Christine Giorgetti

Ab initio GW many-body effects in graphene

We present an ab initio numerical many-body GW calculation of the band plot in freestanding graphene. We consider the full ionic and electronic structure introducing e-e interaction and correlation effects via a self-energy containing non-Hermitian and dynamical terms. With respect to the density-functional theory local-density approximation, the Fermi velocity is renormalized with an increase of 17%, in better agreement with the experiment. Close to the Dirac point the linear dispersion is modified by the presence of a kink, as observed by angle-resolved photoemission spectroscopy. We demonstrate that the kink is due to low-energy pi-->pi* single-particle excitations and to the pi plasmon. The GW self-energy does not open the band gap.

Nanometric Resolved Luminescence in h-BN Flakes: Excitons and Stacking Order

Romain Bourrellier, Michele Amato, 2 Luiz Henrique Galvão Tizei, Christine Giorgetti, Alexandre Gloter, Malcolm I. Heggie, Katia March, Odile Stéphan, Lucia Reining, Mathieu Kociak, and Alberto Zobelli ∗ Laboratoire de Physique des Solides, Univ. Paris-Sud, CNRS UMR 8502, F-91405, Orsay, France Laboratoire des Solides Irradiés, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau and European Theoretical Spectroscopy Facility (ETSF), France Department of Chemistry, University of Surrey, Guildford GU2 7XH, United Kingdom

Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State

Abstract Photo-induced magnetisation has been recently evidenced by Hashimoto et al. in CoFe cyanide[1]. We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect. The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated. The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed.

X-ray absorption spectroscopy and magnetic circular x-ray dichroism in

The effect of nitrogen insertion on the electronic properties of cerium and iron in has been studied by means of x-ray absorption spectroscopy (XAS) and magnetic circular x-ray dichroism (MCXD). The evolution of the electronic structure of cerium upon nitrogen insertion has been studied. It emerges from this MCXD study at the edges of cerium that the cerium atoms bear a 5d magnetic moment of about that is coupled antiferromagnetically to that of iron. The spin polarized EXAFS signal has been observed; an attempt to evidence the magnetic environment has been made in the light of previous neutron diffraction results. Comparison of EXAFS and spin polarized EXAFS is revealed to be an efficient new tool to distinguish between magnetic and non-magnetic atoms in the environment of the absorber. The analysis of the Fe K edge MCXD spectra shows that insertion of nitrogen induces a trend to strong ferromagnetism with an almost completely filled majority spin subband.

Hard X-rays magnetic EXAFS

Abstract The results of a systematic study of Magnetic EXAFS at K edges of transition metals (TM) and L edges of rare earth (RE) compounds are presented. At the K edges of TM the magnetic EXAFS is in phase with the normal EXAFS in all studied cases and its intensity is related to the variation of the mean magnetization. At L edges, magnetic EXAFS is not correlated with the complex magnetic dichroism observed at the edge. By studying the phase difference between the magnetic and normal EXAFS, we observe that there exists an interplay between the contribution of the central atom (giving a contribution to the phase in quadrature with normal EXAFS) and the magnetic neighbours (giving a contibution in phase). Multiple excitations are frequently observed in the spectra with a greater sensitivity than in normal EXAFS.

Anomalous angular dependence of the dynamic structure factor near Bragg reflections: graphite.

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q, omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

X-ray magnetic circular dichroism as a tool for magnetic studies of 4f compounds

Abstract A short review is given of the theoretical fundamentals, applications and limitations of X-ray magnetic circular dichroism (XMCD) at the M 4,5 and L 2,3 edges of rare earth compounds.

X-ray phase plate for energy-dispersive and mononchromatic experiments

Diamond crystals are excellent phase plates for the forward diffracted X-rays in the Bragg geometry. The phase-shift between the (sigma) and (pi) components of the transmitted wave varies with the incident angular offset from the center of the reflection profile and can be adjusted to any desired value. Due to the low absorption coefficient of diamond, rather thick crystals (about 1 mm thick) can be used and operated in a region where the variation of the phase-shift with the angle of incidence is not too strong. This property was exploited to obtain a high degree of circular polarization in an energy dispersive absorption spectrometer at LURE (Orsay) where the source is rather broad.

Quadrupolar transitions by MCXD at L edges? Search of evidence

To explain the low-energy features of magnetic circular X-ray dichroism (MCXD) at L edges of rare earth compounds, quadrupolar electric transitions (E2) from 2p to 4f levels have been invoked. Such transitions should be distinguished from dipolar electric (E1) ones by looking at the different behaviour of the E1 and E2 MCXD cross sections as a function of the temperature and the angle the quantization axis and the photon wave vector. However, convincing experimental proof of such transitions has not yet been given. A good candidate for these key tests is Yb, where the E2 transitions are allowed at the L2 edge and forbidden at the L2 edge. In YbFe2, we observe an important change in the shape of the spectrum with temperature with temperature at the L3 edge of Yb.