104 research outputs found
Propriétés électroniques et magnétiques sous excitation laser femtoseconde, du Gd monocristallin aux alliages ferrimagnétiques
Ces travaux de thèse rentrent dans le cadre de l étude de la dynamique ultra rapide de l aimantation. Tout d abord sont présentés des aspects théoriques, puis les aspects expérimentaux de ces expériences. Pour ce faire, nous avons étudié la réponse d alliages ferrimagnétiques à composition variables à l aide d un dispositif de mesure d effet Kerr résolu en temps, puis dans une seconde partie, la dynamique de l aimantation et de la bande de valence du gadolinium épitaxié sur tungstène. Dans ce cadre rentre une étude de l oxydation de ce matériau, limitant dans le temps les études approfondies. Pour finir, il est mis l accent sur un phénomène contraignant lors des études de dynamique électronique en photoémission, l effet de charge-espace. Ceci a pour effet de générer des photoélectrons à partir de métaux, à l aide d un processus multiphotonique. Nous proposons dans cette partie un modèle théorique expliquant ce phénomène.Ces travaux sont inscrits dans le cadre du développement du synchrotron SOLEIL, pour permettre le développement du FEMTOSLICING, qui permettra prochainement de mesurer des dynamiques rapides résolues en éléments, à une résolution de l ordre de la centaine de femtosecondes.Those thesis works are included in the framework of the study of ultrafast magnetization dynamics. First of all I introduce theoretical aspects, then experimental aspects of this kind of experiments.In this aim, we have studied the answer of ferromagnetic alloys of different compositions with a bench of time resolved magneto optical Kerr effect measurement, then in a second part, the magnetization and valence band dynamics of the epitaxial Gadolinium on tungsten. In this framework, we studied the oxidization of the Gd, which limits in the time the studies. In the end, we focus on a disturbing process that happens during the study of electrons dynamics in photoemission, the space charge effect. This can generate photoelectrons from metals, with a multiphotonic process. We propose in this last part a theoretical model to explain this phenomenon.These works are included in the development of SOLEIL synchrotron facility, in order to allow the development of the FEMTOSLICING, that will next allow to perform element resolved experiments within a time resolution of a hundredth of femtoseconds.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
Superconducting Thick-Films From a Y-Ba-Cu-O Precursor
We have prepared screen-printed films of the Y-Ba-Cu-O compound starting from a spray-pyrolysis
precursor powder. BeO ceramic substrates are confirmed to be inert with respect to the film up to
about 1000ºC. Electrical properties of oxygen annealed films are investigated, evidencing excellent
superconducting behaviour, both in terms of Tc(≃91K)
and Jc (>102 A.cm-2 at 77 K)
Direct Observation of Massless Domain Wall Dynamics in Nanostripes with Perpendicular Magnetic Anisotropy
Domain wall motion induced by nanosecond current pulses in nanostripes with
perpendicular magnetic anisotropy (Pt/Co/AlO) is shown to exhibit
negligible inertia. Time-resolved magnetic microscopy during current pulses
reveals that the domain walls start moving, with a constant speed, as soon as
the current reaches a constant amplitude, and no or little motion takes place
after the end of the pulse. The very low 'mass' of these domain walls is
attributed to the combination of their narrow width and high damping parameter
. Such a small inertia should allow accurate control of domain wall
motion, by tuning the duration and amplitude of the current pulses
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Preventing carbon contamination of optical devices for X-rays: the effect of oxygen on photon-induced dissociation of CO on platinum
Platinum is one of the most common coatings used to optimize mirror
reflectivity in soft X-ray beamlines. Normal operation results in optics
contamination by carbon-based molecules present in the residual vacuum of
the beamlines. The reflectivity reduction induced by a carbon layer at the mirror
surface is a major problem in synchrotron radiation sources. A time-dependent
photoelectron spectroscopy study of the chemical reactions which take place at
the Pt(111) surface under operating conditions is presented. It is shown that the
carbon contamination layer growth can be stopped and reversed by low partial
pressures of oxygen for optics operated in intense photon beams at liquidnitrogen
temperature. For mirrors operated at room temperature the carbon
contamination observed for equivalent partial pressures of CO is reduced and
the effects of oxygen are observed on a long time scale
Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2x1)
We report results on the self-assembly of silicon nanoribbons on the (2x1)
reconstructed Au(110) surface under ultra-high vacuum conditions. Upon
adsorption of 0.2 monolayer (ML) of silicon the (2x1) reconstruction of Au(110)
is replaced by an ordered surface alloy. Above this coverage a new
superstructure is revealed by low electron energy diffraction (LEED) which
becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons
all oriented along the [-110] direction as revealed by LEED and scanning
tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy
indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In
addition the silicon atoms show signatures of two chemical environments
corresponding to the edge and center of the ribbons.Comment: Under publication in Applied Physics Letter
Direct observation of Oersted-field-induced magnetization dynamics in magnetic nanostripes
We have used time-resolved x-ray photoemission electron microscopy to
investigate the magnetization dynamics induced by nanosecond current pulses in
NiFe/Cu/Co nanostripes. A large tilt of the NiFe magnetization in the direction
transverse to the stripe is observed during the pulses. We show that this
effect cannot be quantitatively understood from the amplitude of the Oersted
field and the shape anisotropy. High frequency oscillations observed at the
onset of the pulses are attributed to precessional motion of the NiFe
magnetization about the effective field. We discuss the possible origins of the
large magnetization tilt and the potential implications of the static and
dynamic effects of the Oersted field on current-induced domain wall motion in
such stripes.Comment: Published in Phys. Rev. B 83, 020406 (2011) (Rapid Communications
Multiple satellites in materials with complex plasmon spectra: From graphite to graphene
International audienceThe 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 π+σ plasmon of graphite, whereas the π 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
Silicon Sheets By Redox Assisted Chemical Exfoliation
In this paper, we report the direct chemical synthesis of silicon sheets in
gram-scale quantities by chemical exfoliation of pre-processed calcium
di-silicide (CaSi2). We have used a combination of X-ray photoelectron
spectroscopy, transmission electron microscopy and Energy-dispersive X-ray
spectroscopy to characterize the obtained silicon sheets. We found that the
clean and crystalline silicon sheets show a 2-dimensional hexagonal graphitic
structure.Comment: Accepted in J. Phys.: Condens. Matte
High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)
International audienceThe van de Waals heterostructure formed by an epitaxial trilayer graphene is of particular interest due to its unique tunable electronic band structure and stacking sequence. However, to date, there has been a lack in the fundamental understanding of the electronic properties of epitaxial trilayer graphene. Here, we investigate the electronic properties of large-area epitaxial trilayer graphene on a 4° off-axis SiC(0001) substrate. Micro-Raman mappings and atomic force microscopy (AFM) confirmed predominantly trilayer on the sample obtained under optimized conditions. We used angle-resolved photoemission spectroscopy (ARPES) and Density Functional Theory (DFT) calculations to study in detail the structure of valence electronic states, in particular the dispersion of π bands in reciprocal space and the exact determination of the number of graphene layers. Using far-infrared magneto-transmission (FIR-MT), we demonstrate, that the electron cyclotron resonance (CR) occurs between Landau levels with a (B)1/2 dependence. The CR line-width is consistent with a high Dirac fermions mobility of ~3000 cm2·V−1·s−1 at 4 K
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