Electronic correlations in the Fe-pnictide BaFe2As2 are explored within LDA+DMFT, the combination of density functional theory with dynamical mean-field theory. While the correlated band structure is substantially renormalized there is only little transfer of spectral weight. The computed k-integrated and k-resolved spectral functions are in good agreement with photoemission spectroscopy (PES) and angular resolved PES experiments. Making use of a general classification scheme for the strength of electronic correlations we conclude that BaFe2As2 is a moderately correlated system.

BaFe 2 As 2 exhibits properties characteristic of the parent compounds of the newly discovered iron (Fe)-based high-T C superconductors. By combining the real space imaging of scanning tun-neling microscopy/spectroscopy (STM/S) with momentum space quantitative Low Energy Electron Diffraction (LEED) we have identified the surface plane of cleaved BaFe 2 As 2 crystals as the As terminated Fe-As layer-the plane where superconductivity occurs. LEED and STM/S data on the BaFe 2 As 2 (001) surface indicate an ordered arsenic (As)-terminated metallic surface without reconstruction or lattice distortion. It is surprising that the STM images the different Fe-As orbitals associated with the orthorhombic structure, not the As atoms in the surface plane.

The electronic structure in alkaline earth AeO (Ae = Be, Mg, Ca, Sr, Ba) and post-transition metal oxides MeO (Me = Zn, Cd, Hg) is probed with oxygen K -edge X-ray absorption and emission spectroscopy. The experimental data is compared with density functional theory electronic structure calculations. We use our experimental spectra of the oxygen K -edge to estimate the bandgaps of these materials, and compare our results to the range of values available in the literature. From the calculated partial DOS we conclude that the position of main O K -edge X-ray emission feature in BeO, SrO and BaO is defined by the position of the np-states of the cation while in the other oxides studied here the main O K -edge X-ray emission feature is defined by the position of the (n-1)d (for CaO) or nd-states of the cation.

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The electronic structures of undoped and fluorine-doped LaFeAsO superconductors and fluorine-doped SmFeAsO superconductors are studied using soft x-ray absorption and emission spectroscopy combined with full potential linearized augmented planes waves ͑FP LAPW͒ calculations. The comparison between the numerical simulations and the experimental Fe L-emission spectra shows that the Fe states are concentrated near E F , suggesting that the materials are not highly correlated systems. The comparison of the O K-edge and F K-edge spectra with the calculated density of states shows that the F dopants do not directly participate in the electronic structure near E F , and so they serve the same purpose as an oxygen vacancy leading to an increase in the number of available charge carriers. Increasing the amount of fluorine doped into the structure of SmFeAsO is shown to cause a narrowing of the bandwidth of occupied O valence states.

Resonant x-ray emission spectroscopy (XES) measurements at Fe L(2,3) edges and electronic structure calculations for LiFeAs and NaFeAs are presented. Experiment and theory show that in the vicinity of the Fermi energy, the density of states is dominated by contributions from Fe 3d states. The comparison of Fe L(2,3) XES with spectra of related FeAs compounds reveals similar trends in energy and the ratio of intensities of the L(2) and L(3) peaks (I(L(2))/I(L(3)) ratio). The I(L(2))/I(L(3)) ratio for all FeAs-based superconductors is found to be closer to that of metallic Fe than that of the strongly correlated FeO. We conclude that iron-based superconductors are weakly or, at most, moderately correlated systems.

In this paper we report the first LDA+DMFT results (method combining Local Density Approximation with Dynamical Mean-Field Theory) for spectral properties of superconductor LaFePO.

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