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International audience; A brief derivation of the cross section in the independent particle approximation for some of the spectroscopies treated in the msspec program package is presented. We solve the related Schrödinger equation with a complex energy-dependent effective potential in the framework of multiple scattering theory to write the cross-section for photoemission and photoabsorption in a physically transparent way that provides insight in their interpretation and analysis. Relativistic corrections are also implemented. In order to be able to apply this theory to a wide variety of systems we use a kind of all-purpose optical potential, depending only on the local density of the system under investigation, and discuss its merits and drawbacks. A Green’s function approach is shown to be necessary to write the photoabsorption cross section in the case of complex potential. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; We derive a general spherical wave multiple scattering expression of the EELS cross-section. Contrarily to most of the previous theoretical frameworks, this approach treats all the electrons involved on an equal footing with respect to multiple scattering. The main point in our results is that the cross-section depends not only on Coulomb integrals on the absorbing atom, but also on contributions from all the other atoms. We show that these external contributions should be restricted to neighbouring atoms. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; We present a scattering theoretic approach to the calculation of the cross-section of (e,2e) impact spectroscopy where all the electrons involved are treated within the real space multiple scattering framework. This approach is particularly suited to the reflection geometry at low kinetic energies, with the ejection of a core-level electron. In this case, we expect (e,2e) spectroscopy can be turned into an extremely sensitive surface structure probe. © 2018, Springer International Publishing AG, part of Springer Nature.

International audience; We present an interface package, called ES2MS, for passing self-consistent charge density and potential from Electronic Structure (ES) codes To Multiple Scattering (MS) codes. MS theory is based on the partitioning of the space by atomic-size scattering sites, so that the code provides the charge densities and potentials for each scattering site. For pseudo potential codes, the interface solves Poisson equation to construct the all-electron potential on the radial mesh which is used to solve the transition operators (T-matrix) and Green’s functions in MS codes. We show the algorithm of the interface and the example for X-ray absorption spectra of graphene.

International audience; Following our recent derivation of a Real Space Full-Potential Multiple-Scattering-Theory (RSFP-MST), we calculate the low-energy part of the K-edge absorption spectra of the transition metals of the first row of the periodic table using the Hedin-Lundqvist (HL) and the Dirac-Hara (DH) complex potentials, which are often used in the literature for this kind of calculations. In both cases the complex part is taken from the HL potential. Taking advantage of the fact that in the FP-MS scheme the Schrödinger Equation is solved exactly without approximations (within the limits of the l-truncation procedure in MST), we intend to test the ab initio validity of these optical potentials in cases that are borderline between the independent particle and the weakly correlated regime. Surprisingly enough, we find better agreement with experiments with the DH potential.

Resonant X-ray scattering (RXS) is a spectroscopy where both the power of site selective diffraction and the power of local absorption spectroscopy regarding atomic species are combined. By virtue of the dependence on the core level state energy and the three dimensional electronic structure of the intermediate state, this technique is specially suited to study charge, orbital or spin orderings and associated crystal distortions. In the case of charge ordering, we exploit the fact that atoms with closely related site symmetries but with small charge differences exhibit resonances at slightly different energies. The sensitivity of this effect allows for quantitative estimations of the charge disproportion. Opposite to fluorescence or absorption measurements, the power of diffraction relies on the capability of detecting differences that are smaller than the inverse lifetime of the core hole level. To account for the uncertainty of the crystallographic structure and the fact that the charge ordering must be disentangled from the associated atomic displacements, a complete methodology is proposed and applied to the low temperature phase of magnetite. Relative sensitivity on spin, toroidal and orbital ordering is also shown and compared in different transition metal oxide compounds, like V2O3 and GaFeO3.