Yasuhiro Seino

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.

Hole Distribution in YBaCuO System from the Analysis of Cu 2P-XPS,Cu 2P-XAS and O 1s-XAS

We analyze the polarization dependence of the Cu 2 p -XAS and the Cu 2 p -XPS observed in the YBa 2 Cu 3 O 7-δ system. To take into account the nonequivalent Cu sites in YBa 2 Cu 3 O 7 , we use a model where the impurity Anderson model and a CuO 4 cluster model are linked, where the former and the latter describe the CuO 2 plane and the CuO chain, respectively, For YBa 2 Cu 3 O 6.5 , the CuO 4 cluster is replaced by a CuO 3 cluster. For 0<δ<0.5, the spectrum is obtained by a superposition of the spectra for δ=0 and 0.5. Obtained results quantitatively reproduce the experimental results of Cu 2 p -XAS, and the electron configuration of the ground state obtained from our spectral analysis suggests that extra holes will enter both the O sites in the CuO chains and the CuO 2 layers. This picture is also supported by the analysis of the polarization dependence in the O 1 s -XAS.

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.

Electronic structure of electron-doped superconductor Nd2-xCexCuO4 from the analysis of Cu 2p-XPS

The role of the Cu 4 s conduction band in the Cu 2 p -XPS (X-ray photoemission spectrum) of Nd 2- x Ce x CuO 4 is investigated on the basis of both a simple CuO 4 cluster model and the impurity Anderson model. The impurity Anderson model consists of the impurity Cu 3 d shell and the valence and conduction bands which are obtained from the O 2 p and Cu 4 s bands by the tight-binding approximation. From the carrier concentration dependence of the Cu 2 p -XPS, the electron doping mechanism is discussed. If the Cu 4 s level is lower in energy than that estimated from the energy band calculation by a few eV, the doped electrons are shown to occupy the Cu 4 s band in the low-doping regime.

Role of Cu 4s Band in Cu 2p-XPS of Nd2-xCexCuO4

The role of the Cu 4 s band in the Cu 2 p -XPS of Nd 2- x Ce x CuO 4 is investigated on the basis of the impurity Anderson model. The model consists of the impurity Cu 3 d shell and the conduction and valence bands formed from the O 2 p and Cu 4 s bands under the tight-binding approximation. Because of the symmetry of the crystal, the 3 d states couple weakly to the Cu 4 s band so that the effect of the 4 s band on the Cu 2 p -XPS is very small in general. However, in the case where the 4 s band is lower by a few eV in energy than that estimated by the first-principle calculation, the 4 s state is important in the electron doping process in Nd 2- x Ce x CuO 4 because the doped electron enters this state.

CALCULATION OF CU 2P RESONANT PHOTOEMISSION SPECTRA IN CUO

We investigated theoretically the Cu 2 p resonance of 3 d , 3 p and 3 s X-ray photoemission spectra (XPS) in CuO using a cluster model including multiplet coupling, crystal field and anisotropic hybridization. The spectra were calculated by taking into account the radiative transitions up to the first order and Coster-Kronig transitions up to the infinite order. The resonant enhancement is so strong that it can be used to interpret small satellite structures in both core level and valence band photoemission spectra. The influence of the configuration interaction between the 3p 2 3 d 10 and 3s 3 d 9 final states is discussed.

Optical Properties of Electron Center in Layered Ionic Crystals (CnH2n+1NH3)2MnCl4:

A new optical absorption band was found at the peak energy of 17 kcm -1 (590 nm) besides the Cl 2 - band (22.5 kcm -1 =445 nm) in (C n H 2 n +1 NH 3 ) 2 MnCl 4 ( n =1, 2, 3) irradiated with X-rays at 15 K. The absorption intensity of the new band increases with the X-ray dosage in proportion to that of the Cl 2 - band. The intensity thermally decreases in two steps at temperatures around 50 K and 80 K accompanying the decrease of Cl 2 - band. The absorption band can not be bleached by the 590 nm light (17 kcm -1 ) at 15 K. The band is assigned to an electron center where an electron is trapped at the ammonium head in the neighborhood of a Cl - vacancy.

n{=}1

Abstract We study the effect of electron doping on the Cu 2 p -XPS in the Nd 2− z Ce z CuO 4 system using the impurity Anderson model which consists of Cu 3 d , Cu 4 s , Cu 2 p and O 2 p states. We discuss the character of the doped electron from the analysis of the doping dependence of the intensity ratio of the satellite to the main peak in the Cu 2 p -XPS.

, 2, 3 Irradiated with X-Rays at 15 K

By theoretically analyzing the core-level spectra, such as X-ray photo-emission (XPS) and X-ray absorption (XAS), with the use of the impurity Anderson model, we discuss the electronic structure of high Tc superconductors and some related materials. From Cu 2p-XPS we obtain the information on the charge transfer energy from 0 2p to Cu 3d states and on the hybridization strength between these states. The polarization dependence of Cu 2p-XAS and 0 1s-XAS provides us with the most direct information on the site and the symmetry of doped holes. An analysis of X-ray emission spectra is also made using the same model system.