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International audience; Comprehension of the electronic properties of nano-objects is a key to defining dedicated properties, which can be adjusted by changing their size. Beyond confinement effects, the presence of interfaces, i.e., places where there is an abrupt change of electronic density, should also play a role. Time-dependent density functional theory (TD-DFT) is a state-of-the-art ab initio formalism in which this effect is accounted for through the so-called local field effects. In an earlier paper [S. Mazzei and C. Giorgetti, Phys. Rev. B 106, 035431 (2022)], we showed that the framework inherited from three-dimensional crystals could not provide reliable absorption spectra. In the present paper, we propose to calculate the macroscopic average of the dielectric tensor of a quasi-twodimensional (2D) object from the response function of the density to the total macroscopic potential in order to avoid use of the so-called Adler and Wiser formula. We evidence that the inclusion of interfaces in the thickness of the slab causes the response function for the out-of-plane component to move sharply from the bulk absorption resonance to the plasmon one. This shows that the longitudinal-longitudinal contraction of the dielectric tensor is no longer equal to the transverse-transverse one in a quasi-2D object for out-of-plane perturbation. Nevertheless, we also show that the macroscopic average of the dielectric tensor of an ultrathin slab calculated within the longitudinal formalism of TD-DFT depicts the properties of the transverse reflectance and transmittance spectra of a thin slab.

International audience; Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard AA′ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated h11BN flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of h11BN. Hence, we show that hBN presents an AA′ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications.

International audience; Electrostatic gating or alkali metal evaporation can be successfully employed to tune the interface of layered black phosphorus (BP) from a semiconductor to a 2D Dirac semimetal. Although Angle Resolved Photoelectron Spectroscopy (ARPES) experiments have captured the collapse of the band gap in the inversion layer, a quantitative estimation of band structure evolution has been hindered by the short escape depth and matrix elements of the probed photoelectrons. Here, we precisely monitor the evolution of electronic states using time-resolved ARPES at a photon energy of 6.3 eV. The probing depth of laser-based ARPES is long enough to observe the buried electronic states originating from the valence band maximum. Our data shows that the band gap has a maximal value of 0.32 eV in the pristine sample, and that it shrinks down monotonically by increasing the carrier concentration in the topmost layer. Most interestingly, the band velocity of the valence band increases by a factor of two along the armchair direction, surpassing the value reported in graphene on silicon carbide (SiC). This control of band structure via external gating will be of interest with regard to the design of optoelectronic devices.

Manipulation of intrinsic electronic structures by electron or hole doping in a controlled manner in van der Waals layered materials is the key to control their electrical and optical properties. Two-dimensional indium selenide (InSe) semiconductor has attracted attention due to its direct band gap and ultrahigh mobility as a promising material for optoelectronic devices. In this work, we manipulate the electronic structure of InSe by in situ surface electron doping and obtain a significant band gap renormalization of ∼120 meV directly observed by high-resolution angle resolved photoemission spectroscopy. This moderate doping level (carrier concentration of 8.1 × 1012 cm-2) can be achieved by electrical gating in field effect transistors, demonstrating the potential to design of broad spectral response devices.

Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy (2PPE), we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Gamma point of the Brillouin zone (BZ). STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction band minimum (CBM) and a very sharp one near the maximum of the valence band (VMB). This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle resolved photoemission spectroscopy (ARPES) investigation. technologies. In fact, a hole effective mass of about m/m0 = -0.95 gammaK direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic

International audience; We investigate black phosphorus by time- and angle-resolved photoelectron spectroscopy. The electrons excited by 1.57 eV photons relax down to a conduction band minimum within 1 ps. Despite the low band gap value, no relevant amount of carrier multiplication could be detected at an excitation density 3–6 × 1019 cm–3. In the thermalized state, the band gap renormalization is negligible up to a photoexcitation density that fills the conduction band by 150 meV. Astonishingly, a Stark broadening of the valence band takes place at an early delay time. We argue that electrons and holes with a high excess energy lead to inhomogeneous screening of near surface fields. As a consequence, the chemical potential is no longer pinned in a narrow impurity band.

High-energy electronic excitations of graphene and ${\mathrm{MoS}}_{2}$ heterostructures are investigated by momentum-resolved electron energy-loss spectroscopy in the range of 1 to 35 eV. The interplay of excitations on different sheets is understood in terms of long-range Coulomb interactions and is simulated using a combination of ab initio and dielectric model calculations. In particular, the layered electron-gas model is extended to thick layers by including the spatial dependence of the dielectric response in the direction perpendicular to the sheets. We apply this model to the case of graphene/${\mathrm{MoS}}_{2}$/graphene heterostructures and discuss the possibility of extracting the dielectric properties of an encapsulated monolayer from measurements of the entire stack.

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the $\overline{M}$ valley. Subsequently, the electronic cooling is dictated by Fr\"ohlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\overline{M}$ valley could travel through a multilayer flake of InSe with a lateral size of 1 $\ensuremath{\mu}\mathrm{m}$. The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

More than 50 years after the first experimental observation of second-harmonic generation, the theoretical description of second-harmonic generation is still under debate, whereas it is well understood from an experimental point of view. This is the gap that this thesis aims to fill. This work aims to improve the theoretical description and understanding of the generation of second-harmonic from the surfaces of crystalline semiconductors. When applying an external electric field to a dielectric material, electric dipoles are created at a microscopic level. These dipoles are responsible for the apparition, inside the material, of an induced field. The fluctuations of the electric field at a microscopic level, the density fluctuations or any kind of microscopic inhomogeneities must be taken into account when describing the optical properties of a system. These effects are often referred as “local-field effects”. These local-field effects have been widely studied in the past and in particular their effects on the optical properties of bulk materials are now well established. In the case of surfaces, the theoretical description and the numerical simulations are more intricate than for bulk materials. The abrupt change in the electronic density leads to a huge variation of the electric field at the interface with vacuum. As a result, strong effects of the local-field are expected, in particular in the direction perpendicular to the plane of the surface. The goal of this thesis is to quantify how important these effects are for the linear and second-order optical properties of surfaces. A macroscopic theory of second-harmonic generation from crystal surfaces has been developed in order to account for local-field effects. The latter are calculated from first-principles, in the framework of the Time-Dependent Density-Functional Theory (TDDFT). The primary interest is the description of non-linear optical responses of surfaces, but new theoretical tools for improving the description of local-field effects in the case of linear optics have also been developed. The numerical simulations have been focused on the Si(001) surface, and the macroscopic formalism developed during this thesis has been applied to three surface reconstructions, namely the clean Si(001)2x1, the monohydride Si(001)2x1:H and the dihydride Si(001)1x1:2H surfaces. Comparison with available experimental results is also reported.

International audience; Low-dimensional materials differ from their bulk counterpart in many respects. In particular, thescreening of the Coulomb interaction is strongly reduced, which can have important consequencessuch as the significant increase of exciton binding energies. In bulk materials the binding energyis used as an indicator in optical spectra to distinguish different kinds of excitons, but this is notpossible in low-dimensional materials, where the binding energy is large and comparable in size forexcitons of very different localization. Here we demonstrate that the exciton band structure, whichcan be accessed experimentally, instead provides a powerful way to identify the exciton character.By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane andsingle-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensionalmaterials, highlighting the different role played by the exchange electron-hole interaction and by theelectronic band structure. Our interpretation is substantiated by a prediction for phosphorene.

International audience; Surface optical and electronic properties are crucial for material science and have implications in fields as various as nanotechnology, nonlinear optics, and spectroscopies. In particular, the huge variation of electronic density perpendicular to the surface is expected to play a key role in absorption due to local-field effects. Numerous state-of-the-art theoretical and numerical ab initio formalisms developed for studying these properties are based on supercell approaches, in reciprocal space, due to their efficiency. In this paper, we show that the standard scheme fails for the out-of-plane optical response of the surface. This response is interpreted using the “effective-medium theory” with vacuum and also in terms of interaction between replicas, as the supercell approach implies a periodicity which is absent in the real system. We propose an alternative formulation, also based on the supercell, for computing the macroscopic dielectric function. Application to the clean Si(001) 2×1 surface allows us to present the effect of the local fields for both peak positions and line shape on the bulk and surface contributions. It shows how local fields built up for the in-plane and out-of-plane dielectric responses of the surface. In addition to their conceptual impact, our results explain why the standard approach gives reliable predictions for the in-plane components, leading to correct reflectance anisotropy spectra. Our scheme can be further generalized to other low-dimensional geometries, such as clusters or nanowires, and open the way to nonlinear optics for surfaces.

International audience; One of the big challenges of theoretical condensed-matter physics is the description, understanding, and prediction of the effects of the Coulomb interaction on materials properties. In electronic spectra, the Coulomb interaction causes a renormalization of energies and change of spectral weight. Most importantly, it can lead to new structures, often called satellites. These can be linked to the coupling of excitations, also termed dynamical effects. State-of-the-art methods in the framework of many-body perturbation theory, in particular, the widely used GW approximation, often fail to describe satellite spectra. Instead, approaches based on a picture of electron-boson coupling such as the cumulant expansion are promising for the description of plasmon satellites. In this work, we give a unified derivation of the GW approximation and the cumulant expansion for the one-body Green’s function. Using the example of bulk sodium, we compare the resulting spectral functions both in the valence and in the core region, and we discuss the dispersion of quasi-particles and satellites. We show that self-consistency is crucial to obtain meaningful results, in particular, at large binding energies. Very good agreement with experiment is obtained when the intrinsic spectral function is corrected for extrinsic and interference effects. Finally, we sketch how one can approach the problem in the case of the two-body Green’s function, and we discuss the cancellation of various dynamical effects that occur in that case.

The simultaneous formation of nanometer sized zinc oxide (ZnO), and acetate zinc hydroxide double salt (Zn-HDS) is described. These phases, obtained using the sol-gel synthesis route based on zinc acetate salt in alcoholic media, were identified by direct characterization of the reaction products in solution using complementary techniques: nephelometry, in situ Small-Angle X-ray Scattering (SAXS), UV-Vis spectroscopy and Extended X-ray Absorption Fine Structures (EXAFS). In particular, the hydrolytic pathway of ethanolic zinc acetate precursor solutions promoted by addition of water with the molar ratio N=[H2O]/[Zn2+] = 0.05 was investigated in this paper. The aim was to understand the formation mechanism of ZnO colloidal suspension and to reveal the factors responsible for the formation of Zn-HDS in the final precipitates. The growth mechanism of ZnO nanoparticles is based on primary particle (radius ≈ 1.5 nm) rotation inside the primary aggregate (radius

The evolution of both oxygen and framework tetrahedral shells around Ni2+ cations in faujasite upon rehydration can be described in real time thanks to in situ dispersive-EXAFS. Such type of analysis permits precise experimental description of the coordination environment of cations in zeolite structures under different hydration conditions.

The RMn 12− x Fe x series of compounds of ThMn 12 structure type exhibit rather complicated and composition sensitive magnetic properties. In a recent past, systematic neutron diffraction and magnetisation experiments allow to relate the main aspects of the magnetic structures to the polarisation state of the d-band. X-ray magnetic circular dichroism (XMCD) analyses have been undertaken on iron rich compounds with R = Y, Er, and Lu, thus probing the L 2,3 edges of Mn and Fe as well as Er, and the M 4,5 edges of Er. These experiments permit to better understand the local magnetic polarisation of the different interacting orbitals versus temperature and applied field as well as to precise the nature of the magnetic couplings acting between the 3d metals (M).

We report here the X-ray magnetic circular dichroism (XMCD) study at the Gd M(4,5)- and L(2,3)-edges of two linear magnetic chains involving Gd(III) cations bridged by nitronyl nitroxide radicals. This spectroscopy directly probes the magnetic moments of the 4f and 5d orbitals of the gadolinium ions. We compare macroscopic magnetic measurements and local XMCD signals. The M(4,5)-edges results are in agreement with the J values extracted from the fits of the SQUID magnetic measurements. The L(2,3)-edges signals show that the electronic density in the Gd 5d orbitals depends on the neighbors of the gadolinium cations. Nevertheless, the 5d orbitals do not seem to play any role in the superexchange pathway between radicals through the metal ion proposed to explain the particular magnetic exchange interactions between the radicals in these chains.

Les deux enantiomeres de l'aimant a precurseur moleculaire bidimensionnel { [N(C 4 H 9 ) 4 ][NiCr(ox) 3 ]} n (ox = C 2 O 4 2-) ont ete etudies. Le dichroisme circulaire magnetique dans le domaine des rayons X a ete mesure a 11 K, en dessous de la temperature de Curie du materiau (16 K). Le signal dichroique est identique pour les deux enantiomeres. Les spectres dichroiques aux seuils K du nickel et du chrome confirment le couplage ferromagnetique entre les deux ions metalliques. Les moments magnetiques remanents portes par les atomes de Ni et de Cr ont ete clairement mis en evidence, analyses et compares a l'aimantation remanente totale mesuree a 11 K par magnetometrie classique. Pour citer cet article : C. Train et al., C. R. Chimie 6 (2003).

International audience; The strong excitonic emission of hexagonal boron nitride (h-BN) makes this material one of the most promising candidate for light emitting devices in the far ultraviolet (UV). However, single excitons occur only in perfect monocrystals that are extremely hard to synthesize, while regular h-BN samples present a complex emission spectrum with several additional peaks. The microscopic origin of these additional emissions has not yet been understood. In this work we address this problem using an experimental and theoretical approach that combines nanometric resolved cathodoluminescence, high resolution transmission electron microscopy and state of the art theoretical spectroscopy methods. We demonstrate that emission spectra are strongly inhomogeneus within individual few layer flakes and that additional excitons occur at structural deformations, such as faceted plane folds, that lead to local changes of the h-BN layers stacking order.

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.

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.

The photoemission spectrum of graphite is still debated. To help resolve this issue, we present photoemission measurements at high photon energy and analyze the results using a Green's function approach that takes into account the full complexity of the loss spectrum. Our measured data show multiple satellite replicas. We demonstrate that these satellites are of intrinsic origin, enhanced by extrinsic losses. The dominating satellite is due to the $\ensuremath{\pi}+\ensuremath{\sigma}$ plasmon of graphite, whereas the $\ensuremath{\pi}$ plasmon creates a tail on the high-binding energy side of the quasiparticle peak. The interplay between the two plasmons leads to energy shifts, broadening, and additional peaks in the satellite spectrum. We also predict the spectral changes in the transition from graphite towards graphene.

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.

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.

Magnetic circular x-ray dichroism on Co $/$Cu and Fe $/$Cu multilayers at the $K$ edge of copper shows (i) that the $p$ band of copper is significantly spin polarized by the adjacent Co or Fe atoms, (ii) that the spin polarization of the copper layers strongly depends on the adjacent magnetic layer, and (iii) that the magnetic polarization is not restricted to the interface layer, i.e., it departs from a simple ${1/t}_{\mathrm{Cu}}$ dependence.

An experimental study of conventional x-ray absorption and of magnetic circular x-ray dichroism has been performed on La/Fe and on Ce/Fe multilayers at the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ edges of La and Ce and at the K edge of Fe to characterize the impact of the interaction of La or Ce and Fe on the local electronic and magnetic structure of the interfaces. The spectra reveal that the interaction is particularly unusual for Ce/Fe, where two effects have been observed. First, on a depth scale of at least 15 \AA{} near the interface, the Ce atoms adopt the electronic structure of the \ensuremath{\alpha} phase, with itinerant 4f states. Second, these \ensuremath{\alpha}-phase-like Ce atoms are magnetically polarized and carry an ordered magnetic 5d spin moment. This moment is antiferromagnetically coupled to the Fe 3d moment. Interface mixing on a length scale of the magnetic polarization can be excluded. For the La/Fe system, a similar magnetic polarization is effective, but in contrast to Ce/Fe it is restricted to the immediate interface with Fe. The results are discussed with reference to recent measurements of the macroscopic magnetization and M\"ossbauer spectra. Comparison with intermetallic compounds of rare-earth and late transition-metal elements points to the importance of hybridization effects for the Fe and Ce or La electronic states near the interfaces in the multilayers.

In this paper we discuss the specific features of K-edge magnetic circular x-ray-dichroism (MCXD) spectra of Fe and Co in bcc Fe-Co alloys and Fe/Co multilayers. In the alloys, the systematic variations of shape and amplitude of the near-edge MCXD signals as a function of concentration can be correlated with the changes in local atomic environment. The difference between the Co K-edge and Fe K-edge spectra of the same alloy shows that the signal is central atom dependent. Well above the absorption edge (E-${\mathit{E}}_{0}$g80 eV) the extended x-ray-absorption fine structure (EXAFS)-like oscillations in the MCXD spectra clearly reflect the local bcc structure and appear not to depend on the absorbing atom. In both alloys and multilayers, the magnetic EXAFS intensity can be correlated to the average magnetization in the neighborhood of the absorbing atom.

A 220 diamond phase plate was combined with an energy-dispersive absorption spectrometer to convert linearly polarized X-rays into circularly polarized photons and to detect circular magnetic X-ray dichroism (CMXD) from ferromagnetic samples. In these experiments, carried out at LURE, the energy-dispersive spectrometer was equipped with a bent Si (311) polychromator and vertically collimating slits accepting essentially a linearly polarized incident beam. The quarter-wave plate was operated in the Bragg geometry but well outside the range of quasi total reflection, with the consequence that the forward-diffracted beam was circularly polarized with a polarization rate approaching 80% over the whole energy bandpass of the polychromator. CMXD spectra of GdFe2 and GdCo2 intermetallic compounds were recorded at ca 7.2 keV near the Gd LIII absorption edge: they are essentially identical to the spectra commonly recorded with elliptically polarized X-ray photons collected out of the orbit plane of the storage ring. It is suggested that the energy-dispersive phase plate will be very useful to detect CMXD spectra with energy-dispersive spectrometers exploiting the well collimated linearly polarized emission of standard undulators installed on the storage rings of the third generation.

Hydrogen insertion in the ${\mathrm{Ce}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{H}}_{\mathit{x}}$ system leads to spectacular changes of the physical properties. A relative increase of the Curie temperature of almost 100% is observed as well as the highest volume increase. In addition to previous neutron diffraction measurements which pointed out an increase of the local iron moments, we present in this paper absorption and magnetic circular x-ray dichroism (MCXD) experiments performed at the ${\mathit{L}}_{2,3}$ edges of Ce and the K edge of iron. These measurements allow us to study the evolution of the Ce electronic structure upon hydrogen insertion. Values for the Ce valence have been derived from both absorption and MCXD signals. The MCXD data are analyzed for their shape, intensity, and sign, and compared to the macroscopic magnetic measurements. It is shown that in these compounds the Ce atoms bear a 5d magnetic moment, which is coupled antiferromagnetically to that on iron. There is no orbital contribution to this 5d moment which is thus solely of spin origin. The values of the 5d moment are deduced as a function of each composition in the ${\mathrm{Ce}}_{2}$${\mathrm{Fe}}_{17}$${\mathrm{H}}_{\mathit{x}}$ system at low and room temperature. At the K edge of iron, the shape and intensity of the dichroic signal have been related to the filling of the majority 3d band.

Magnetic circular x-ray dichroism (MCXD) studies at L 2,3 absorption edges in intermediate-valence (IV) compounds CeFe 2 , Ce(Fe 0.8 Co 0.2 ) 2 , CeCo 2 , Ce 2 Co 17 , and trivalent 4f 1 systems (CeRu 2 Ge 2 , Ce 3 Al 11 , CePd, CeFe 2 H 3.8 ) at the Ce L 2,3 edges are presented. The comparison with reference compounds (La, Lu, Hf, Gd) confirms the existence, in such IV systems, of an ordered 5d magnetic moment on the Ce site antiferromagnetically coupled eeith the 3d Fe (Co) moment. Moreover, this study suggests, provided the 4f orbital moment is almost quenched, the existence of a simple proportionality between the MCXD intensity and the magnetic moment of the probed atom

International audience; 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,ω). 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.

International audience; We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion.

We present an {\it ab initio} many-body GW calculation of the self-energy, the quasiparticle band plot and the spectral functions in free-standing undoped graphene. With respect to other approaches, we numerically take into account the full ionic and electronic structure of real graphene and we introduce electron-electron interaction and correlation effects from first principles. Both non-hermitian and also dynamical components of the self-energy are fully taken into account. With respect to DFT-LDA, the Fermi velocity is substantially renormalized and raised by a 17%, in better agreement with magnetotransport experiments. Furthermore, close to the Dirac point the linear dispersion is modified by the presence of a kink, as observed in ARPES experiments. Our calculations show that the kink is due to low-energy $\pi \to \pi^*$ single-particle excitations and to the $\pi$ plasmon. Finally, the GW self-energy does not open the band gap., Comment: 5 pages, 4 figures, 1 table

The existence of oscillations far from the edge in magnetic circular dichroism (MCD) spectra in the X-ray range were first observed by Schlitz and coll. [1]. A number of subsequent experimental and theoretical works where devoted to this magnetic Extended X-ray Absorption Fine Structure (EXAFS) [2, 3, 4, 5, 6]. This phenomenon seems to be quite general, and was observed at the K-edge of 3d transition metals and at the Lii,m-edges of rare-earths and 5d transition metals. The intensity of this structure varies strongly between compounds. Magnetic EXAFS is expected to help understanding the magnetic coupling between atoms. The high energy region of MCD spectra is interesting because it is simpler to interpret and directly related to structural properties, as is the case for absorption spectra. In particular, no contribution of quadrupole transitions or multiplet effects is expected. The first article on the subject [I] proposed that a standard EXAFS analysis could be used to determine the number and distance of the magnetic neighbours only. A more detailed analysis tends to show that the analysis of magnetic EXAFS is somewhat more complex, especially at the K-edge. This chapter attempts to summarize a few known facts about magnetic EXAFS. The first part studies the Lii,m-edges, where the core hole spin-orbit is large and the spin-orbit coupling on the photoelectron was neglected. The second part treats the case of the K-edge where photoelectron spin-orbit is essential. For both parts, a formal introduction is given, followed by a specific example (gadolinium and iron) where the relation between MCD and the spinpolarisation of the density of states is examined. The present discussion is made within the multiple-scattering approach. Alternative points of view are presented in this book by Prof. G. Schiitz.

This paper reports on two new technical developments concerning sample environments for X-ray magnetic circular dichroism (XMCD). The first measurements under high pressures of up to 30 GPa are described. The difficulties of combining the techniques of high pressure and XMCD are commented on. The second development involves the use of a fast-switching magnetic field. A new superconducting device is used to perform XMCD measurements on paramagnetic compounds in magnetic fields of up to 6 T. The small amplitude of the XMCD signal imposes, for a given signal-to-noise ratio, a noise less than a few 10−5. The signal-to-noise ratio is improved by the use of a series of acquisitions, switching the magnetic field between each acquisition. A very fast switching mechanism has been built based on mechanical rotation of a superconducting coil, with the sample kept in place inside the coil. The XMCD signals at the L II,III-edges of paramagnetic rare-earth compounds have been measured at 4.5 K in fields of up to 6 T with a switching time of 11 s.

We have measured core-level x-ray-absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Gd-L2,3 edges to characterize the low-lying Gd-5d derived conduction-band states in thin films of the 4f ferromagnet GdN with a unit-cell volume 8.6% above that of bulklike layers. The nonequilibrium structure is obtained by N+ plasma-assisted reactive sputter deposition at room temperature. The Curie temperature TC, a key quantity for magnetism, amounts to only half the bulk value of ~60 K indicating a significant reduction of the effective exchange interaction between the 4f states. An intricate observation is that the ratio of the dichroic signal amplitudes in the lattice-expanded layers, |L3/L2|, is up to three times higher than the value expected from the degeneracy of the 2p3/2 and 2p1/2 core states, which is observed for the bulklike layers. This is mainly due to a reduced L2 XMCD amplitude. We suggest that the effect may be related to the different weight the crystal-field split Gd-5d final states (t2g and eg) have in the absorption process at the L2 and L3 edges, and to the special electronic band structure of this strongly correlated material and its modification upon lattice expansion. This hypothesis is supported by the observation that the L2 absorption edge is shifted to lower energies upon ferromagnetic ordering while the L3-edge position remains inert

X-ray magnetic circular dichroism (XMCD) measurements have been performed at the ${\mathit{L}}_{\text{II},\text{III}}$ edges of Er in the intermetallic compounds ${\text{ErMn}}_{2}$, ${\text{ErFe}}_{2}$, ${\text{ErCo}}_{2}$, ${\text{ErNi}}_{2}$, and ${\text{ErAl}}_{2}$, as well as in the ionic compound ${\text{Er}}_{2}{({\text{SO}}_{4})}_{3},8{\mathrm{H}}_{2}0$, in order to study the evolution of the XMCD spectra as a function of the $5d$ states filling and∕or their hybridization with magnetic or nonmagnetic electronic states. Almost all the spectra present the same general features, confirming that the $4f\text{\ensuremath{-}}5d$ intra-atomic coupling is a key point to explain XMCD spectra. Thus, we analyze the models based on this interaction, proposed to account for the unexpected sign of $\mathit{E}1\text{\ensuremath{-}}\text{XMCD}$ measurements, as well as the nonstatistical branching ratio between XMCD at the ${\mathit{L}}_{\text{II}}$ and ${\mathit{L}}_{\text{III}}$ edges. We underline the impossibilities of reproducing our measurements using these models. Moreover, we point out the special role played by Fe which leads, at the ${\mathit{L}}_{\text{II}}$ edge of Er, to a significant modification of the shape of XMCD spectrum and an unexpected temperature dependence. The XMCD revealed an unexpected behavior of the $5d$ magnetic polarization in these compounds which is not visible in macroscopic measurements. This also demonstrates the non-negligible role played by the $5d\text{\ensuremath{-}}3d$ hybridization.

The RMn 12− x Fe x series of compounds of ThMn 12 structure type exhibit rather complicated and composition sensitive magnetic properties. In the recent past, systematic neutron diffraction and magnetisation experiments allow to relate the main aspects of the magnetic structures to the polarisation state of the d band. X-ray magnetic circular dichroism (XMCD) analyses have been undertaken on iron rich compounds with R=Er, thus probing the L 2,3 edges of Mn and Fe as well as Er, and the M 4,5 edges of Er. These experiments permit to better understand the local magnetic polarisation of the different interacting orbitals versus temperature and applied field as well as to specify the nature of the magnetic couplings acting between the 3d metals.

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.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Magnetic circular X-ray dichroism (MCXD) studies at the CeL2,3 and M4,5 absorption edges in the CeRh3B2 and CeFe2 compounds are presented. Experiments at the L2,3 edges allow to get information on the 5d magnetism whereas the M4,5 edges witness the behavior of the 4f electrons. It will be shown that in CeRh3B2, the properties of the 4f orbitals are between localization and intermediate valence. Values of the magnetic moment on the Ce atom can be estimated from MCXD measurements. The 4f orbital magnetic moment is found to be strongly reduced by crystal field effects as compared to the value expected for fully localized electrons.

An experimental study of conventional x-ray absorption and of magnetic circular x-ray dichroism has been performed on Ce/Fe Multilayers at the Ce L2,3 and the Fe-K edge, to characterize the impact of the interaction of Ce and Fe on the local electronic and magnetic structure of the interface. The spectra reveal that the interaction has two effects. First, on a depth scale of up to 15 A near the interface, the Ce atoms adopt the electronic structure of the oc-phase, with itinerant 4f states. Second, a large portion (=10 Å) of these cc-like Ce atoms is magnetically polarized and carries an ordered Magnetic 5d Moment. The Fe-3d and Ce-5d Moments are antiferromagnetically coupled. Interface Mixing on a length scale of the magnetic polarization can be excluded.

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.

Measurement of very small differences of the total cross section is the current demand for the spectrometers dedicated to time‐dependent experiments carried out under various time‐ramped parameters. The dispersive optics and more precisely the full x‐ray‐absorption spectrometer is mechanically movement‐free during data collection which can last over 12 h at LURE‐DCI to be sensitive to relative change of the absorption of the order of 10−5. In this range, artefacts due to the drift of silicon lattice spacing under temperature change of the crystal, and drifts of the detector position because of liquid–nitrogen evaporation contained in the cryostat, are sources of errors which have been identified and cured or ... by‐passed. The accuracy in difference measurements is now of the order of 10−5 for a total cross section measured equal to 1. In term of optics stability a difference signal of 10−4 out of 1 can be generated by an absorption edge shift caused by a 0.05 K drift of the temperature of the silicon crystal at 7 keV. These performances are essential for the measurement of XMCD in the hard‐x‐ray range. Water cooling of the dynamically bent crystal reduces dramatically the change of the Si temperature. Adequate geometry makes the spatial drift of the position of the photodiode array much less concerning. The focusing efficiency is also a key parameter to push high‐pressure x‐ray‐absorption spectroscopy (55 GPa), and high‐temperature XAS (2000 K), and the combination (15 GPa, 800 K). Simple devices, taking advantage of the focusing geometry, have been successfully tested these last two years.

My Ph.D. thesis is about the development of novel theoretical and numerical approaches for a comprehensive description of dielectric response in in ultra thin films. After an introductory part (chapters 1 to 5), I present investigations of electronic excitations in specific systems (SrTiO3 in chapter 6 and graphite-related systems in chapters 7 and 8). In the last two chapters I present a numerical development for the simulation of isolated nano films (chapter 9) and a formal derivation of a dynamical theory for 2-particle excitations (chapter 10).In chapter 6, I employ several theories and approximations to analyse the absorption spectra of bulk SrTiO3. The most remarkable result is the observation of an exceptionally sharp peak at 6.4 eV of excitonic origin, absent in actual measurements. Its sharpness is ascribed to peculiar dispersion properties of Ti 3eg states, while its absence in measured data is ascribed to the lack of dynamical coupling between excitations in state-of-the-art computational methods.In order to overcome this limitation, in chapter 10 I derive an approach to compute the 2-particle correlation function equivalent but alternative to the Bethe-Salpeter equation (BSE). This is inspired by the cumulant ansatz for the 1-particle Green’s function. In this framework, the 2-particle equation can be solved without altering its dependence on time variables. The resulting formula for the polarizability presents three corrective terms to the static BSE solution: The first is a weight renormalization, the second implies a shift of the BSE peaks and the third creates additional peaks at higher energy accounting for the coupling with other neutral excitations of the system (e.g. exciton-exciton or exciton-phonons).I also investigate the influence of the environment on the dielectric response of graphite-based materials. In chapter 7 I focus on the differences induced by the staking of graphene layers (AA, ABA and ABC), while in chapter 8 I focus on the evolution of spectral properties when passing from bulk graphite (inter-plane distance d0) to isolated graphene (d0 tending to infinite). In isolated films, optical spectra and loss function coincide. However the speed of convergence to the isolated spectrum depends on the quantity computed (absorption/loss function) and on the direction and the amplitude of the exchanged momentum q. The comparison of calculations in several configurations allows me to indicate which spectrum converges faster. Calculations in chapters 7 and 8 are done within the supercell approach, which actually increase enormously the computational workload of plane-wave based calculations. So in chapter 9 I make a thorough study of the Coulomb cutoff method, which permits calculations of isolated systems by artificially extinguishing all out of plane interactions. I implemented successfully the technique in the DP and EXC simulation codes, adding an analytical algorithm to treat the q=0 limit, and I apply it successfully to compute the loss function and the spectral function of isolated graphene sheets. Moreover, I thoroughly discuss some delicate analytical aspects, which make the method meaningful when sums over q-points are involved (like in the Dyson equation of the polarizability, or calculations of the self-energy) but ill-defined when specific values of q are required.; Ma thèse concerne le développement de nouveaux approches théoriques et numériques pour une description complète de la réponse diélectrique dans des couches ultra-minces. Après une introduction (chapitres 1 à 5), je présent des études des excitations électroniques dans des systèmes spécifiques (SrTiO3 au chapitre 6 et systèmes graphitiques aux chapitres 7 et 8). Dans les derniers deux chapitres je présent un développement numérique pour la simulation de nano-couches isolées (chapitre 9) et une dérivation formelle d'une théorie dynamique des excitations à deux particules (chapitre 10).Dans le chapitre 6 j'utilise plusieurs théories et approximations pour analyser les spectres d'absorption du SrTiO3 volumique. Le résultat le plus remarquable est l'observation à 6.4 eV d'un pic excitonique exceptionnellement net, absent en fait dans les mesures. Sa netteté est attribuée aux propriétés de dispersion particulières des états 3eg du Ti, alors que son absence dans les données expérimentales est expliquée par l'absence dans les méthodes de simulation à l'état de l'art de couplage dynamique entre les excitations.Afin de dépasser ces limitations, dans le chapitre 10 je dérive une approche pour calculer la fonction de corrélation à deux particules équivalente mais alternative à l’équation de Bethe-Salpeter (BSE). Cette approche est inspirée par l’ansatz de cumulant pour la fonction de Green à une particule. Dans ce cadre, l’équation à deux particules peut être résolue sans altérer sa structure temporelle. La formule pour la polarizabilité que l’on obtient a trois termes qui corrigent la solution de la BSE statique. Le premier est une renormalization du poids spectrale, le deuxième implique un changement de l’énergie des pics, et le troisième est à l’origine de pics additionnels à plus haute énergie qui prennent en compte le couplage avec d’autres excitations neutres du système (e.g. exciton-exciton ou exciton-phonon). Je me suis aussi intéressé à l’influence de l’environnement sur la réponse de matériaux à base de graphite. Dans le chapitre 7 je m’intéresse aux différences induites pas l’empilement de couches de graphène (AA, ABA et ABC). Le chapitre 8 porte sur l’évolution des propriétés spectrales quand l’on passe de la graphite volumique (avec une distance entre plans d0) au graphène isolé (avec d0 qui tend à l’infini). Dans les couches isolées, le spectre optique et la fonction de perte coincident. Toutefois, la vitesse de convergence au spectre isolée depend de la quantité que l’on calcule (absorption ou fonction de perte) et de la direction et l’amplitude du moment échangé q. La comparaison de calculs en différent configurations me permet de indiquer quel spectre converge le plus vite.Les calculs dans les chapitres 7 et 8 ont été faits avec la méthode de la supercellule, qui augmente énormément le poids des calculs en ondes planes. Donc, au chapitre 9 je conduis une étude détaillée de la méthode du cutoff de Coulomb, qui permet le calcul dans des systèmes isolés grâce à une extinction artificielle de l’interaction entre plans. J’ai implémenté avec succès cette technique dans les codes de simulation DP et EXC, en y ajoutant un algorithme analytique pour traiter la limite à q=0 et je l’ai appliqué avec succès pour calculer la fonction de perte et la fonction spectrale de feuillets de graphène isolés. En outre, je discute aussi des aspects délicats des formules analytiques qui rendent la méthode licite quand on fait des sommes sur les points q (comme dans l’équation de Dyson pour la polarizabilité ou pour le calcul de la self-energie) mais mal définie quand on recherche des valeurs spécifiques de q.

In this thesis, collective excitations of carbon systems with different dimensionality have been studied both theoretically and numerically. First, a method combining effective medium theory and ab-initio calculations has been developed which allows to describe the collective excitations of nanostructures (like carbon nanotubes) starting from the polarisability of their building blocks (graphene). In contrast to standard dielectric theory, the Maxwell equations are solved for the full frequency- and momentum-dependent microscopic dielectric function ε(q,q',ω) of the bulk material. The latter is calculated from first-principles within the Random Phase Approximation. Second, this method has been applied to calculate electron-energy loss spectra (EELS) for graphene and single-wall carbon nanotubes (SWCNT). The results are in good agreement with full ab-initio calculations of these systems and corresponding experiments. The linear dispersion of the π plasmon in graphene and SWCNTs has been analysed and explained. The directional dependence of the electron energy-loss spectra of SWCNTs is understood in terms of its normal mode excitations. Third, the use of ab-initio calculations for the prediction of spatially-resolved EELS is discussed. Therefore, the mixed dynamic form factor has been studied and a discontinuous behavior at certain Bragg reflections is predicted. This effect has been explained in terms of crystal local field effects and verified by IXS experiments on graphite and silicon. Finally, the building block approach is applied for spatially-resolved EELS., Cette thèse présente les résultats d'études théoriques et numériques des excitations collectives de systèmes de carbone de dimensionnalité différente. D'abord, une méthode combinant la théorie du milieu effectif et des calculs ab-initio a été développée pour décrire les excitations collectives de nanostructures (comme les nanotubes de carbone) à partir de la polarisabilité de leurs blocs élémentaires (graphène). Contrairement à la théorie diélectrique standard, les équations de Maxwell sont résolues pour la fonction diélectrique microscopique dépendant du temps et du moment ε(q,q',ω) du matériau massif. Cette dernière a été calculée à partir des premiers principes dans l'approximation de la phase aléatoire (RPA). Cette méthode a ensuite été appliquée pour calculer les spectres de perte d'énergie d'électrons (EELS) pour le graphène et les nanotubes de carbone monoparoi (SWCNT). L'accord avec les calculs complètement ab-initio sur ces systèmes et avec les expériences correspondantes est très bon. La dispersion du plasmon π dans le graphène et les SWCNT a été analysée et expliquée. La dépendance avec la direction du moment des spectres de perte d'énergie d'électrons des SWCNT est comprise en termes d'excitations des modes normaux. Finalement, l'utilisation des calculs ab-initio en vue de prédire le EELS résolu spatialement a été étudiée. Le facteur de forme mixte dynamique a été analysé et une discontinuité de son comportement pour certaines réflections de Bragg a été prédite. Cet effet a été expliqué par les effets de champ cristallin et a été confirmé par des expériences de IXS sur le graphite et le silicium. L'approche blocs élémentaires a été appliquée au EELS résolu spatialement.