48 research outputs found

### First-Principles Study on Electron-Conduction Properties of C$_{60}$ Chains

The electron-conduction properties of fullerene chains are examined by
first-principles calculations based on the density functional theory. The
conductivity of the C$_{60}$ dimer is low owing to the constraint of the
junction of the molecules on electron conduction, whereas the C$_{60}$ monomer
exhibits a conductance of $\sim$ 1 G$_0$. One of the three degenerate $t_{u1}$
states of C$_{60}$ is relevant to conduction and the contributions of the
others are small. In addition, we found a more interesting result that the
conductance of the fullerene chain is drastically increased by encapsuling
metal atoms into cages.Comment: 10pages and 5 figure

### 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 $s$-$d_{z^2}$ 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 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$_2$ 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$_2$
model is insensitive to the DB state

### First-principles study on dielectric properties of NaCl crystal and ultrathin NaCl films under finite external electric field

We present a first-principles study on the dielectric properties of an NaCl
crystal and ultrathin NaCl films under a finite external electric field. Our
results show that the high-frequency dielectric constant of the films is not
affected by the finite size effect from crystal surfaces and is close to that
of the crystal, whereas the static one is sensitive to the thickness of the
film due to the difference in the atomic configurations between the surface and
inside of the film.Comment: 11 pages and 4 figure

### 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

### First-principles study on scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces

Scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
are studied using first-principles calculations. Our results show that the
calculated filled-state images and local density of states are consistent with
recent experimental results, and the empty-state images appear significantly
different from the filled-state ones. To elucidate the origin of this
difference, we examined in detail the local density of states, which affects
the images, and found that the bonding and antibonding states of surface
silicon atoms largely affect the difference between the filled- and empty-state
images.Comment: 4 pages, and 4 figure