M. Pompa

Atomic and electronic structure probed by X-ray absorption spectroscopy : full multiple scattering analysis with the G4XANES package

Abstract G4XANES, a software package calculating X-ray Absorption Near Edge Structure (XANES) spectra in the frame of the one-electron full multiple scattering formalism, is presented. The G4XANES package performs one-electron K,L-XANES “ab initio” simulations on different kinds of compounds (small molecules, crystals, free clusters, metal sites in biological compounds) by using a minimal set of input data. Polarized spectra as a function of the size of the scattering cluster are calculated, allowing to extract information on both the electronic and atomic structure around the selected absorbing atom. Recently reported spin-dependent XANES spectra have been also successfully treated. A description of the package, as well as its applications for such important classes of materials as high temperature superconductors, magnetic materials and metalloproteins, are presented.

One-electron excitations and shake up satellites in Cu K-edge X-ray absorption near edge structure (XANES) of La2CuO4 by full multiple scattering analysis in real space

Abstract The polarized Cu K-edge X-ray Absorption Near Edge Structure (XANES) of La2CuO4 has been interpreted by the multiple scattering approach. The size of the cluster of neighbouring atoms having good agreement with the XANES experimental data is determined by 45 atoms surrounding the absorbing Cu. The polarized spectra can be predicted in term of a one-electron dipole (Δl=+1) transition Cu 1s→ϵp, probing the unoccupied p-like (l=1) density of states projected on a Cu site with orbital angular momentum ml=0 in the E∥z spectra, and the ml=1 for the E⊤c spectra. Thus we show that the electronic structure of the high energy conduction bands, beyond the Cu 3d band, of La2CuO4 are well described in terms of the one-electron approximation. It is shown that XANES spectra are consistent with the contraction of the Cu-apex oxygen distance with doping. Final state effects induced by the core hole have been indentified: (i) the core transitions take place in the fully relaxed potential, (ii) the satellite at 7 eV above the main K-XANES peak in both polarizations is assigned to a multielectron shake up excitation. Finally the shoulder on the rising absorption edge, present in many Cu compounds and usually assigned to a shake down multi-electron excitation, is shown to be due to a one-electron transition to a state delocalized over a large cluster.

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.

Structure of the Different Cu Sites in the Corrugated CuO2 Plane in High Tc Superconductors

By using x-ray absorption spectroscopy different configurations of the Cu site structure in the CuO2 plane of the Bi 2:2:1:2 high Tc superconductor are found. The different Cu sites are characterized by short 2.3 A and long 2.45 A Cu-O(apical) distances. The linear arrays of different Cu sites forming domains with a corrugated-iron-foil shape is proposed to be a key feature of superconducting domains in the CuO2 plane. The wavelength λ of the modulation is close to the superconducting coherence length ξ and kFλ~2π. The ordering of the distorted Cu sites is suggested to be evidence for ordering of polarons driven by the pseudo Jahn Teller electron lattice interaction.

Aluminum site structure in serum transferrin and lactoferrin revealed by synchrotron radiation X-ray spectroscopy

The Al site structure of serum transferrin and lactoferrin is investigated using X-ray absorption near edge structure (XANES) spectroscopy. Al K-edge spectra in the mono- and dialuminum forms of the proteins have been recorded for the first time. Our results show that the aluminium ion is hexa-coordinated in an octahedral-like symmetry and that the monoaluminum form, where only the C-terminal binding site is saturated, has an increased structural distortion around the metal site.

Full multiple scattering analysis of linearly polarized Cu L3-edge XANES of La2CuO4

Abstract The linearly polarized Cu L3-edge X-ray absorption near edge structure (XANES) of a La2CuO4 single crystal has been measured and the spectrum is interpreted by the full multiple scattering approach in the real space. The polarized XANES spectra over a range of 20 eV can be predicted in term of a one-electron dipole (δl = +1) transitions Cu 2p→ϵd, probing the unoccupied d-like (l=2) density of states of the high energy conduction bands. The white line is shown to be due to Cu 2p→3d transitions giving a bound final state Cu 2p53d10 formed by the 5.5 eV Coulomb interaction between the core hole and the 3d electron. The probability of ml=2 orbital angular momentum for the localized Cu 3d hole is shown to be in the range 95–100%.

On the orbital angular momentum of Cu 3d hole states in superconducting La 2 −xSrxCuO4

The symmetry of the 3d9L* states induced by doping has been studied in a superconducting La2-xSrxCuO4 (x = 0.08, T(c) = 20 K) single crystal by polarized Cu L3-edge X-ray absorption spectroscopy (XAS) by using synchroton radiation. The integral of the unpolarized white line at threshold increases by 15% going from the insulating La2CuO4 to the superconductor La1.92Sr0.08CuO4 due to the additional 3d9L* states formed in the gap. The weight of the 3dx2-y2(9)L* (3d3z2-y2(9)L*) states is obtained from the variation of the polarized E perpendicular-to c and E parallel-to c spectra is found to be 70% (30%) of the total 3d9L* states. These experimental results show that the orbital angular momentum of the 3d holes is not frozen in the m(l) = 2 (3dx2-y2) symmetry in the doped cuprate perovskite suggesting the presence of a second band with mostly 3d3z2-r2 character close to the Fermi level.

Self-absorption effect in fluorescence XAS of polarization dependent white lines

Abstract The application of fluorescence yield (FY) in the soft energy domain, and the problem of self-absorption corrections in the presence of polarization dependent white lines are discussed here. A comparison of X-ray FY and total electron yield (EY) X-ray absorption spectra at the Bi2Sr2CaCu2O8 CuL3 edge is presented.

Coupling between the charge carriers and lattice distortions via modulation of the orbital angular momentum Mℓ=0 of the 3d holes by polarized xas spectroscopy

Abstract The change of the orbital angular momentum M l of the Cu 3d holes going from the insulating to the metallic phase has been studied in several families of high Tc superconductors. The symmetry of the 3d9 L states in the metallic phase has been studied by quantitative analysis of the variation of polarized Cu L3 x-ray absorption spectra. At a doping level nh∼15% we have found 10% of Zhang-Rice singlets 3d x 2 -y 2 L (b 1 ) and 5% of 3d 3z 2 -r 2 L (a 1 ) states. Therefore the percentage of the 3d3z2−r2 L states on the total number of the 3d9 L states is about 30% i.e. much larger than the probability of single hole states 3d3z2−r2 in the insulating phase. The EXAFS and XANES studies of the Cu site structure and dynamics in Bi2Sr2Ca1−xYxCu2O8 system point toward the coupling of the charge carriers with distortions of the Cu sites driven by the M l=0 character of the Cu 3d holes that can be called a 3dz2−r2 polaron.

ELECTRONIC STRUCTURE OF LA2CUO4 BY JOINT ANALYSIS OF O K AND CU K AND L3 EDGE X-RAY ABSORPTION SPECTRA

Abstract The symmetry of the conduction bands over a range of 15 eV above the Fermi level in La2CuO4 has been studied by the joint analysis of the polarized X-ray absorption spectra at several edges: O K-edge, Cu K-edge, and Cu L3-edge. The partial density of states of the conduction bands DX (l, ml) projected on the atomic site X (Cu and O) with selected orbital angular momentum l, and its projection ml along the c-axis have been obtained. The oxygen K-edge X-ray absorption near edge structure (XANES) spectrum has been calculated by multiple scattering approach. A good agreement between the one-electron calculations in the fully relaxed final state potential and the experimental spectra for the final states in the high energy conduction bands has been found.