Valence fluctuations and oxygen dimerization in high-temperature superconductors from X-ray photoemission spectroscopy

The high-temperature superconducting Cu oxides — which are intrinsically Mott insulators — are strongly correlated metals because of valence fluctuations involving Cud ⁹–d ¹⁰ configurations. Temperature-dependent X-ray photoemission spectroscopy of theO 1s and Cu 2p levels in the range 10–300 K provides evidence for O-dimerization, with corresponding increase of the Cud ¹⁰O 2p ⁵ well-screened XPS-final state belowT _c . This supports the suggestion of dynamical mixed-valence and double-exchange antiferromagnetic interaction as an important mechanism for the origin of highT _c superconductivity.     

X-ray absorption and photoemission spectroscopy of 3C- and 4H-SiC

We have studied the electronic properties of 3C- and 4H-SiC with X-ray absorption (XAS). Particular emphasis is placed on the conduction bands because they exhibit larger differences between the various SiC polytypes than valence bands. XAS spectra at the Si2p and C1s edges provide projections onto Si3d, 4s and C2p conduction band states. We explain the observed differences in the Si L_2,3 XAS data to arise from transition into dispersive bands which occur at the M and K point of the hexagonal Brillouin zone. The XAS data are sensitive to a difference in the dispersion of the two lowest conduction bands. For 3C-SiC the dispersion is larger than for 4H-SiC in agreement with theory. We compare the XAS data at the Si L edge with CFS and CIS spectra and find that the SiLVV Auger is dominant.     

Medieval glass from the Cathedral in Paderborn: a comparative study using X-ray absorption spectroscopy, X-ray fluorescence, and inductively coupled laser ablation mass spectrometry

We have investigated four stained glass samples recovered from an archaeological excavation at the Cathedral in Paderborn (Germany) between 1978 and 1980. On two of the samples there are parts of paintings. Concentrations of major elements were determined using two independent techniques: LA–ICP–MS (a UV laser ablation microsampler combined with an inductively coupled plasma mass spectrometer) and synchrotron radiation X-ray excited X-ray fluorescence (SR-XRF). The SR-XRF data were quantified by using the program package PyMCA developed by the software group of the ESRF in Grenoble. Significant differences were found between the concentrations determined by the two techniques that can be explained by concentration gradients near the surface of the glasses caused, for example, by corrosion/leaching processes and the different surface sensitivities of the applied techniques. For several of the elements that were detected in the glass and in the colour pigments used for the paintings X-ray absorption near edge structure (XANES) spectra were recorded in order to determine the chemical speciation of the elements of interest. As was expected, most elements in the glass were found as oxides in their most stable form. Two notable exceptions were observed: titanium was not found as rutile—the most stable form of TiO₂—but in the form of anatase, and lead was not found in one defined chemical state but as a complex mixture of oxide, sulphate, and other compounds.     

Laser spectroscopy applied to the study of hyperfine interactions

The impact of laser-spectroscopy techniques on the study of hyperfine interactions is illustrated by chosen examples. A brief survey of the theory of hyperfine interactions is given as well as an overview of the available experimental techniques. Results from alkali atoms, alkaline-earth elements and Group III atoms are given with special emphasis on the studies of electronic properties in sequences of Rydberg states and the investigation of nuclear properties for long chains of isotopes of the same element. Recent developments on the theoretical side, particularly what regards the many-body perturbation approach, are also discussed.     

ESR spectroscopy applied to the study of radiation mechanisms

ESR spectroscopy is a powerful tool for detecting radicals or radical-ions trapped in solids, and in many cases “primary” electron-gain and-loss centres can be identified and their structures elucidated. Sometimes these primary centres are trapped in pairs, separated byca. 5 to 15 Å giving triplet state ESR spectra. The significance of pair-trapping in relation to current theories for the action of ionizing radiation will be discussed with reference to several recent examples. Generally, the primary centres are well separated and ESR does not give useful information about spurs or local regions of high radical densities. Reasons for trapping will be summarized with specific reference to DNA and proteins. The question of electron-return will be discussed, and several examples of this will be reported.     

Hard X-ray photoemission spectroscopy: Variable depth analysis of bulk,surface and interface electronic properties

The electronic properties of surfaces and buried interfaces can vary considerably in comparison to the bulk. In turn, analyzing bulk properties, without including those of the surface, is understandably challenging. Hard X-ray photoelectron spectroscopy (HAXPES) allows the well known ability of photoemission to interrogate the electronic structure of material systems with bulk volume sensitivity. This is achieved by tuning the kinetic energy range of the analyzed photoelectrons in the multi-keV regime. This unique ability to probe truly bulk properties strongly compliments normal photoemission, which generally probes surface electronic structure that is different than the bulk selected examples of HAXPES and possible implications towards the study of complex oxide-based interfaces and highly correlated systems are discussed.     

The characterization of Cr secondary oxide phases in ZnO films studied by X-ray spectroscopy and photoemission spectroscopy

X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS) were used to characterize the Cr secondary oxide phases in ZnO films that had been prepared using a co-sputtering method. Analysis of the Cr L_3,2-edge XANES spectra reveals that the intensity of white-line features decreases subtly as the sputtering power increases, indicating that the occupation of Cr 3d orbitals increases with Cr concentration in (Zn, Cr)O films. The O K-edge spectra show that the intensity of XANES features of (Zn, Cr)O films is lower than those of ZnO film, suggesting enhanced occupation of O 2p-derived states through O 2p-Cr 3d hybridization. The XES and XPS spectra indicate that the line shapes in the valence band of (Zn, Cr)O films are quite different from those of ZnO and that the Cr₂O₃ phase dominates the spinel structure of (Zn, Cr)O films increasingly as the Cr sputtering power is increased. Over all results suggest that the non-ferromagnetic behavior of (Zn, Cr)O films can be attributed to the dominant presence of Cr₂O₃, whereas the bulk comprise phase segregations of Cr₂O₃ and/or ZnCr₂O₄, which results them the most stable TM-doped ZnO material against etching.     

A method for the experimental determination of the net atomic charge via X-ray photoemission spectroscopy

A method for the experimental determination of the net charge distribution over the atoms in a large class of compounds is described. The method is based on X-ray photoemission spectroscopy and requires the determination of two core-level energies per constituting atomic species. Two special cases of large conceptual and practical interest are discussed: the silicon charge in CoSi₂, Si_1-xGe_x (0≤x≤0.3), SiC, Si₃N₄ and SiO₂, and the aluminum charge in aluminum-doped zeolites. Received: 20 March 2000 / Accepted: 28 March 2000 / Published online: 21 March 2001     

X-ray photoemission study of the outermost levels of the rare earth metals

Standard X-Ray photoemission technique has been applied to the study of 4f and valence states of the complete lanthanide series. The energy range of 15     

Direct observation of the mass renormalization in SrVO3 by angle resolved photoemission spectroscopy

Band dispersions and Fermi surfaces of the three-dimensional Mott-Hubbard system SrVO3 are directly observed by angle-resolved photoemission spectroscopy. An observed spectral weight distribution near the Fermi level (E(F)) shows cylindrical Fermi surfaces as predicted by band-structure calculations. By comparing the experimental results with calculated surface electronic structures, we conclude that the obtained band dispersion reflects the bulk electronic structure. The enhanced effective electron mass obtained from the energy band near E(F) is consistent with the bulk thermodynamic properties and hence with the normal Fermi-liquid behavior of SrVO3.     

Energy-dispersive dependences of the X-ray photoemission of electrons from implanted silicon

The possibilities of interpreting the secondary-electron energy spectra and recording the X-ray photoemission of single-crystal silicon samples are examined. Detected shifts in the energy-spectrum extremum, as well as its height, and the entire energy-distribution form for different types of implanted and annealed Si samples, indicate the degree of ordering of surface atoms. The possibilities of employing the Friedel- oscillation model are discussed.     

Electronic structure of the YH3 phase from angle-resolved photoemission spectroscopy

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on angle-resolved photoemission experiments of the Y trihydride phase. Most importantly, we find the absence of metal d bands at the Fermi level and a set of flat, H-induced bands located at much higher binding energy than predicted, indicating an increased electron affinity at H sites.     

LIPS and linear correlation analysis applied to the classification of Roman pottery Terra Sigillata

Archaeological ceramics Terra Sigillata manufactured in different production centres have been studied by laser-induced plasma spectroscopy (LIPS). The aim of this work was to establish a procedure for the rapid classification of these archaeological ceramics in function of their provenance through combination of LIPS and statistical methodologies. Representative emission spectra of the Hispanic, Gaulish and African groups of pottery were selected as references. The use of linear correlation allowed one to cluster the samples by quantitative comparison of LIP spectra, leading to a reliable assignment of Terra Sigillata pieces to origin centres. PACS 42.62.Fi; 32.30.Jc; 02.50.-r; 81.05.Je     

Influence of elastic scattering on the measurement of core-level binding energy dispersion in X-ray photoemission spectroscopy

We explore the interplay between the elastic scattering of photoelectrons and the surface core level shifts with regard to the determination of core level binding energies in Au(111) and Cu₃Au(100). We find that an artificial shift is created in the binding energies of the Au 4f core levels, that exhibits a dependence on the emission angle, as well as on the spectral intensity of the core level emission itself. Using a simple model, we are able to reproduce the angular dependence of the shift and relate it to the anisotropy in the electron emission from the bulk layers. Our results demonstrate that interpretation of variation of the binding energy of core-levels should be conducted with great care and must take into account the possible influence of artificial shifts induced by elastic scattering.     

A study of the firing temperature of archeological pottery by X-ray diffraction and electron paramagnetic resonance

A fragment of an archeological funerary urn from Campos dos Goytacazes, Brazil, was studied using electron paramagnetic resonance (EPR) and X-ray diffraction (XRD). The thermal stability of all paramagnetic species was studied with isothermal treatment. In the present study, the iron signal (Fe³⁺) cannot be used as a firing temperature reference for archeological pottery. The intensification of this signal with temperature is a consequence of Fe²⁺ oxidation, but this reaction occurs in a short-lived treatment at high temperature or in an extended treatment at lower temperature. However, the iron signal and three other paramagnetic species indicate that the urn was fired for an extended time (up to three days). The thermal stability of the three paramagnetic species indicates a firing temperature of around 500 °C in the inner layer, between 400 and 500 °C in the middle layer, and between 500 and 800 °C in the outer layer. The presence of kaolinite structures only in the middle portion is consistent with the temperature values estimated. A firing method for the funerary archeological urn is suggested.     

Buildup of the InSe/M interface (M Pd, Au) studied by X-ray photoemission and X-ray absorption spectroscopy

The electronic properties of InSe/M (M  Pd, Au) interfaces have been studied by X-ray photoemission measurements. For the InSe/Pd interface, it has been found that Pd atoms diffuse into the InSe lattice at early stages of Pd coverage, acting as acceptor centers. As the Pd coverage increases, a Pd-InSe reaction determines the electronic behaviour of the interface. However, for Pd coverages higher than 1 ML, the barrier formation tends to be controlled by an emerging bulklike Pd overlayer. Despite the atomic structure of this system is far from that expected for an ideal Schottky one, the final electronic barrier value is close to that expected for an abrupt InSe/Pd Schottky interface. On the contrary, the InSe/Au system appeared to behave as a quasi-ideal abrupt Schottky interface. Annealing processes performed at temperatures higher than 600 K alter this scheme, as revealed by X-ray absorption spectroscopy measurements, enhancing diffusion of Au atoms into InSe. In any case, the electronic barrier results to be determined by the Au overlayer formed.