Hubert Renevier

Pre-edge structure analysis of Ti K-edge polarized X-ray absorption spectra in TiO2 by full-potential XANES calculations.

The pre-edge region of Ti N-edge polarized XANES spectra in TiO2-mtile is investigated by full-potential calculations based on the finite-difference method. Both dipolar and quadrupolar transitions are considered. The use ofnon muffin-tin potential allows a clear interpretation of the pre-edge features. The results are consistent with Full-potential LAPW band structure calculations, and are also compared with multiple-scattering calculations.

In situ resonant x-ray study of vertical correlation and capping effects during GaN∕AlN quantum dot growth

Grazing incidence anomalous x-ray scattering was used to monitor in situ the molecular beam epitaxy growth of GaN∕AlN quantum dots (QDs). The strain state was studied by means of grazing incidence multiwavelength anomalous diffraction (MAD) in both the QDs and the AlN during the progressive coverage of QDs by AlN monolayers. Vertical correlation in the position of the GaN QDs was also studied by both grazing incidence MAD and anomalous grazing incidence small angle scattering as a function of the number of GaN planes and of the AlN spacer thickness. In a regime where the GaN QDs and the AlN capping are mutually strain influenced, a vertical correlation in the position of QDs is found with as a side effect an average increase in the QD width.

Quantitative structural characterization of GaN quantum dot ripening using reflection high-energy electron diffraction

Reflection high-energy electron diffraction (RHEED) was used to monitor the strain of GaN/AlN quantum dots (QDs) grown by molecular beam epitaxy. Quantitative and absolute values of the in- and out-of-plane strains of the QDs were determined and compared to reference values, obtained by x-ray diffraction measurements. A very good agreement was found between RHEED and x-ray measurements. The growth and thermal ripening of the dots were analyzed. A progressive strain relaxation was observed during the ripening stage, suggesting a morphology evolution of the dots.

Diffraction anomalous fine structure of forbidden Bragg reflections: charge localization and structure of the octahedral site in magnetite.

Resonant X ray scattering has been used to investigate charge localization on the octahedral iron atoms in magnetite below and above the Verwey temperature. We have measured the DAFS spectra of the 002 and 006 forbidden Bragg reflections permitted by the anisotropy of the iron anomalous scattering factor. We performed ab initio calculations which are in fair agreement with the experiment in the near edge region and demonstrate the sensitivity of the DAFS spectra to tiny structural and electronic changes. No change is observed, in the energy and azimuthal dependences, when the sample is cooled down below the Verwey temperature. Charge ordering can be definitely excluded and different charge localisation schemes discarded. Ab initio simulations, performed by using the refined crystallographic structure proposed for the room temperature phase, do not show a good agreement with the experiment in the extended region of the DAFS spectrum. This point is being investigated.