Shinnosuke Furuya

Absence of Dirac Electrons in Silicene on Ag (111) Surfaces

We report first-principles calculations that clarify stability and electronic structures of silicene on Ag(111) surfaces. We find that several stable structures exist for silicene/Ag(111), exhibiting a variety of images of scanning tunneling microscopy. We also find that Dirac electrons are absent near Fermi level in all the stable structures due to buckling of the Si monolayer and mixing between Si and Ag orbitals. This is the first theoretical investigation that clarify the absence of Dirac electrons in silicene due to the strong interaction with substrates. We instead propose that either BN substrate or hydrogen-processed Si surface is a good candidate to preserve Dirac electrons in silicene.

Ab Initio Calculation of the Electric Properties of Al Atomic Chains under Finite Bias Voltages

We have analyzed electric conduction through Al atomic chains between two jellium electrodes by ab initio calculations, focusing on their dependences on bias voltages and the number of atoms. We have found that the electric current increases more slowly at high bias voltages (Vbias>3 V) as the number of atoms increases, whereas the current is roughly independent of the number of atoms at low bias voltages. This result can be understood if we assume that resonant conduction through states spreading over the entire chain is destroyed in the cases of Al4 and Al5 at high bias voltages.

Efficient Algorithm for Linear Systems Arising in Solutions of Eigenproblems and Its Application to Electronic-Structure Calculations

We consider an eigenproblem derived from first-principles electronic-structure calculations. Eigensolvers based on a rational filter require solutions of linear systems with multiple shifts and multiple right hand sides for transforming the spectrum. The solutions of the linear systems are the dominant part of the eigensolvers. We derive an efficient algorithm for such linear systems, and develop implementation techniques to reduce time-consuming data copies in the algorithm. Several experiments are performed on the K computer to evaluate the performance of our algorithm.

Ab initio calculation of capacitance of semi-infinite jellium electrodes with a nanoscale gap

We have examined the capacitance of two parallel jellium electrodes with a nanoscale gap using an ab initio method within the density functional theory. As the gap becomes smaller, first the calculated capacitance exhibits behavior qualitatively similar to that estimated using classical electrostatics, but is systematically larger than the classical one. Then, the calculated capacitance decreases when the gap approaches zero. The value of the maximum capacitance is larger for a smaller Wigner-Seitz radius of the electrodes. These results can be understood from two quantum effects, namely, electron spill from the electrodes and tunneling between the electrodes.

First-Principles Study of Apparent Barrier Height

We have analyzed apparent barrier height measured by scanning tunneling microscopy (STM), using calculations within the density functional theory including the effects of electric fields and currents. In particular, we have compared the apparent barrier height of an Al(100) surface containing a vacancy cluster in the second layer with that of a surface without defects. We have found that the site with the defect, appearing as hillocks in the STM topographic image, is imaged as dark spots in the apparent barrier height image. These results can be attributed to two factors, the difference of the density of states (DOS) near EF and the difference of the decay rate of electron density around the sample surface.

First-Principles Calculation of Vibrational Properties of a Nanostructure in Electric Fields

We have studied the vibrational properties of a nanostructure, a single Al atom on an Al(100) surface, in strong electric fields of 5 V/nm, using first-principles density functional calculation taking account of the semi-infinite electrode and applied electric field. We have found a linear response on the change of the stable position of the Al adatom when different electric fields are applied. We have also found that the force acting on the adatom is nearly proportional to its displacement when only displacements in the direction normal to the surface are considered. The estimated vibrational frequency of the stretching mode of the adatom is of the order of ~1013 Hz, and hardly depends on the intensity of the electric field. We have also found that structural relaxation causes a considerable change in field emission currents.

Electron Confinement Due to Stacking Control of Atomic Layers in SiC Polytypes: Roles of Floating States and Spontaneous Polarization

We report on first-principles total-energy electronic-structure calculations that clarify the stability and electronic structures of heterocrystalline superlattices consisting of SiC polytypes. The calculated local density of states unequivocally reveals substantial effects of spontaneous polarization in hexagonal polytypes. The polarization in the hexagonal region renders the band lineup slanted in real space along the stacking direction in the superlattice; furthermore, the counterpolarization in the cubic region makes it slanted in the reverse direction. We find that electrons are confined near the interface in the cubic region and that holes are under a negligible band offset. We also find that the slanted band lineup causes a downward (upward) shift of the conduction (valence) band edge and the band gap becomes narrower than that in the bulk polytype, offsetting the band gap increase due to the quantum confinement. The calculated Kohn–Sham orbitals of the conduction band bottoms distribute not at ato...

Theoretical Analysis of Apparent Barrier Height on an Al Surface: Difference by Measurement Methods

We have analyzed the difference in apparent barrier height (ABH) by the approach and modulation methods, using the boundary-matching scattering-state density functional method, in which electric states under an applied bias voltage are self-consistently calculated. We found that the approach and modulation methods provide nearly the same calculated ABH values for a wide range of tunneling conductances from 10-11 to 10-5 Ω-1. The change in ABH in the tunneling conductance range from 10-11 to 10-7 Ω-1 is smaller than that in the tunneling conductance range from 10-7 to 10-5 Ω-1, suggesting that some of the observed discrepancies between the two methods may be due to the difference in tip-sample distance. Furthermore, we also found that the change in modulation amplitude, which is caused by the force acting on the tip atom due to the applied bias voltage, can account for the observation that the modulation method provides a smaller ABH value than the approach method.

Dependence of Electric Properties of Al Atomic Chains on Structure of Chain–Electrode Junction

We carried out ab initio calculations of the electric properties of Al atomic chains connected to electrodes by the boundary-matching scattering-state density functional method developed by our group. We examined two adsorption sites on Al(001), hollow and on-top sites, and found that the current–voltage characteristics of the chain are significantly different between the two sites. We also found that the current–voltage characteristics, potential shift and energy shift of the peak of the local density of states are correlated with one another.

Ab initio calculation of stable structures of a Na atomic chain under bias voltages

Abstract The stable geometries of a three-atom Na chain connected to two semi-infinite jellium electrodes are studied under bias voltages by performing ab initio force calculations within the density functional theory. At a low bias voltage of 0.01 V, the stable geometry is found to be symmetric, while it becomes asymmetric for higher bias voltages. The displacements of Na atoms in the chain are proportional to the applied bias voltages up to 1 V, while their behavior changes drastically above 1 V. These results can be understood from the behavior of charges induced by the applied voltage. It is also found that the structural relaxation due to the bias voltage also affects the potential drop along the chain and the current—voltage characteristics of the chain.