In this article, we review our recent activities of quantum transport simulations from atomistic levels, especially focusing on the multi-scale viewpoints. First we present the first-principles electron transport calculations based on the density-functional theory (DFT) and the non-equilibrium Green's functional theory (NEGF). Then we present our recent development of an order-N transport calculation method, called as the time-dependent wave-packet diffusion method (TD-WPD), which is based on the linear-response Kubo formula and is applicable for up to 100 million atomic systems. As an example, we show some results on the transport of CNT.
We review effects of an external electric field on the magnetocrystalline anisotropy (MCA) in transition-metal monolayers and surfaces, and at metal-insulator interfaces, as obtained from the first-principles full-potential linearized augmented plane-wave method. The results indicate that the MCA can be modified by the electric field through a change in band structure around Fermi level, which originates from the redistribution of charge/spin density and/or atomic displacements at surfaces and interfaces. These findings greatly advance our understanding of the electric-field-induced MCA modifications in itinerant ferromagnets.
Electronic structure study under an external electric field is one of the important subjects in the electronic device development and sustainable energy technologies. We developed a calculation method, called effective screening medium (ESM) method, to include the external electric field or bias potential within the density functional theory. In this article, the details of the ESM method are explained and two recent researches are reviewed. First, a simulation of the electrochemical reaction on a metal-water interface is reviewed. We have elucidated an electronic state modulation of metal surface during the reaction. Second, a study about the electronic structure of an ABC-stacked graphite thin film with or without an external electric field is reviewed. We have found that ferrimagnetic spin polarization occurs on the both surface sides of the thin film. The polarization is associated with the peculiar surface localized state that possesses the same characteristics as the edge-localized states of graphene nanoribbons. The spin polarization is, therefore, controllable by applying an external electric field.
Non-equilibrium dynamics induced by the femtosecond laser shot has attracted high attentions in condensed matter community. Interaction between a strong field of light and materials cannot be treated by conventional perturbation theory that assumes both electron wave functions and ionic positions as insensitive to the field. Direct approach for computing real-time propagations of electron wave packet and ionic positions under presence of strong optical field is one of promising solutions. In this review, I present theoretical approaches based on the time-dependent density functional theory and show some examples of ultra-fast dynamics in materials induced by the femtosecond laser shots: exfoliation of graphene layer from graphite and photochemical reaction of molecules encapsulated inside carbon nanotubes.
We review a real-time computational method for photoionization in molecules based on the time-dependent density-functional theory in linear response regime. In the method, an impulsive external field is applied to a molecule, and the induced electric polarization in the molecule is calculated as a function of time. Then the frequency-dependent polarizability, which characterizes linear optical properties of molecules, is obtained as the ratio between Fourier transform of the induced polarization and that of the applied impulsive field. After presenting the formalism, we explain practical aspects of the procedure taking ethylene and fullerene molecules as examples. We also explain extensions of the real-time formalism for other optical properties such as magnetic circular dichroism, a linear optical property of molecules under a static magnetic field, and dielectric function of periodic bulk materials.
Recent status of optical response spectra calculations is reviewed, illustrating various bulk, surface, and interface systems. One can obtain fruitful information on atomic and electronic structures using the freedom of light such as energy (wave length) and polarization. Moreover, by explaining the spectral shape origin in reflectance difference spectroscopy, we show that the optical response of surface and interface has quite different features and needs different calculation methods from those of bulk systems, reflecting the breakdown of translational symmetry.
The electronic properties of transition metal oxide (TMO) films in resistance random access memory (ReRAM) devices have been gaining research interest in relation to their relevant application in the field of non-volatile memory devices. TMO films sandwiched between two metallic electrodes in ReRAM devices have the ability to switch its electronic properties between insulator and metal upon being subjected to pulse voltages. However, this switching mechanism has not been fully clarified. As part of this big endeavor to fully elucidate this switching mechanism in ReRAM, we introduce, in this review, the proposed effect of electric field on the metal electrode-TMO interface's electronic properties. We use the Ta/CoO/Ta system in our investigation and evaluated its density of states (DOS) to analyze changes in the bandgap of the interface layers in the presence of an electric field. Analysis shows that the presence of the electric field causes a change in the bandgap of the TMO interface layer near the anode side while no change have been seen at the interface layer near the cathode side. This presumes that the anode side has an important role in switching mechanism as compared with the cathode side.