P. Lagarde

L2,3 XANES of the High Tc Superconductor YBa2Cu3O~7 with Variable Oxygen Content

Abstract The copper L2,3 soft x-ray absorption spectra of the single phase high temperature (Tc=91 K) superconductor YBa2Cu3O≈7 have been measured by using synchrotron radiation at LURE. The spectra show at the absorption threshold a white line at 931.1 eV due to Cu 2p 3d10 final states (the underline indicates the hole) for the Cu 3d9 initial state, and a broad high energy shoulder with the maximum at 933 eV assigned to Cu 2p 3d10 L and to Cu 2p 3d10 L 4s1 final states ( L indicates a hole in the oxygen 2p derived valence band), i.e. to the initial states 3d9 L . The intensity of this last feature has been found to decrease with decreasing oxygen content x in YBa2Cu3O6.5+x. No evidence of Cu 2p 3d9 final states has been found. This experiment gives direct evidence that additional oxygen x does not give Cu3+ ions with Cu 3d8 configuration but gives 3d9 L electronic configuration (i.e. Cu2+O- pairs) and no gap is observed between the 3d9 (Cu+) and 3d10 L configuration (i.e. Cu+O- pairs). Therefore in the metallic phase which exhibits superconductivity a continous set of itinerant states 3d9 L and 3d10 L is observed.

Evidence of 3d9-ligand hole states in the superconductor La1.85Sr0.15CuO4 from L3 X-ray absorption spectroscopy

The Cu L3 X-ray absorption spectra of La2−xSrxCuO4−y (x=0, 0.15 and 0.88) have been measured with high resolution. For comparison the L3 X-ray absorption spectrum of the formally trivalent Cu compound La2Li0.5Cu0.5O4 is reported. The semiconductor La2CuO4, with Cu formal valence v=2, shows a single white line due to the 2p3d10 final state arising from the Cu ed9 initial state. The superconducting sample with x=0.15 (Tc=37 K) shows the additional 2p3d10L final states arising from the initial states 3d9L (where L is a hole in the oxygen derived band, ligand hole). The characteristic feature of the superconducting material is that there is no energy gap between the final states 2p3d10 and 2p3d10L. In the semiconducting compound, formed by increasing Sr concentration (x=0.88), a red shift of 0.2 eV of the 2ped10 final state is enough to create a gap between the two states. The conductivity in these ceramic materials is assigned to the itinerant 3d9L states (probed by the 2p 3d10L final state) which indicate the presence of oxygen hole carriers. These results support a microscopic mechanism for high Tc superconductivity due to pairing of oxygen holes coupled with localized Cu 3d9 localized states i.e. of two itinerant 3d9L states.

Determination of the symmetry of the 3d9L states by polarized CuL3XAS spectra of single crystal YBa2Cu3O≈6.9

Abstract The symmetry of the charge carriers 3d 9 L in the high T c superconductor YBa 2 Cu 3 O 6.9 has been determined by measuring the po larized L 3 x-ray absorption spectra of a single crystal. The intinerant holes of the 3d 9 L configuration formed by oxidation of the divalent Cu compound are found to have only 3d 3z 2 -1 2 character at the Cu site. Therefore the charge carriers induced by doping are not formed in the two dimensional band derived by the Cu 3d x 2 -y 2 but in a band derived by the Cu 3d 3z 2 -1 2 orbital orthogonal to the ab plane of the CuO 2 layers. This result indicates that the ligand holes are in a band involving also the BaO layers and the theories of high T c superconductivity have to take account of a second band with different symmetry, beyond a single band models of the electronic structure.

EXPERIMENTAL EVIDENCE OF ITINERANT Cu 3d9 - OXYGEN HOLE MANY BODY CONFIGURATION IN THE HIGH-TC SUPERCONDUCTOR YBa2Cu3O~7

Cu L3 x-ray absorption near edge structure (XANES) and Cu L3 x-ray photoelectron spectroscopy (XPS) of YBa2Cu3O6.5+x are compared. The breakdown of one-electron picture of its electronic structure is reported. The data are interpreted by mixing of Cu 3d9 and of (where is a hole in the oxygen derived band, ligand hole) many body configuration in the initial state. The localization of Cu 3d9 configuration is indicated by the bare Coulomb interaction Udd~6 eV. The conductivity is assigned to the itinerant configuration. The experimental evidence that the additional oxygen x, giving higher Tc, increases the weight of the configuration is reported. The presence of holes on the oxygen atoms is confirmed by the Ols XPS spectra. The Cu3+(Cu 3d8) configuration is not observed in L3 XANES in agreement with valence band XPS giving the energy of the 3d8 excited state at about 12 eV above the ground state. An energy scheme of the many body configurations in YBa2Cu3O~7 is obtained. These experiments give experimental evidence that the high Tc superconductivity is due to pairing of holes in the oxygen valence band interacting with localized electrons at the Cu sites.

Na K-edge XANES spectra of minerals and glasses

Na K-edge X ray absorption near-edge structure (XANES) spectra for minerals and glasses were collected using synchrotron radiation. Na-K XANES is a good structural probe to determine the coordination state of this element in important minerals and glasses. From the XANES spectra on crystalline reference phases with different chemical environment, information can be obtained and compared with the structure of the minerals and glasses. The different classes of studied minerals correspond to distinct Na K-edge spectra which have been examined. The XANES spectra for the crystalline references are used as fingerprint to interpret those obtained on silicate and borate glasses. Experimental spectra for jadeite and carnegieite crystals can be satisfactorily reproduced by theoretical calculations using the Full Multiple Scattering formalism.

XANES Study of Structural Disorder in Amorphous Silicon

An investigation of the structure of several amorphous silicon (a-Si) films is presented. Samples were prepared by using the ion beam sputtering technique at different substrate deposition temperatures. X-ray absorption spectroscopy and multiple scattering formalism have been used to detect structural variations of the a-Si films. The analysis of the XANES (X-ray absorption near-edge structure) spectra shows that increasing the substrate deposition temperature leads to a structural change toward a higher-level short-range order.

Correlation between mixing of Cu d orbitals and Tc determined by polarized Cu L3 XAS: Experimental evidence for pairing mediated by d-d excitations

Abstract Experimental evidence for the increasing probability of the Cu 3d z 2 holes with increasing T c has been found in BiCaSrCuO superconductors by high resolution Cu L 3 XAS. The weight of the Cu 3d z 2 holes increases by about 5% going from the normal phase at 300K to the superconducting phase at 10K. The energy splitting Δ z between the d z 2 and d x 2 −y 2 hole states decreases by increasing T c by doping within each class of high T c superconductors. The range of values of Δ z increases going from one class of high T c superconductors to another class with higher T c . These results give a positive experimental test for pairing mechanisms involving the Cu 3d z 2 holes and in particular for pairing mediated by virtual d-d excitations.

Structural studies of amorphous CoP and NiP by EXAFS

Abstract The EXAFS spectra of amorphous CoP and NiP alloys have been recorded and analyzed on both edges of the metal and metalloid. The spectra cannot be directly fitted by the classical neutrons or X-rays results: these appear to be the average values of asymmetric radial distribution functions that we have to use in EXAFS analysis. Then we show that the environment of the phosphorus atoms in the amorphous state is very reminiscent of that of the Co2P or Ni3P crystalline materials.

Probing matrix isolated SiO molecular clusters by X-ray absorption spectroscopy

TheK absorption edge of Si in matrix isolated SiO molecular clusters was studied for various dilutionsR(Ar/SiO). The spectra from the clusters at different dilutions show a dramatic evolution in their relative intensity. Two types of spectral features from Si atoms present in the clusters could be detected in the spectra. The first is due to the Si atoms which are tetrahedrally coordinated to Si and O atoms responsible for the presence of different Si oxidation states. The Si+, Si3+ and Si4+ oxidation states in SiO clusters and in bulk were readily identified. The Si2+ oxidation state whose abundance in SiO clusters is predicted by structural considerations is not clearly observed. The second type of feature, whose intensity decreases with the dilution R, can be attributed to the presence of Si atoms possessing unsaturated bonds. In order to explain this the clustering of 4–6 SiO molecules is simulated to yield different tetrahedral bonding microstructures in which the well coordinated as well as under coordinated Si atoms are present. The behaviour of the spectral features resulting from these microstructures is discussed.

Speciation of Mg in biogenic Calcium Carbonates

A selection of marine biominerals, mostly aragonitic coral skeletons were probed at the Mg K-edge by XANES spectroscopy coupled to μXRF methods and compared to an extensive set of relevant model compounds (silicates, carbonates, oxides and organic). Extensive methodologies are required to better describe the speciation of Mg in those minerals. A combination of ab-initio XANES calculations for defective clusters around Mg in aragonite together with wavelets analyzes of the XANES region are required to robustly interpret the spectra. When using those methodologies, the speciation of Mg ranges from a magnesite-type environment in some scleractinian corals to an organic-type environment. In all environments, the Mg-domains probed appear to be less than 1 nm in size.