Mamoru Sakaue

A first-principles study on defect association and oxygen ion migration of Sm3+ and Gd3+ co-doped ceria

First-principles calculations based on density functional theory were performed to investigate the co-doping effects of Sm and Gd in ceria on its oxygen ion conduction. The focus of this study is on the interactions between the cation dopants and an oxygen vacancy within the two adjacent tetrahedral sites of fluorite structure surrounding the oxygen migration path. Vacancy formation energies, dopant-vacancy association energies, and migration energies were calculated to elucidate the doping effects on oxygen ion conduction. The migration energies show remarkable dependences on the ionic radii of the cations located at the edges of the migration path. A simple relation between migration energy and vacancy formation energy is proposed. This work provides an informative insight into vacancy diffusion that could be useful in optimizing doping materials for improving oxygen ion conductivity in doped ceria.

Theory of time-resolved two-photon photoemission from a metal surface: the effect of Coulomb interactions between electrons

The effect of Coulomb interactions between electrons on time-resolved two-photon photoemission from a metal surface is investigated theoretically. It is shown by nonequilibrium perturbation theory that the correlation trace (photoelectron intensity as a function of the pump-probe delay time) depends on lifetimes of both photoexcited electrons and holes due to the effect of Coulomb interactions. The numerical results of the correlation trace reproduce the qualitative features of the experimental results.

The Effect of Interband Excitations on Time-Resolved Two-Photon Photoemission via a Localized State at a Metal Surface

The effect of interband excitations on time-resolved two-photon photoemission from an adsorbate-surface system is investigated. The system is assumed to be composed of two subbands in the bulk and a localized state of the adsorbate, which is in resonance with the upper energy subband. In the two-photon photoemission process, first, an electron in the lower energy subband is excited into the localized state by the pump photon, and subsequently excited above the vacuum level by the probe photon. The first excitation can be divided into two processes, i.e., the direct and the indirect one involving the interband excitation. The excitation probability due to the latter process reaches a maximum at a certain photon energy because of momentum conservation. A two-photon photoemission spectrum shows a peak due to the indirect process at an energy position where the initial state is fixed, as well as one due to the direct process where the intermediate state is fixed. The maximum of the cross-correlation trace of ...

Analysis of structural and electronic properties of Pr2NiO4 through first-principles calculations

The structural and electronic properties of bulk Pr(2)NiO(4+δ) (δ = 0 and 0.031) were analyzed using first-principles calculations based on the density functional theory (DFT) for application to electrode materials in solid-oxide fuel cells (SOFCs). Two structures of Pr(2)NiO(4) were analyzed: one in space group I4/mmm associated with the high temperature tetragonal (HTT) structure, and the other in Bmab with the low temperature orthorhombic (LTO) structure. The main difference between the two structures is the pronounced tilting of the nickelate octahedra found in the Bmab structure. Here, we will show that the difference in the electronic properties between the two structures, i.e. half-metallic for the I4/mmm structure and metallic for the Bmab structure, is attributed to the tilting of the nickelate octahedra. Furthermore, we found that the presence of interstitial O atoms at the Pr(2)O(2) bilayers is responsible for the tilting of the octahedra and thus is a dominant factor in the transition from the I4/mmm structure to the Bmab structure. These results would be of great significance to materials design related to the enhancement of O diffusivity in this material.

Theory of dynamics of electron wave packets in time-resolved two-photon photoemission via image states

Time-resolved two-photon photoemission (TR2PPE) from the Cu(100) surface is investigated by the Keldysh Green function method in order to analyze ultrafast dynamics of an electron wave packet in the image states of the surface. By numerical analysis, the quantum beats in the TR2PPE spectrum due to interference among the image states are reproduced, and the motion of the electron wave packet in front of the surface is demonstrated. It is discussed on the basis of the obtained results as to how the motion of the electron wave packet is evaluated from the TR2PPE spectrum.

Interplay between Plasmon Luminescence and Vibrationally Resolved Molecular Luminescence Induced by Scanning Tunneling Microscopy

The effects of coupling between a molecular exciton and a surface plasmon (exciton–plasmon coupling) on the luminescence properties of the molecule and surface plasmons are investigated using the nonequilibrium Greens function method. Molecular absorption and enhancement by molecular electronic and vibrational modes (molecular modes) lead to dip and peak structures in the luminescence spectra of the surface plasmons. These structures will correspond to the peak and dip structures observed in a recent experiment. In addition to the molecular dynamics, it is found that the reabsorption by surface plasmons plays an important role in determining the luminescence spectral profiles.

Effects of Interference between Energy Absorption Processes of Molecule and Surface Plasmons on Light Emission Induced by Scanning Tunneling Microscopy

A theoretical analysis of plasmon and molecular luminescence induced by scanning tunneling microscopy is carried out in the case of luminescence from clean metal surfaces using a molecule-covered tip and from molecular layers. The effects of coupling between molecular exciton and surface plasmon (exciton–plasmon coupling) on the luminescence properties of the molecule and surface plasmons are investigated using the nonequilibrium Greens function method. The enhancement by molecular electronic and vibrational modes (molecular modes) and the energy absorption by the creation of molecular excitons lead to peak and dip structures in the luminescence spectra of surface plasmons. The re-emission of energy in surface plasmons by exciton annihilation leads to a dent structure in their luminescence spectra. It is found that the processes of energy absorption by the creation of molecular excitons and the subsequent re-emission by exciton annihilation interfere with each other, resulting in the enhancement and supp...

Controllability of Electrical Conductivity by Oxygen Vacancies and Charge Carrier Trapping at Interface between CoO and Electrodes

Recently, the role of resistance random access memory (RRAM) is becoming extremely important in the development of nonvolatile memories. RRAM works by changing the resistance of the transition metal oxide contained in RRAM after the application of a sufficiently high voltage, however, this switching mechanism has not been fully clarified. In this study, by performing first principles calculations based on the density functional theory, we first investigate the change in the property of bulk CoO resulting from oxygen vacancies and charge carrier trapping in the vicinity of the oxygen vacancies. Next, we perform calculations for slab models of CoO in contact with Ta, W, and Pt electrodes and hence investigate the effects of oxygen vacancies at the interface between the CoO layer and the electrode layer. On the basis of the obtained results, we conclude that W is the most suitable electrode material compared with Ta and Pt.

Effect of Relaxation of Secondary Electrons and Holes on Time-Resolved Two-Photon Photoemission from Cu(111)

We investigate the time-resolved two-photon photoemission (time-resolved 2PPE, TR2PPE) spectrum of the image state on Cu(111) by nonequilibrium perturbation theory. We show that, in the 2PPE process, the scattering of photoexcited quasiparticles (electrons and holes) due to Coulomb interactions between electrons causes excitation of secondary quasiparticles in the vicinity of the Fermi level as well as electron transitions to the image state. We demonstrate quantitatively on the basis of numerical results how the effect of relaxation of secondary quasiparticles is manifested in the TR2PPE spectrum.

Transient Population of Photoexcited Electrons in a Localized State at a Metal Surface

A simple model is introduced to investigate the temporal evolution of the density of electrons which are excited from a metal substrate to a localized state at the surface by a laser pulse. The electron system consists of two subbands in the substrate and a localized state at the surface, and the localized state is in resonance with the upper subband. The obtained expression for the electron density in the localized state indicates that the excitation process can be divided into two, i.e., direct and indirect processes. In the latter, the transition probability reaches a maximum at a certain photon energy ( absorption peak energy ) because of the momentum conservation. This process contributes substantially to the electron density and hence strongly affects its temporal evolution at the incident photon energy around the absorption peak energy .