Ryu Takayama

Krylov subspace method for molecular dynamics simulation based on large-scale electronic structure theory

For large scale electronic structure calculation, the Krylov subspace method is introduced to calculate the one-body density matrix instead of the eigenstates of given Hamiltonian. This method provides an efficient way to extract the essential character of the Hamiltonian within a limited number of basis set. Its validation is confirmed by the convergence property of the density matrix within the subspace. The following quantities are calculated; energy, force, density of states, and energy spectrum. Molecular dynamics simulation of Si(001) surface reconstruction is examined as an example, and the results reproduce the mechanism of asymmetric surface dimer.

Excitation Spectra of the Anderson Model with Charge Screening through d- f Coulomb Interaction

Excitation spectra of the extended impurity Anderson model that includes the intra-atomic d - f Coulomb screening are studied based on the numerical renormalization group method. The low-energy fixed point is usually classified as the local Fermi liquid state. There is a regime in which the single particle and the magnetic excitation spectra can be scaled by the effective Kondo temperature, T K , which is reduced due to the screening effect. In this regime, the optical conductivity spectrum shows the excitonic shift in the energy region of the f → d atomiclike transition, and the extreme increase of the intensity in the low-energy region comparable to T K . In the valence fluctuation regime, the coupled charge fluctuations of f and d electrons occur.

High resolution Ce 3d–4f resonant photoemission study of CeNiSn and CePdSn

Abstract We have measured Ce 3d resonant photoemission spectra of CeNiSn and CePdSn with high resolution of E/ΔE∼4000. Although the Ce valences are close to 3+ for both compounds, the Ce 3d resonant spectra show stronger f1 final states near the Fermi level compared with Ce 4d resonant spectra. Calculations based on the single impurity model give consistent spectral functions to the experimental results. The valence band states except for the Ce 4f states in the bulk are essentially the same as those in the surface and consistent with the result of a band-structure calculation.

High resolution photoemission study of CeRu2Si2

Abstract High resolution Ce 4d → 4f resonance photoemission study was done for CeRu2Si2. The Ce 4f spectrum has shown two prominent peaks around 0.3 and 2.5 eV. The 0.3 eV peak has clearly shown an additional structure closer to the Fermi level, whose intensity decreases at higher temperatures. This intensity reduction is ascribed to the tail of the Kondo resonance peak.

Large-scale electronic-structure theory and nanoscale defects formed in cleavage process of silicon

Abstract Several methods are constructed for large-scale electronic structure calculations. Test calculations are carried out with up to 10 7 atoms. As an application, cleavage process of silicon is investigated by molecular dynamics simulation with 10-nm-scale systems. As well as the elementary formation process of the ( 1 1 1 ) –( 2 × 1 ) surface, we obtain nanoscale defects, that is, step formation and bending of cleavage path into favorite (experimentally observed) planes. These results are consistent to experiments. Moreover, the simulation result predicts an explicit step structure on the cleaved surface, which shows a bias-dependent STM image.

Single Impurity Anderson Model with Coulomb Repulsion between Conduction Electrons on the Nearest-Neighbour Ligand Orbital

We study how the Kondo effect is affected by the Coulomb interaction between conduction electrons on the basis of a simplified model. The single impurity Anderson model is extended to include the Coulomb interaction on the nearest-neighbour ligand orbital. The excitation spectra are calculated using the numerical renormalization group method. The effective bandwidth on the ligand orbital, D eff , is defined to classify the state. This quantity decreases as the Coulomb interaction increases. In the D eff > Δ region, the low energy properties are described by the Kondo state, where Δ is the hybridization width. As D eff decreases in this region, the Kondo temperature T K is enhanced, and its magnitude becomes comparable to Δ for D eff ∼Δ. In the D eff < Δ region, the local singlet state between the electrons on the f and ligand orbitals is formed.

High Resolution 4d-4f Resonance Photoemission Spectroscopy of CePdX (X=As, Sb)

We have studied the electronic structures of ternary Ce compounds CePdX (X=As, Sb) by means of high resolution 4 d -4 f resonance photoemission spectroscopy. In the Ce 4 f spectra, the relative int...

Acoustic de Haas-van Alphen effect of LaB6

Acoustic de Haas-van Alphen (dHvA) effect of LaB 6 has been investigated by the ultrasonic velocity measurement. The oscillation intensities of the elastic constants have been found to be highly anisotropic even for magnetic field directions with crystallographic equivalence. This anisotropy depends strongly on the polarization and propagation directions of the ultrasonic modes. The observed oscillation intensities have been analyzed by means of the theory of the acoustic dHvA effect which bases on the effective mass approximation. These analyses enable us to know the change of the cross-sectional areas of the Fermi surfaces by the ultrasonic wave and therefore the electron-lattice interaction. The characteristic features of the electron-lattice interactions for the main α Fermi surfaces at X points and for the small ρ Fermi surfaces on Σ lines are presented.

Excitation spectra of the Anderson model with Coulomb interaction between localized and conduction electrons

Abstract Excitation spectra of the extended Anderson model that includes Coulomb screening by delocalized electrons were studied based on the numerical renormalization group method. The model also includes the energy dependence of mixing intensity with account of the realistic band structure.

Excitation spectra of the degenerate Anderson model with finite f-f Coulomb interaction

Abstract Spectral densities of local single particle, magnetic and charge excitations are calculated for the impurity Anderson model based on the renormalization group approach. Calculations are made for various magnitudes of the f-f Coulomb interaction. The role of the multiplet splitting and of the so-called negative U interaction is also examined. The width of the peak at the Fermi level in the single particle spectrum has always a scale comparable to that of the magnetic excitation and is never larger than the hybridization width. The broad band-like peak in BIS of U-compounds is interpreted by a broad shoulder structure which appears when the f-f interaction is not small and the f-electron number is intermediate and larger than 1.