T. Gourieux

Sequence and symmetry of hole injection in YBa2Cu3O6+x in situ and ex situ experiments on powders and single crystals using X-ray absorption spectroscopy

Abstract Very accurate XAS in situ and ex situ measurements at the Cu K-edge on powders and single crystals of YBa2Cu3O6+x have been carried out using the time-resolved dispersive spectrometer at LURE-DCI. The whole range of oxygen content has been investigated and the x-dependence of the amount of Cu(I) has been evaluated at room and at high temperatures (from 400 to 750°C). Owing to the balance of charges, we derived the amount of the oxygen 2p holes. The sequence of injection was separated into steps, associated with three oxygen bands of distinct symmetries and with the well-known steps in the superconducting behavior.

Valence changes of copper under oxygen uptake and removal in YBa2Cu3O7−δ observed in situ by X-ray absorption spectroscopy

Abstract By using X-ray absorption spectroscopy (XAS) in dispersive mode, we have studied the evolution of the Cu K edge upon changing in situ the oxygen stoichiometry of Y1Ba2Cu3O7−δ. We observed directly the immediate transformation of the | 3d9 (ulbar|L) 〉 into the | 3d9 〉 configuration upon the initial increase of δ within the high oxygenated superconducting phase. Furthermore, even before the compound becomes semiconducting, in the range of δ from 0.25 to 0.35, a dramatic amount of monovalent copper is produced. That means that the hole concentration in that range is greater than that given by the chemical formula. When the compound reaches a poor oxygenated semiconducting phase a large amount of monovalent copper, i.e. | 3d10 〉 configuration is present. A nonlinear variation of the | 3d10 〉 configuration with δ is deduced.

Oxygen adsorption observed during the epitaxy of V(110) on the (1120) α-Al2O3 surface

Abstract Oxygen structures ( c (6 X 2) and p (11 X 4)) have been observed during the epitaxy of V(110) on a (11 2 0) α-Al 2 O 3 surface. The p (11 X 4)− O structure is reported for the first time. Results based on Auger spectroscopy show that the oxygen coverage for the p (11 X 4) structure must be ∼ 20% smaller than that for the c(6 X 2) structure. In-plane parameters deduced from RHEED patterns observed on the initial sapphire surface are in accordance with the parameters of the oxygen sub-lattice of sapphire. A clean V(110) surface is obtained after several hours of succesive annealing and ion bombardment.

Growth and structure of samarium overlayers on a cobalt (0001) single crystal

The growth and electronic behaviour of Sm ultra thin films deposited on a Co(0001) substrate have been studied by Auger spectroscopy, high energy electron diffraction and X-ray photoemission experiments. When the Co substrate is held at room temperature, a quasi two-dimensional growth is observed, whereas at higher substrate temperature (610 K), deposition of Sm leads to the formation of an epitaxial trivalent Sm-Co compound. At room temperature, Sm 3d photoemission spectra show an important time dependence.

The Eu/Pd(111) interface: spectroscopic and structural studies

Abstract Growth of Eu ultra thin films on a Pd(111) single crystal, kept at room temperature, has been studied by spectroscopic (XPS, UPS, AES) and structural (RHEED) methods. Eu atoms of the first two deposited monolayers are divalent and form a p(2 × 2) arrangement on the Pd(111) surface. During the completion of the third Eu monolayer, this ordered structure smears out in connection with the appearance of trivalent Eu atoms at the interface. The fine structure modifications of Pd and valence band XPS spectra indicate the 4d band filling, via a charge transfer mechanism, of the Pd atoms involved in the interface. For intake Eu coverages a diffusion process leads to formation of disordered divalent EuPd alloys. After annealing the sample at 820 K, RHEED diffraction patterns reappear (p(2 × 2)) and, simultaneously, an increase in the Eu mean valence is observed. XPS results show that segregation of divalent Eu occurs at the surface of the new ordered bulk-trivalent EuPd compound which forms at the interface. This behaviour occurs whatever the thickness of the Eu deposit is.

High Angle Magnetic X-ray Diffraction

The magnetic scattering of x-rays was first performed 30 years ago and was considered as a curiosity. However, due to the advent of high brilliance synchrotron radiation sources, this technique has become a standard microscopic probe for the investigation of magnetic properties. The magnetic x-ray scattering amplitude contains two terms: the nonresonant term that exists at all photon energies and the resonant term, induced by multipole electric transitions, that is peaked near absorption edges. In this paper, the basic concepts of x-ray magnetic scattering are first introduced for both non-resonant and resonant regimes. Then, selected examples are presented in order to illustrate several specific properties of the technique: i) high resolution in the reciprocal space; ii) ability to separate the orbital momentum contribution to magnetization; iii) chemical and electronic selectivity; iv) possibility to investigate weak moment systems and observe resonant signal for polarized non-magnetic ions; v) multi-q structure detection.

Historical and interpretative aspects of quantum mechanics: a physicists' naive approach

Many theoretical predictions derived from quantum mechanics have been confirmed experimentally for the last 80 years. However, interpretative aspects have long been subject to debate. Among them, the question of the existence of hidden variables is still open. We review these questions, paying special attention to historical aspects, and argue that one may definitively exclude local realism based on the present experimental outcomes. Nevertheless other interpretations of Quantum Mechanics are not excluded.

Ultra-thin films of terbium deposited on cobalt (101̄0)

Abstract The growth and structure of ultra thin films of terbium deposited on Co(1010) have been studied by X-ray photoemission, Auger spectroscopy and low energy electron diffraction at 600 K. Two stages are observed in the growth mode: the growth of about two Tb layers is followed by an interdiffusion process, leading to the formation of an alloy. We also show that two-dimensional ordering of the film occurs at 600 K in the monolayer range.

Helical magnetic structures in epitaxial [110] Eu films

In this paper, we investigate how the symmetry breaking, the lattice clamping and eventually the epitaxial strains in a thin film may affects the magnetic arrangement.

Photoemission studies of the cerium electronic properties at the CeFe(100) interface

Abstract Electronic properties of cerium ultra-thin films deposited on Fe(100) surfaces at room temperature have been investigated by Ce 4f resonant photoemission and Ce 3d X-ray photoemission experiments. For thicknesses lower than 4 monolayers, cerium is found to be in a strongly hybridized phase. By covering cerium films with three samarium layers, we have suppressed the Ce-vacuum interface in which the Ce atoms are less strongly hybridized. In this way, it is also possible to obtain the Ce 3d photoemission contributions from each atomic layer. It is then highlighted that the electronic structure of these layers are very different from each other. By combining basic contributions, it is possible to reconstruct the Ce 3d experimental spectra obtained for all cerium film thicknesses.