Oral session


An Interpretation of ps Transient XANES of WO3 in Photoabsorption and Decay Processes

Kiyotaka Asakura

Catalysis Research Center, Hokkaido University

WO3 is a water splitting catalyst using visible light in a so-called Z-scheme.(1) In our previous pump-probe experiment using a laser pump and a X-ray probe, we found a large decrease in the white line of XANES spectrum 100 ps after the photoexcitation.(2) The difference spectrum shows that a negative peak appears at a position well corresponding to eg orbital as WO3 is regarded approximately as an Oh symmetry. The white line is a transition from 2p3/2 to 5d orbital. WO3 has a d0 structure in the first approximation. Therefore it provides a large white line peak. The photoabsorption induces the excitation to the empty d orbitals and hence the white line peak is decreased. However we have a problem why the photoabsorption occurs not to the t2g but eg state. The excitation energy of laser is about 3 eV a little above the band gap between valence band(mainly oxygen atom) and conduction band(d orbitals in W). The bottom of the conduction band is mainly composed of t2g d orbitals though eg orbital lies far above so that it is impossible to find the excited electron at the eg orbitals. In order to understand the phenomenon we carried out faster XANES measurement using XFEL. We found that the positive peak at the edge position in addition to the negative peak at 700 fs after the photoirradiation. The shape is similar to the first derivative of the XANES though the peak intensities are rather different. Then the negative peak gradually increases with time till 200 ps. One possible interpretation is the shift occurs due to the electron transition from O valence to W t2g orbitals by the pump laser. Consequently, W is reduced to W5+ and edge shift occurs with simultaneous occupation of the t2g orbitals by excited electrons so that the electron filling cancel out the increase in the absorption due to the edge shift to give a smaller peak in the difference spectrum around the edge region. But we still have another problem why there is a slow increase in the negative peak till 200 ps.(3) We would like to discuss this point in my talk.
(1) R. Abe, T. Takata, H. Sugihara, K. Domen, Chemical Communications 2005, 3829-3831
(2) Y. Uemura, H. Uehara, Y. Niwa, S. Nozawa, T. Sato, S. Adachi, B. Ohtani, S. Takakusagi, K. Asakura, Chemistry Letters 2014, 43 977-979.
(3) Y. Uemura, D. Kido, Y. Wakisaka, H.Uehara, T. Ohba,YNiwa, S.Nozawa, T. Sato, K. Ichiyanagi, R.Fukaya, S. Adachi, T, Katayama, T, Togashi, S. Owada, K. Ogawa, M. Yabashi, K. Hatada, S. Takakusagi, T. Yokoyama, B. Ohtani, K. Asakura, in preparation

Theory of pump-probe ultrafast photoemission and X-ray absorption

Takashi Fujikawa

Graduate School of Advanced Integration of Science, Chiba University, Yayoi-cho 1-33, Inage, Chiba, JAPAN, tfujikawa@faculty.chiba-u.jp

Keldysh Green's function approach is extensively used in order to derive practical formulas to analyze the pump-probe ultrafast photoemission and X-ray absorption spectra. Here the pump pulse is strong enough whereas the probe X-ray pulse can be treated by use of a perturbation theory. The present theoretical formulas allow us to handle both the intrinsic and extrinsic losses. In the ultrafast XPS and XAFS analyses the intrinsic and extrinsic effects can interfere as well. In the latter careful analyses are necessary to handle extrinsic losses in terms of damped photoelectron propagation.
The nonequilibrium dynamics after the pump pulse irradiation is well described by use of the time-dependent Dyson orbitals. Well above the edge threshold, ultraist photoelectron diffraction and EXAFS provide us with transient structural change after the laser pump excitation. Near threshold detailed information could be obtained for the combined electronic and structural dynamics.

XAFS analyses for hierarchic nanoparticles and magnetic nanocrystalline alloy

Takafumi Miyanaga

Department of Advanced Physics, Hirosaki University, Hirosaki, Aomori, 036-8561 Japan

We present XAFS analyses for nanoparticles of Te and Bi prepared by island deposition with NaCl, which have hierarchical structure in the crystal state. For Te particle, the break down of 2NN interchain interaction makes 1NN intrachain Te-Te interaction strengthen as particle size decreases. Analogously for Bi particles the small change of 1NN interatomic distance can be observed but large decrease of its coordination number indicates the break down of 1NN interaction with decreasing of particle size. Second we present the XAFS results for soft magnetic materials of FeSiPBCu prepared by rapid cooling with melt spinning method. The local environment of Cu atoms appear to be bcc structure in the high magnetic performance phase. However whether it is bcc Cu nanocrystalline or embedded Cu atom into bcc Fe is still unclear. We discuss the EXAFS and XANES results for these systems.

Development and structural analysis of base metal nanocluster catalyst

Nobuyuki Ichikuni

Graduate School of Engineering, Chiba University, ichikuni@faculty.chiba-u.jp

Nanoclusters have been accepted the great attention due to their potential applications in both industrial and academic areas. However, development of nanoclusters was mainly focused on the precious metals. Based on the idea for the sustainable society, development of the substituting materials for noble metals is an urgent issue. In this report, the development and structural analysis of supported Ni nanocluster and Co oxide nanocluster catalysts will be introduced: (1) Size regulated Ni nanocluster onto Al2O3 support was synthesized by using alcoholate-stabilized Ni colloid as precursor. Obtained catalyst showed the volcano type size dependent catalysis toward water gas shift reaction. This behavior can be explained in terms of the support effect. (2) Co oxide nanocluster supported on SiO2 was prepared by using alcoholate-stabilized Co nanocluster, followed by the heat treatment in air. It was revealed that the nature of the surface oxygen of Co oxide nanocluster was depended on the cluster size.

Ambient Pressure Hard X-ray Photoelectron Spectroscopy of Polymer Electrolyte Fuel Cells under Working Conditions

Toshihiko YOKOYAMA and Yasumasa TAKAGI

Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

We have exploited an ambient pressure hard x-ray photoelectron spectrometer (HAXPES) that uses the high-performance undulator beamline BL36XU of SPring-8 and works at a pressure up to 3000 Pa. We designed a polymer electrolyte fuel cell (PEFC) that allows us to perform HAXPES measurements of an electrode under working conditions. The Pt 3d5/2 core levels were measured for Pt nanoparticle cathode catalysts in various working PEFC. It is found that the Pt oxidation state clearly depends on the externally applied voltage between the anode and cathode and that a large positive bias (~1.4 V) to the cathode leads to large hysteresis loops in the cyclic voltammogram, which may induce irreversible elution of Pt and consequent degradation of the PEFC.

Theory of Pressure-induced Valence Transition and Sm L3 XAS Spectra for SmX (X = S, Se, Te)

A. Kotani

Photon Factory, IMSS, High Energy Accelerator Organization, Tsukuba, Ibaraki 305-0801, Japan

At ambient pressure, SmX (X = S, Se, Te) are in the insulating state with almost divalent Sm ions, but by applying pressure they exhibit the pressure-induced valence transition which accompanies the insulator-metal transition. Recently, Jarrige et al.[1] measured the Sm L3 XAS spectra for SmX by changing pressure, and revealed the details of the pressure dependence of the valence. In this study, I propose the mechanism of the pressure-induced valence transition for SmX and calculate the pressure dependence of the Sm L3 XAS spectral shape, taking into account the density of states (DOS) of the Sm 5d band. The electron promotion from Sm 4f to 5d states plays an important role in the pressure-induced valence transition, and the difference in the Coulomb interaction between the 4f and 5d states Ufd, as well as that in DOS of the Sm 5d band, for X = S, Se, Te explains well the first-order transition for SmS and the second-order transition for SmSe and SmTe. Calculated XAS spectra reproduce the experimental ones, and we can estimate the pressure dependence of Ufd, which originates from the screening of Ufd by the 5d conduction electrons, by comparing the calculated and experimental XAS spectra. [1] I. Jarrige et al., Phys. Rev. B 87, 115107 (2013).

Theoretical Study of ARPES spectra in undoped and Co-doped BaFe2As2 via the KKR-GF method

Gerald Derondeau, Federico Bisti, Jurgen Braun, Hubert Ebert, Vladimir Strocov and Jan Minar

LMU, Munich, Germany

We have shown recently that the Korringa-Kohn-Rostoker-Green function (KKR-GF) method, in combination with the coherent potential approximation (CPA), indeed provides a very suitable platform to describe the impact of substitutional disorder on the electronic structure of iron pnictide superconductors. [1] Based on this we focused on ARPES calculations for the undoped and Co-doped Ba-122 compound, revealing so far unexplained and crucial parts of the electronic structure seen in experimental ARPES data. [2, 3] First we are able to reproduce the unexpectingly strong in-plane anisotropy of the antiferromagnetic state seen in measurements of detwinned crystals at low photon-energies. [2] In the second part we find an explanation for the uncommen propeller like strucutres of the Fermi surface in BaFe2As2 for higher Brillouin zones coupled with experimental SX-ARPES data. [3]
[1] G. Derondeau et al., Phys. Rev. B 90, 184509 (2014). [2] M. Yi et al., PNAS 108, 6878 (2011). [3] F. Bisti, V. N. Strocov et al., unpublished work.

New Electron States At The Bi/InAs(111) Interface

L. Nicolai (1,2,3), J-M. Mariot(4), U. Djukic(2), M. C. Richter(2,3), O. Heckmann(2,3), T. Balasubramanian(5), M. Leandersson(5), J. Sadowski(5), J. Denlinger(6), I. Vobornik(7), S. Borek(1), J. Braun(1), H. Ebert(1), K. Hricovini(2,3) and J. Minar(1,8)

(1)LMU, Munich, Germany (2)LPMS, Universite de Cergy-Pontoise, France (3)DSM, IRAMIS, SPEC, CEA-Saclay, France (4)LCP?MR, UPMC Univ. Paris 06/CNRS, France (5)MAX-lab, Lund University, Sweden 6Advanced Light Source, LNBL, Berkeley, USA (7)Elettra-Sincrotrone Trieste, Basovizza, Trieste, Italy (8)University of West Bohemia, Plze?, Czech Republic

The Bi(111) surface is a prototype system to study Rashba-split surface states. Theoretical studies [1] predicted non-trivial topological surface states appearing on a single bi-layer of Bi(111) and a more complex behaviour was suggested for a variable film thickness as a function of layer thickness [2]. Here we present combined theoretical and ARPES studies of the electronic structure of Bi(111) films grown on InAs(111). Bismuth growth is epitaxial and a monocrystal of very high quality is obtained after depositing several monolayers. The ARPES experiments on these samples show several new types of electronic states. We show that a proper description of the photoemission process is necessary to understand them. In particular, it is shown that a part of these new states corresponds to novel bulk-like features. These features are well reproduced by the one-step model of photoemission as implemented in the SPR-KKR package [3].
[1] M. Wada et al., Phys. Rev. B 83, 121310 (2011).
[2] Z. Liu et al., Phys. Rev. Lett. 107, 136805 (2011).
[3] H. Ebert, D. Kodderitzsch, and J. Minar, Rep. Prog. Phys. 74, 096501 (2011).

A complete multiple scattering approach to core-state EELS cross-section

Didier Sébilleau (1), Junqing Xu (2), Rakesh Choubisa (3) and Calogero Natoli (4)

(1) Institut de Physique de Rennes, UMR CNRS-Université 6251, Université de Rennes-1, 35042 Rennes-cedex, France
(2) National Synchrotron Radiation Laboratory, University of Science and Technology of China, 230029 Hefei, Anhui, China
(3) Birla Institute of Technology and Science,Pilani, Rajasthan, India
(4) Theory Group, INFN-Laboratori Nazionali di Frascati, c. p. 13, I-00044 Frascati, Italy

We propose a multiple scattering formulation of the core-state EELS cross-section where, in addition to the excited electron, the incoming and the scattered electron are also described within the multiple scattering framework. We show that because of the long-range nature of the Coulomb interaction potential, contribution of neighbouring atoms have to be taken into account in the description. We discuss the relative importance of these contributions.

Atomic resolution holography and 3D atomic image reconstruction algorithm

Tomohiro Matsushita

Japan Synchrotron Radiation Research Institute (JASRI/SPring-8)

Recently, atomic resolution holography, such as photoelectron holography, x-ray fluorescence holography, has been dramatically developed. These experimental methods can visualize the three-dimensional atomic structure around the target atomic site, such as dopant, adsorbate and so on. The principle is as follows. When the x-ray is irradiated to sample, an atom absorbs the x-ray and emits the spherical wave such as photoelectron or x-ray fluorescence. The atomic site can be identified by the selection of the energy of the emitted wave. The emitted wave is scattered by the surrounding atoms. The scattered wave interferes with the initial wave and forms the atomic resolution hologram. This is called as the normal mode hologram. The inverse mode, which utilizes the time-reversal of this process, is also utilized as the experimental technique. Therefore, there are many types of the experimental techniques, photoelectron holography, Auger electron holography, inverse x-ray fluorescence holography, internal detector electron holography and so on. The analysis method has been also developed. Beyond the Barton's method, which utilizes a kind of Fourier transform, scattering pattern extraction method using maximum entropy method (SPEA-MEM) was developed. This method can reconstruct the 3D atomic image from the single-energy photoelectron hologram, x-ray fluorescence hologram.

Multiple scattering formulation of the Ballistic Electron Emission Microscopy and STM

Keisuke Hatada

Institut de Physique de Rennes, UMR CNRS-Université 6251, Université de Rennes-1, 35042 Rennes-cedex, France

The Ballistic Electron Emission Microscopy (BEEM) is a powerful technique to investigate interfaces of bulk systems. The experimental setup of BEEM is basically based on the STM, so that the focusing resolution is in order of atomic size as STM. The characteristic point is that the sample has a metal thin layer on semiconductor, which creates Schottky barrier in addition to the tunneling barrier from the tip to the sample. Due to these barriers, there will be created a hot electron, whose kinetic energy is about 1 eV, which travels in the bulk "ballistically", namely the scattering should be elastic without loss.
Theoretical studies have been done by k-space band calculation or tight binding method. K-space method can be useful for understanding in qualitative level, however it is not feasible to have the diffusion process of the hot electron in such huge system. While the tight binding method can be treated in the real space, the approximation is not appropriate for the long traveling delocalized state. We developed a theory for BEEM based on the full potential multiple scattering theory in real space, which quantitatively describes the transport phenomena.


Poster session


Effect of precursor in the metamagnetic transition of FeRh thin films from a local viewpoint

Yuki Wakisaka

Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

The metamagnetic transition in CsCl-type ordered FeRh thin films has been investigated via temperature dependent x-ray absorption fine structure spectroscopy in order to gain correlations between the magnetization and the local electronic and geometric structures, especially with respect to the existence of precursors. According to the Fe and Rh K-edge x-ray absorption near edge structure (XANES), strong hybridization was found to exist between Fe and Rh. Besides, this Fe-Rh hybridization was revealed to decrease during the phase transition from the antiferromagnetic (AFM) to ferromagnetic (FM) phases from the systematic change observed in the Fe K-edge XANES. As for structural fluctuations, only the Debye-Waller factor of the Fe-Fe pair in the AFM phase is considerably enhanced when compared with that in the FM phase, which was ascribed to the precursor effect of the phase transition. This precursor effect supports the recent theoretical studies proposing the fluctuations of spin and structure considering the different features of the exchange interactions in Fe-Rh and Fe-Fe. Therefore, we consider that the local Fe-Fe distance and the spin fluctuations play an important role for driving the metamagnetic transition, whereas the Fe-Rh hybridization is important for determining the static stability of each magnetic order.

Time resolved XAFS study on photoexcited state of tungsten trioxide in picosecond and femtosecond time scales

Yohei Uemura

Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

Photoexcitated states of WO3 were studied by ultrafast XAFS using PF-AR and SACLA. In PF-AR, the differential spectra with 100-ps time resolution showed the decrease of transition from W 2p3/2 to W 5d, which could be caused by structural changes in the excitaed state. The excited state decayed within 10 ns. Faster photoexcitation processes were followed by femtosecond X-rays emitted from SACLA. The fine structures appeared in the high S/N XAFS spectra using SACLA, which provided more precise information about the transient state of WO3 after the photoexcitation.

The effect of spin-orbit interaction on surrounding atoms in XMCD for transition metals

Akihiro Koide(1), Kaori Niki(1), Seiji Sakai(2) and Takashi Fujikawa(1)

(1)Graduate School of Advanced Integration Science, Chiba University,
Japan, (2)Advanced Science Research Center, Japan Atomic Energy Agency, Japan

Unexpected large XMCD spectra for light elements have been reported in some systems [1]. Single scattering calculation of Fe K-edge XMCD for Fe metal had shown that only spin-orbit interaction (SOI) on an absorbing atom was important [2]. However, the observed XMCD intensity for light elements was not obtained by full multiple scattering calculations including SOI only on an absorber [1]. To improve our calculation, SOI on surrounding atoms is taken into account carefully in the present theory. Before the investigation of light element XMCD spectra, we study Fe K-edge XMCD and show that these SOI are also important even for Fe metal: the SOI on far scattering sites (~6 Angs.) still has certain amount of contribution to XMCD intensity.
[1] Y. Matsumoto et al. J. Mater. Chem. C 1 (2013) 5533.
[2] T. Fujikawa and S. Nagamatsu, Jpn. J. Appl. Phys. 41 (2002) 2005.

On the theory of Photoelectron Yield Spectroscopy

Shohei Tadano and Peter Krüger

Nanomaterial Science Department, Chiba University, Chiba 263-8522,Japan

In Photoelectron Yield Spectroscopy (PYS) the total number of photoelectrons ejected from the surface is measured as a function of the incident light energy. The absolute value of the ionization potential can be found from the threshold energy of the PYS spectrum. For this purpose, PYS is a simpler and more sensitive method than UPS. In order to determine the ionization potential accurately, we need to know the theoretical shape of the PYS spectrum in the threshold region. Calculating the spectrum theoretically for each type of samples (metal, semiconductor and organic) is our purpose. We made a simple model about the electronic surface transmission and introduce new formula which contains a transmission probability function. In this formula we only need information of density of state. We calculated spectra using typical DOS and our formula and obtained a good agreement with experiment. Moreover, I will explain how to get information about DOS from PYS spectrum.