Yoshiyuki Egami

Even-odd oscillation in conductance of a single-row sodium nanowire

We present a first-principles calculation of the electronic conduction properties of single-row sodium nanowires suspended between semi-infinite electrodes. The conductance of the nanowire is ~1 G0 (=2e^2/h) and oscillates with a two-atom period as the number of the atoms within the nanowire varies. Moreover, we observed bunches of high electron density with a two atom-lengths in the channel density distribution. The relation between the period of the conductance oscillation and the length of bunches are examined by using simplified models and is found to be largely affected by the characteristics of the infinite wire.

Time-saving first-principles calculation method for electron transport between jellium electrodes.

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(z²) 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.

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.

Real-space calculations for electron transport properties of nanostructures

Recent developments in the fabrication and investigation of conductors of atomic dimensions have stimulated a large number of experimental and theoretical studies on these nanoscale devices. In this paper, we introduce examples presenting the efficiencies and advantages of a first-principles transport calculation scheme based on the real-space finite-difference (RSFD) formalism and the overbridging boundary-matching (OBM) method. The RSFD method does not suffer from the artificial periodicity problems that arise in methods using plane-wave basis sets or the linear dependence problems that occur in methods using atomic basis sets. Moreover, the algorithm of the RSFD method is suitable for massively parallel computers and, thus, the combination of the RSFD and OBM methods enables us to execute first-principles transport calculations using large models. To demonstrate the advantages of this method, several applications of the transport calculations in various systems ranging from jellium nanowires to the tip and surface system of scanning tunneling microscopy are presented.

First-principles study of the electronic structures and dielectric properties of the Si/SiO2 interface

A first-principles study of the electronic structures and dielectric properties of Si/SiO(2) interfaces is implemented. Comparing the interfaces with and without defects, we explore the relationship between the defects and the dielectric properties, and also discuss the effect of the defects on the leakage current between the gate electrode and silicon substrate. We found that the electrons around the Fermi level percolate into the SiO(2) layers, which reduces the effective oxide thickness and is expected to enhance the leakage current. The dangling bonds largely affect the dielectric properties of the interface and the termination of dangling bonds by hydrogen atoms is successful in suppressing the increase of the dielectric constant.

First-principles calculation of transport properties of single-row aluminium nanowires suspended between semi-infinite crystalline electrodes

We present first-principles calculations based on the density-functional theory for electronic structures and conductances of single-row aluminium nanowires attached to semi-infinite crystalline electrodes. The nanowire does not exhibit conductance quantization, which is consistent with experiments. The resultant conductance of is in agreement with that of other first-principles studies. The transmissions and the local density of states sensitively vary depending on the energy of incident electrons, which supports more irregular conductance of the aluminium nanowire than those of gold and other monovalent metals.

A coherent relation between structure and conduction of infinite atomic wires

We demonstrate a theoretical analysis concerning the geometrical structures and electrical conduction of infinite monatomic gold and aluminium wires in the process of their elongation, based on first-principles molecular-dynamics simulations using the real-space finite-difference method. Our study predicts that the single-row gold wire ruptures up to form a dimer coupling structure when the average interatomic distance increases up to more than 3.0 A, and that the wire is conductive before breaking but changes to an insulator at the rupturing point. In the case of the aluminium wire, it exhibits a magnetic ordering due to the spin polarization, and even when stretched up to the average interatomic distance of 3.5 A, a dimerization does not occur and the wire keeps a metallic nature.

First-principles calculation method for electron transport based on the 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 Greens 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 Greens 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 jellium electrodes. The results confirm that the leakage current through the (001)Si-SiO_{2} model becomes much larger when the dangling-bond state is induced by a defect in the oxygen layer, while that through the (001)Ge-GeO_{2} model is insensitive to the dangling bond state.

First-Principles Study on Dynamic Electron-Transport Property through Low Dimensional System

Recently, several first-principles studies on the electron-transport property of low dimensional nanomaterials, such as graphene sheets, nanotubes, and atomic or molecular chains, have been demonstrated. However, almost of them discuss the transport properties in the steady state, and there remains a lot of uncertainty on the dynamics of electrons flowing through the materials. In this study, we examined the dynamic transport properties of the nanomaterial suspended between semi-infinite electrodes. The dynamic behavior of electrons are simulated by the impulse response (IR) method [2] based on the real-space finite-difference approach [1] within the framework of the time-dependent density functional theory. It is reported that a phenyl chain absorbed on the gold substrate with thiol anchor groups exhibits two types of conduction channels in our previous study using a static transport property simulator [3]. Furthermore, adopting the IR method to phenyl chain systems, we found one channel can be ignored at a finite temperature since the response time of the channel is quietly slow (Fig. 1) compared with the period of intramolecular vibration mode at ~THz. Other applications of the IR method will be presented in the conference. Static IR

First-principles study on quantum-transport properties of single molecule depending on adsorption conditions

We present a first-principles electron-transport simulation within the framework of the density functional theory for a 1,4-benzenedithiol molecule suspended between semi-infinite Au electrodes. The transport properties are demonstrated under the several adsorption conditions. It is found that the conducting electrons have two types of resonant-tunneling transport properties with different responses to changes in adsorption conditions.