Haruhiko Ogasawara

Electronic structure investigation of CoO by means of soft X-ray scattering

The electronic structure of the heavy fermion compound CeB6 is probed by resonant inelastic soft X-ray scattering using photon energies across the Ce 3d and 4d absorption edges. The hybridization between the localized 4f orbitals and the delocalized valence-band states is studied by identifying the different spectral contributions from inelastic Raman scattering and normal fluorescence. Pronounced energy-loss structures are observed below the elastic peak at both the 3d and 4d thresholds. The origin and character of the inelastic scattering structures are discussed in terms of charge-transfer excitations in connection to the dipole allowed transitions with 4f character. Calculations within the single impurity Anderson model with full multiplet effects are found to yield consistent spectral functions to the experimental data.

Theory of 3d Core Photoemission Spectra in Rare Earth Oxides Series

Systematic analysis is carried out for 3 d core photoemission spectra of rare earth oxides R 2 O 3 (R=La∼Yb) and RO 2 (R=Ce, Pr, Tb) with the use of the impurity Anderson model. From the comparison of the calculated and experimeutal spectra, we estimate the values of some key parameters such as the charge transfer energy, the strength of the hybridization interaction, etc. We propose a possible mechanism of a satellite in the spectra for Eu 2 O 3 and Yb 2 O 3 , which is different from that for La 2 O 3 ∼Nd 2 O 3 . We also point out the reason why the RO 2 structure is stable only for R=Ce, Pr and Tb.

Theory of Magnetic Circular Dichroism of the Ce L2,3 X-ray Absorption Spectra in Mixed Valence Vompound CeFe2

Magnetic circular dichroism (MCD) of x-ray absorption spectroscopy (XAS) at Ce L 2,3 edges in the typical mixed valence compound CeFe 2 is theoretically studied by using the impurity Anderson model combined with a cluster approach. It is found that the energy-dependent polarization of Ce 5 d states due to mixing with spin-polarized Fe 3 d states is indispensable for explaining the spectra. Our calculations reproduce well the two-peak structure observed in XAS and MCD experiments, especially their energy position, spectral width and energy separation of the two peaks. For a detailed analysis of these, the Coulomb interactions - U d c (between a 5 d electron and a core hole) and U f d (between a 4 f electron and a 5 d electron) play a special role.

Calculation of Magnetic Circular Dichroism of Rare-Earth Elements

We present atomic calculations for the magnetic circular dichroism (MCD) in the 4 d →4 f excitation region of Eu, Gd, Tb, Dy, and Ho metal. The spectral shapes at T =0 K for non-polarized, right and left circularly polarized light, and their difference spectra are given. The interference effect between the 4 d →4 f excitation followed by 4 d -4 f 4 f super-Coster-Kronig decay process and the 4 f → e g direct excitation process, which is known as the Fano effect, is considered in the calculations. The result reproduces the overall features of recently measured experiments. It is essential to consider the Fano effect for the understanding of the heavy rare-earth 4 d →4 f absorption and their MCD.

Magnetic Circular Dichroism of Resonant X-Ray Emission Spectroscopy in Longitudinal and Transverse Geometries

Theoretical calculations and experimental measurements are performed for magnetic circular dichroism in resonant X-ray emission spectroscopy (MCD-RXES) in Gd 3+ systems. It is shown that when the i...

Spectroscopy Studies of Temperature-Induced Valence Transition on EuNi2(Si1-xGex)2 around Eu 3d–4f, 4d–4f and Ni 2p–3d Excitation Regions

The resonant photoemission spectra and X-ray absorption spectra of temperature-induced valence transition material EuNi 2 (Si 1- x Ge x ) 2 around Eu 3 d –4 f , 4 d –4 f and Ni 2 p –3 d resonant excitation regions have been measured. From the comparison between the resonant photoemission spectra around the 3 d –4 f (bulk sensitive) and 4 d –4 f (surface sensitive) excitation regions, the Eu divalent component originating from the surface is separated from the bulk one. The divalent and the trivalent features of 4 f electrons are obviously distinguished depending on the excitation conditions. It was confirmed that the intensity ratio of the 4 f electronic structures between the Eu divalent and trivalent ions changes as a function of the temperature. The mean valence values estimated from the Eu M -edge X-ray absorption spectra and from the photoemission spectra were rather smaller than those obtained previously from the L -edge X-ray absorption spectra. The reason of the discrepancy is discussed. The spect...

Magnetic Circular Dichroism of Resonant X-Ray Emission Spectroscopy in the Transverse Geometry

We perform theoretical calculations and experimental measurements for magnetic circular dichroism in resonant X-ray emission spectroscopy (MCD-RXES) in Gd 3+ systems. A general expression of MCD-RXES is obtained theoretically for arbitrary directions of incident and emitted X-rays with an atomic model. It is shown that when the incident X-ray is perpendicular to the magnetization of the sample the MCD-RXES originates from the quantum mechanical interference between the excitation and decay processes. The behavior of MCD-RXES calculated for the 2 p →5 d excitation and 3 d →2 p radiative decay in Gd 3+ is in good agreement with that measured experimentally for a Gd 33 Co 67 amorphous alloy.

Theory of Polarization Dependence in Resonant X-Ray Emission Spectroscopy of Ce Compounds

We calculate the Ce 4 f →3 d resonant X-ray emission spectroscopy (4 f →3 d RXES) of Ce compounds, CeF 3 and CeO 2 , by means of ionic and impurity Anderson model with full multiplet effects, respectively. The calculation is based on the second-order optical formula, and the effect of the X-ray polarization is taken into account. The calculated results of 4 f →3 d RXES show the strong dependence on the X-ray polarization, as well as on the scattering angle. Using the selection rule of the dipole transition with polarized X-rays, the calculated spectral features are explained.

X-ray magnetic circular dichroism at rare-earth L 2 , 3 edges in R 2 Fe 14 B compounds ( R = La , Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu)

Magnetic circular dichroism (MCD) in the x-ray absorption spectroscopy (XAS) at the L2,3 edges for almost entire series of rare-earth (RE) elements in RE2Fe14B, is studied experimentally and theoretically. By a quantitative comparison of the complicated MCD spectral shapes, we find that (i) the 4f-5d intra-atomic exchange interaction not only induces the spin and orbital polarization of the 5d states, which is vital for the MCD spectra of the electric dipole transition from the 2p core states to the empty 5d conduction band, but also it accompanies a contraction of the radial part of the 5d wave function depending on its spin and orbital state, which results in the enhancement of the 2p-5d dipole matrix element, (ii) there are cases where the spin polarization of the 5d states due to the hybridization with the spin polarized 3d states of surrounding irons plays important roles, and (iii) the electric quadrupole transition from the 2p core states to the magnetic vale! nce 4f states is appreciable at the pre-edge region of the dipole spectrum. Especially, our results evidence that it is important to include the enhancement effect of the dipole matrix element in the correct interpretation of the MCD spectra at the RE L2,3 edges.

Theory of the 4d→ 2p X-Ray Emission Spectroscopy in Ce2O3, Pr2O3 and Dy2O3

The 4 d → 2 p X-ray emission spectra (XES) of Ce 2 O 3 , Pr 2 O 3 and Dy 2 O 3 have been calculated with an impurity Anderson model with the full multiplet couplings, following the Kramers-Heisenberg formula in the second order optical process. Experimental results have been well reproduced with this model by using a constant value for the 4 d core hole lifetime damping Γ (4 d ) in the case of Ce 2 O 3 and Pr 2 O 3 , while in the case of Dy 2 O 3 it is necessary to take into account the term dependence of Γ (4 d ), which is consistent with the previous theoretical analyses of 4 d X-ray photoemission spectra. It was also shown that both the spin-orbit couplings of the 4 d core level in the final state and the 4f level in the initial state are key factors to cause the branching ratio in the L γ line larger than that in the L β line. The phase matching of the wave functions between the intermediate and final states smears out the hybridization effect in the 4 d → 2 p XES in Ce 2 O 3 and Pr 2 O 3 .