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The electronic structures of Pt and Pt-Co nanoparticles with O2 adsorption and O2/H2O co-adsorption were investigated by in situ hard X-ray photoelectron spectroscopy (HAXPES) and in situ high resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) to clarify the effects of water adsorption on fuel cell cathode catalysis surface. The experimental results suggest that under the pressure of 1 mbar, the adsorption of H2O hinders the successive O2 adsorption on Pt surface, while under the pressure of 1 bar, the adsorption of H2O enhances the adsorption of O2 on Pt surface. This water effect is found to be more significant on Pt surface than on Pt-Co surface. These results would be helpful to understand how the water affects the fuel cell performance and why Pt-Co nanoparticles show higher oxygen reduce reaction (ORR) activity than Pt nanoparticles.

Bent band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction and photovoltaic devices. This report presents experimental evidence for ferroelectric band bending, which was observed in the depth profiles of atomic orbitals of angle-resolved hard x-ray photoemission spectra of ferroelectric BaTiO3 thin films. The ferroelectric bent band structure is separated into three depth regions; the shallowest and deepest regions are slightly modulated by the screening effect at surface and interface, respectively, and the intermediate region exhibits the pure ferroelectric effect. In the pure ferroelectric bent band structure, we found that the binding energy of outer shell electrons shows a larger shift than that of inner shell electrons, and that the difference in energy shift is correlated with the atomic configuration of the soft phonon mode. These findings could lead to a simple understanding of the origin of electric polarization., Comment: 16 pages, 7 figures

The valence-band electronic structures of high-pressure-phase PdF2-type (HP-PdF2-type) platinum-group metal dioxides MO2 (M = Ru, Rh, Ir, and Pt) were studied by synchrotron radiation photoelectron spectroscopy and first-principles calculations. The obtained photoelectron spectra for HP-PdF2-type RuO2, RhO2, and IrO2 agree well with the calculated valence-band densities of states (DOSs) for these compounds, indicating their metallic properties, whereas the DOS of HP-PdF2-type PtO2 (calculated in the presence and absence of spin–orbit interactions) predicts that this material may be metallic or semimetallic, which is inconsistent with the electric conductivity reported to date and the charging effect observed in current photoelectron measurements. Compared with the calculated results, the valence-band spectrum of PtO2 appears to have shifted toward the high-binding-energy side and reveals a gradual intensity decrease toward the Fermi energy EF, implying a semiconductor-like electronic structure. Spin-dependent calculations predict a ferromagnetic ground state with a magnetization of 0.475 μB per formula unit for HP-PdF2-type RhO2., ファイル公開：2019/04/15

We have conducted hard X-ray photoelectron spectroscopy investigations of the electronic structure changes and electron correlation phenomena which take place upon the photoinduced reversible phase transition between $\beta$- and $\lambda$-Ti$_3$O. From valence band spectra of $\beta$- and $\lambda$-Ti$_3$O$_5$, we have identified the bipolaron caused by the $\sigma$-type bonding of $d_{xy}$ orbitals in $\beta$-Ti$_3$O$_5$ and the $\pi$ stacking between the $d_{xy}$ orbitals between different Ti sites in $\lambda$-Ti$_3$O$_5$, previously predicted by \textit{ab initio} calculations. On the other hand, the Ti $2p$ and Ti $1s$ core level spectra exhibit nonlocal screening satellite features, which are typical spectroscopic signs of strong electron correlation in the coherent Ti $t_{2g}$ states. Correlation in the valence band also manifests to reduce the plasmon energy, which results in an enhancement of the valence electron mass by a factor of 2.7.

Mn$_{3-x}$Co$_{x}$Ga epitaxial thin films were grown on MgO substrates by magnetron co-sputtering. Structures were tetragonal or cubic depending on Co content. Composition dependence of saturation magnetization and uniaxial magnetic anisotropy $K_u$ of the films were investigated. A high $K_u$ (1.2 MJ m$^{-3}$) was achieved for the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film with the magnetic moment 0.84$\mu_B$. Valence band spectra were obtained by hard X-ray photoelectron spectroscopy. Sharp peaks in the cubic case, which were absent in the tetragonal case, prove that a van Hove singularity causes a band Jahn-Teller effect with tetragonal distortion. Observations agree well with the first-principles calculations.

This work reports the measurement of magnetic dichroism in angular-resolved photoemission from in-plane magnetized buried thin films. The high bulk sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) in combination with circularly polarized radiation enables the investigation of the magnetic properties of buried layers. HAXPES experiments with an excitation energy of 8 keV were performed on exchange-biased magnetic layers covered by thin oxide films. Two types of structures were investigated with the IrMn exchange-biasing layer either above or below the ferromagnetic layer: one with a CoFe layer on top and another with a Co$_2$FeAl layer buried beneath the IrMn layer. A pronounced magnetic dichroism is found in the Co and Fe $2p$ states of both materials. The localization of the magnetic moments at the Fe site conditioning the peculiar characteristics of the Co$_2$FeAl Heusler compound, predicted to be a half-metallic ferromagnet, is revealed from the magnetic dichroism detected in the Fe $2p$ states.

This work reports on the electronic and crystalline structure and the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended x-ray absorption fine-structure spectroscopy and anomalous x-ray diffraction. The compound exhibits a well-ordered L21 structure as is typical for Heusler compounds with 2:1:1 stoichiometry. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab initio calculations. Transport measurements and hard x-ray photoelectron spectroscopy were performed to explain the electronic structure of the compound. The obtained valence band spectra exhibit small energy shifts that are the result of the photoexcitation process, whereas electron-electron correlation in the ground state is negligible. The vibration and mechanical properties of the compound were calculated. The observed hardness values are consistent to a covalent-like bonding of Co2MnGe.

The substitutional series of Heusler compounds NiTi1−xMxSn (where M=Sc,V and 0

This work reports on the bulk properties of the quaternary Heusler alloy Co$_2$Mn$_{1-x}$Fe$_x$Si with the Fe concentration $x=$. All samples, which were prepared by arc melting, exhibit $L2_1$ long range order over the complete range of Fe concentration. Structural and magnetic properties of Co$_2$Mn$_{1-x}$Fe$_x$Si Heusler alloys were investigated by means of X-ray diffraction, high and low temperature magnetometry, M{\"o\ss}bauer spectroscopy, and differential scanning calorimetry. The electronic structure was explored by means of high energy photo emission spectroscopy at about 8 keV photon energy. This ensures true bulk sensitivity of the measurements. The magnetization of the Fe doped Heusler alloys is in agreement with the values of the magnetic moments expected for a Slater-Pauling like behavior of half-metallic ferromagnets. The experimental findings are discussed on the hand of self-consistent calculations of the electronic and magnetic structure. To achieve good agreement with experiment, the calculations indicate that on-site electron-electron correlation must be taken into account, even at low Fe concentration. The present investigation focuses on searching for the quaternary compound where the half-metallic behavior is stable against outside influences. Overall, the results suggest that the best candidate may be found at an iron concentration of about 50%., Comment: 26 pages, 9 figures Phys. Rev. B accepted

Hard x-ray photoemission spectroscopy (PES) of Cu core electronic states, with a probing depth of ∼60, is used to show that the Zhang-Rice singlet feature is present in La2CuO4 but is absent in Nd2CuO4. Hole and electron doping in La2-xSrxCuO4 (LSCO) and Nd2-xCexCuO4 (NCCO) result in new well-screened features which are missing in soft x-ray PES. Impurity Anderson model calculations establish screening from doped states as its origin, which is strongly suppressed within 15 of the surface. Complemented with x-ray absorption spectroscopy, the small chemical-potential shift in core levels (∼0.2eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap (∼1eV) upon hole and electron doping in LSCO and NCCO.

We investigate the electronic structure of a perovskite-type Pauli paramagnet SrMoO 3 (t 2g 2 ) thin film using hard x-ray photoemission spectroscopy and compare the results to realistic calculations that combine density functional theory within the local-density approximation (LDA) with dynamical mean-field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially filled t 2g bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. This interpretation is supported by DMFT model calculations including Hund's rule coupling, which show a renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around −2.5 eV that is not present in the LDA+DMFT results, pointing to a source of correlations that is not present in our calculations that include only on-site interactions. We interpret this weight as a plasmon satellite, which is supported by the measured core-level spectra that all show satellites at this energy.

We have investigated the electronic and magnetic properties of Co-doped Pr 0.8 Ca 0.2 MnO 3 thin films using various spectroscopic techniques. X-ray absorption and hard x-ray photoemission spectroscopy revealed that the substituted Co ions are in the divalent state, resulting in hole doping on the Mn atoms. Studies of element-selective magnetic properties by x-ray magnetic circular dichroism found a large orbital magnetic moment for the Co ions. These spectroscopic studies reveal that the substituted Co ions play several roles of hole doping for Mn, ferromagnetic superexchange coupling between the Co 2+ and Mn 4+ ions, and orbital magnetism of the Co 2+ ions. Competition among these complex interactions produces the unique electronic and magnetic behaviors including enhanced coercivity of the Co-doped Pr 0.8 Ca 0.2 MnO 3 .

Yb 3d and valence-band photoemission spectra of the first-order valence-transition compound YbInCu4 have been measured with hard x ray at an excitation energy of 5.95 keV. Abrupt changes are clearly observed in both spectra around the transition temperature TV = 42 K, in comparison with ultraviolet and soft x-ray photoemission (VUV-PES and SX-PES) spectra. From the Yb 3d spectra, the Yb valence has been estimated to be ~2.90 from 220 down to 50 K and ~2.74 at 30–10 K. We propose that Yb 3d hard x-ray photoemission spectroscopy is a very powerful method to estimate the valence of Yb with high accuracy. On the other hand, the Yb2+ 4f-derived peaks in the valence-band spectra exhibit a remarkable enhancement below TV. The shape of the valence-band spectra is different from those of the VUV-PES and SX-PES spectra above TV, reflecting the In 5s and 5p contributions.