A. Kotani

Photoemission on 3d core levels of Cerium: An experimental and theoretical investigation of the reduction of cerium dioxide

Abstract The oxidation state of ceria (CeO2) used as a support of catalysts is investigated by X-Ray Photoelectron Spectroscopy (XPS) on the 3d core level of Cerium. A sequence of 3d-XPS spectra is obtained for a ceria converted progressively (and partially) to Ce2O3 under increasing temperatures of reduction (up to 900°C). The spectra are analysed theoretically by using a filled-band impurity Anderson model and by chosing appropriate parameter values.

High-Energy Spectroscopy in Oxide Superconductors

We consistently analyze the Cu 2 p -XPS, L 3 -XAS, 3 d -XPS and resonant XPS in high- T c superconducting materials, such as (La 1- x Sr x ) 2 CuO 4 and YBa 2 Cu 3 O 7-δ , on the basis of the impurity Anderson model. We introduce five valence bands into the model to take account of the anisotropic nature of the Cu 3 d orbitals and the different 2 p energy levels for nonequivalent O sites. The key parameter values which characterize these Cu 3 d and O 2 p states are estimated from the analysis, and their trends are discussed. We study the effect of hole doping on the 2 p -XPS and L 3 -XAS, especially on the polarization dependence of L 3 -XAS. For reference, we also analyze the spectra of CuO.

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.

Anomalous behavior of the surface state in ferromagnetic superconductors

Abstract The effect of the electromagnetic interaction between the persistent current and the magnetic moments to the surface state in ferromagnetic superconductors is studied theoretically. This interaction causes an oscillatory decay of the penetrated magnetic field near the magnetic phase transition temperature. Below a critical temperature, the spontaneous surface magnetization is stabilized by the magnetic field induced by the surface persistent current.

Anomalous properties of thin films of ferromagnetic superconductors

Abstract Anomalous magnetic and superconducting behavior which originates from the electromagnetic interaction between the persistent current and the rare earth magnetic moments is theoretically predicted in thin films of ferromagnetic superconductors. It is shown that the ferromagnetic ordering can coexist with superconductivity for films of thickness smaller than the London penetration depth. The ferromagnetic phase transition is of the second order and its critical temperature depends on the thickness of the film.

Effect of Rhombic Distortion on the Polarized X-Ray Absorption Spectra in High Tc Superconductors

We theoretically study the polarization dependence of the Cu 2 p -XAS (X-ray absorption spectrum) in high T c superconductors with the use of a simple CuO 4 cluster. We take account of both the Cu 3 d ( x 2 - y 2 ) and 3 d (3 z 2 - r 2 ) orbitals, and study the effect of a possible rhombic distortion by which the two 3 d orbitals are mixed with each other. The polarized XAS is calculated both in the undoped “Cu 2+ ” system and the hole-doped “Cu 3+ ” system. A considerably large intensity ratio between E // c and E ⊥ c absorption spectra is obtained in the “Cu 3+ ” system as a result of the rhombic distortion, where E and c are directions of the X-ray electric field and the crystalline c -axis, respectively. These results help to explain the increase in the polarized E // c Cu 2 p -XAS white line going from the insulating to the hole-doped phases in Bi-Sr-Ca-Cu-O (2212) systems.

Systematic analysis of 3d core photoemission spectra in rare earth oxides

Abstract Systematic analysis is carried out for 3 d core photoemission spectra of rare earth oxides R 2 O 3 and RO 2 for various rare earth elements R with the use of the impurity Anderson model. We estimate the values of the charge transfer energy, the hybridization strength, etc, and discuss the mechanism of the splitting of these photoemission spectra. We also point out the reason why the RO 2 structure is stable only for R = Ce 1 , Pr and Tb.

Mean-Field Theory of Peierls and Spin-Peierls Instabilities –Commensurability, Harmonics and Dimerized-State Pinning–

Mean-field study of the Peierls instability is made at zero temperature with the electron occupation number varied over a wide range between half-occupancy and nearly zero-occupancy. Total energy of the system, order parameters and the lattice displacement pattern are calculated by taking account of all the accompanying harmonics. The lowering of the total energy per atom is shown to be very large in the neighborhood of the half- and 1/15-occupancies. The amplitudes of harmonics increase drastically as the electron number approaches the zero-occupancy. The spin-Peierls transition is also studied with an XY model. Although the effect of magnetic field on the spin-Peierls system has a close similarity to that of the electron number variation on the Peierls system, an important difference between them is pointed out on the pinning of a dimerized state.

Electron-Electron Correlation, Resonant Photoemission and X-Ray Emission Spectra

In this short review paper we essentially focus on the high energy spectroscopies which involve second order quantum processes, i.e., resonance photoemission, Auger and X-ray emission spectroscopies, denoted respectively by RXPS, AES and XES. First, we summarize the main 3P-RXPS and AES results obtained in Cu and Ni metals; especially we recall that the satellite near the 3P-threshold in the spectra, which arises from a d-hole pair bound state, needs a careful treatment of the electron-electron correlation. Then we analyze the RXPS spectra in a few Ce compounds (Ce02, Ce20a and CeFa) involving 3d or 4d core levels and we interpret the spectra consistently with the other spectroscopies, such as core XPS and XAS which are first order quantum processes. Finally within the same one-impurity model and basically with the same sets of parameters, we review a theory for the Ce 5p-+ 3d XES, as well as for the corresponding RXES, where (1) the incident X-ray is tuned to resonate with the 3d -+4! transition and (2) the X-ray emission due to the 5p-+3d transition is actually observed. The paper ends with a general discussion.

Theory of Cu 2p core XPS, XES and XAS in high-Tc superconducting materials

We theoretically analyze the spectra of X-ray photoemission, X-ray emission and X-ray adsorption associated with the Cu 2p core level in high-Tc superconducting materials. We use the single-site impurity Anderson model with five Cu 3d orbitals and O 2p valence bands specified by the irreducible representations of the D4h symmetry around the Cu atom. We also take account of Coulomb and exchange interactions between Cu 2p and 3d states, those interactions between Cu 3d states, and the spin-orbit interaction.