20年1月29日(火)於長崎大学 Invited Lectur
Ab initio Study on Electron-transport Properties of Graphene Sheet Depending on Transport Length
ナノダイナミクス国際シンポジウム 平成22年1月21日(木) 於長崎大学Nagasaki Symposium on Nano-Dynamics 2010 (NSND2010), January 21, 2010, Nagasaki University, Nagasaki, Japan, Invited Lectur
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