We present experimental and theoretical results on the momentum distribution and the quasiparticle renormalization factor in sodium. From an x-ray Compton-profile measurement of the valenceelectron momentum density, we derive its discontinuity at the Fermi wavevector. This yields an accurate measure of the renormalization factor that we compare with quantum Monte Carlo and G0W0 calculations performed both on crystalline sodium and on the homogeneous electron gas. Our calculated results are in good agreement with the experiment.

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International audienceThe band structure of the diluted magnetic semiconductor (Ga,Mn)N, and the x-ray absorption near-edge structure (XANES) at the K edge of Mn, were calculated using the linearized augmented plane wave method. The calculated K-edge spectra fit well with experimental data obtained on samples of Ga1-xMnxN with a wide range of Mn content, from x=0.3% to 5.7%. These samples were grown by molecular beam epitaxy. X-ray diffraction measurements and extended x-ray absorption fine structure studies were used to confirm the wurtzite structure of the samples, the absence of any secondary phase, and the substitutional position of Mn in the gallium sublattice of GaN. The shape of the measured XANES spectra does not depend on the Mn content, implying the same valence state and local atomic structure around the Mn atom in all samples. The comparison between the measured spectra and the results of the ab initio calculation offers a clear interpretation of the preedge structure: It is mainly due to dipolar transitions, with a single peak in the case of Mn2+ and an additional peak for Mn3+. Such a behavior of the XANES preedge of Mn2+ was confirmed experimentally on Ga,MnAs and Zn,MnTe. We conclude that the valence state of Mn in wurtzite (Ga,Mn)N is 3+, a conclusion which is also supported by infrared optical transmission and magnetization data obtained on the same samples

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This study presents the detailed nature of iron clusters formed on Fe 3+ -Nafion membranes. The catalytic nature of these clusters during immobilized Fenton processes was observed to be a function of the deposition method of Fe ions on the Nafion. The nonbiodegradable azo-dye Orange II and 2-propanol were utilized as convenient organic model compounds in photoassisted Fenton degradation processes. The highest photocatalytic activity was observed when samples were prepared by ion exchange between iron(III) aquacomplexes and H + or Na + as counterions of the Nafion SO3group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H2O)6] 3+ and binuclear complexes [Fe(H3O2)Fe] 5+ and [Fe-O-Fe] 4+ . If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing R-Fe2O3 and [Fe-O-Fe] 4+ . Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchanged and also base-treated membranes showing R-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe 3+ -Nafion by femtosecond laser spectroscopy. The R-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with R-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of R-Fe2O3 in SO3 --water clusters was observed with time constants close to 320 fs, 1.5 ps, and 31 ps after the excitation pulse. Samples of Fe 3+ -Nafion membranes with high activity show different transient dynamics relative to the Fe 3+ -Nafion with low activity. Correlation of the photocatalytic activity of Fe 3+ -Nafion with UV-vis, Fourier transform infrared, Mo ¨ssbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H2O)6 ] 3+ , binuclear complexes [Fe(H3O2)Fe] 5+ and oxo-bridged [Fe-O-Fe] 4+ found in the membranes.

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We use extensive first-principles simulations to show the major role played by interfaces in the mechanism of phase separation observed in semiconductor multifunctional materials. We make an analogy with the precipitation sequence observed in oversaturated AlCu alloys, and replace the Guinier-Preston zones in this new context. A class of materials, the α phases, is proposed to understand the formation of the coherent precipitates observed in the GeMn system. The interplay between formation and interface energies is analyzed for these phases and for the structures usually considered in the literature. The existence of the α phases is assessed with both theoretical and experimental arguments.

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