First-Principles Study on Electron-Conduction Properties of C60_{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

Quantum Transport Calculations Using Periodic Boundary Conditions

An efficient new method is presented to calculate the quantum transports using periodic boundary conditions. This method allows the use of conventional ground state ab initio programs without big changes. The computational effort is only a few times of a normal ground state calculation, thus it makes accurate quantum transport calculations for large systems possible.Comment: 9 pages, 6 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

New structural model for GeO2/Ge interface: A first-principles study

First-principles modeling of a GeO2/Ge(001) interface reveals that sixfold GeO2, which is derived from cristobalite and is different from rutile, dramatically reduces the lattice mismatch at the interface and is much more stable than the conventional fourfold interface. Since the grain boundary between fourfold and sixfold GeO2 is unstable, the sixfold GeO2 forms a large grain at the interface. On the contrary, a comparative study with SiO2 demonstrates that SiO2 maintains a fourfold structure. The sixfold GeO2/Ge interface is shown to be a consequence of the ground-state phase of GeO2. In addition, the electronic structure calculation reveals that sixfold GeO2 at the interface shifts the valence band maximum far from the interface toward the conduction band.Comment: 18 pages, 5 figures, and 2 table

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 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

Transport properties in network models with perfectly conducting channels

We study the transport properties of disordered electron systems that contain perfectly conducting channels. Two quantum network models that belong to different universality classes, unitary and symplectic, are simulated numerically. The perfectly conducting channel in the unitary class can be realized in zigzag graphene nano-ribbons and that in the symplectic class is known to appear in metallic carbon nanotubes. The existence of a perfectly conducting channel leads to novel conductance distribution functions and a shortening of the conductance decay length.Comment: 4 pages, 6 figures, proceedings of LT2

Unsolved problems in the lowermost mantle

Many characteristics of D '' layer may be attributed to the recently discovered MgSiO3 post-perovskite phase without chemical heterogeneities. They include a sharp discontinuity at the top of D '', regional variation in seismic anisotropy, and a steep Clapeyron slope. However, some features remain unexplained. The seismically inferred velocity jump is too large in comparison to first principles calculations, and the sharpness of the discontinuity may require a chemical boundary. Chemical heterogeneity may play an important role in addition to the phase transformation from perovskite to post-perovskite. Phase transformation and chemical heterogeneity and the attendant changes in physical properties, such as rheology and thermal conductivity, are likely to play competing roles in defining the dynamical stability of the D '' layer. Revealing the relative roles between phase transition and chemical anomalies is an outstanding challenge in the study of the role of D '' in thermal-chemical evolution of the Earth