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The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming ${\mathrm{O}}_{2}+{\mathrm{H}}_{2}$ reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in ${\mathrm{H}}_{2}$ produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the ${\mathrm{O}}_{2}+{\mathrm{H}}_{2}$ reaction involve a cyclic transformation between 3D-NiO and 2D-NiO.

Heterostructures of two-dimensional materials have shown unusual properties and rich physical phenomena. This paper reports on micrometer-scale angle-resolved photoemission spectroscopy of van der Waals heterostructures of graphene and MoS${}_{2}$ monolayers. The authors directly measured the electronic structure of monolayer stacking and its tunability due to the twist-angle between the layers. They show that the electronic states of graphene and MoS${}_{2}$ are not hybridized, and the band gap of MoS${}_{2}$ can be engineered by changing the orientation of the two layers.

International audience; Topological protection is an elegant way of warranting the integrity of quantum and nanosized systems. In magnetism one example is the Bloch-point, a peculiar object implying the local vanishing of magnetization within a ferromagnet. Its existence had been postulated and described theoretically since several decades, however it has never been observed. We con rm experimentally the existence of Bloch points, imaged within domain walls in cylindrical magnetic nanowires, combining surface and transmission XMCD-PEEM magnetic microscopy. This opens the way to the experimental search for peculiar phenomena predicted during the motion of Bloch-point-based domain walls.

The inelastic mean free path (IMFP) of electrons is derived using a new approach based on the low-energy electron reflectivity from ultrathin films. The thickness-dependent quantum size oscillations as a function of electron energy observed in the reflectivity of slow electrons are modeled using an absorbing Fabry-P\'erot interferometer consisting of vacuum, film, and substrate. The absorbing properties of the film are represented by the imaginary part of the complex refractive index associated with the IMFP which determines the amplitude of the electron reflectivity oscillations. Using this formalism for an Fe film on W(110), the IMFP in Fe is found in the energy range from 4 to 18 eV above the vacuum level. In contrast to the common notion, the IMFP in Fe is shown to have a very weak energy dependence at low energy. The results are in good agreement with independent IMFP measurements found in thickness-dependent photoemission experiments.

International audience; We investigate the chemical and morphological structure of the Au nanodots on Ge(111) which serve as catalysts for the formation of epitaxial Ge nanowires. The spatial localization of Au is investigated by X-ray spectromicroscopy and transmission electron microscopy. We show that dewetting of an Au film on Ge(111) gives rise to a thin Au-Ge wetting layer and Au-Ge dots. These dots are crystallized but not with a single crystallographic orientation. Thanks to the spatially resolved X-ray and transmission electron microscopy measurements, a chemical characterization of both binary Au-Ge catalysts and wetting layer is obtained at the nanoscale. We show that Ge vertical growth is achieved even without external Ge supply.

The microscopic structure of the $90\ifmmode^\circ\else\textdegree\fi{}$ in-plane magnetic reorientation transition in Fe(110) films is examined using photoemission x-ray microscopy. At the nanoscale, sharp magnetic boundaries are detected. They are indicative of a first-order transition and are consistent with Fe magnetic anisotropy constants. At the micron scale, the magnetic boundary breaks up into triangular patterns whose characteristic angular dependence is revealed by experiments on conical microwedges. This effect, fully accounted by micromagnetic simulations, opens the possibility to control the sharpness of the transition at the microscopic scale.

We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second permalloy structure with a domain wall and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6*10^{-14}T/(A/m^2), which is more than an order of magnitude larger than for conventional current induced domain wall motion. Theoretically we reproduce this high efficiency, which arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning., 11 pages, 3 figures, accepted for publication in Phys. Rev. Lett

Received 28 July 2009; revised manuscript received 18 September 2009; published 23 November 2009 The connection between magnetization and structure of ultrathin films is studied at room temperature for the case of Fe films on W110 by inserting a 2-monolayer-thick growth-modifying Au layer between film and substrate using spin-polarized low-energy electron microscopy and low-energy electron diffraction. Ferromagnetic order with the easy axis pointing in the 11 ¯ 0 direction appears upon percolation at 1.6 monolayers. Shortly thereafter, the easy axis rotates into the 001 direction. With further increasing thickness the magnetization oscillates between the 001 and the 11 ¯ 0 direction with a maximum deviation from the 001 direction at seven monolayers where the magnetic signal has a maximum. The changes in the magnetization are associated with changes in the structure, strain, and morphology that are deduced from the diffraction patterns and which strongly influence the competition between interface anisotropy, magnetoelastic anisotropy, and dipolar surface anisotropy.

We present an experimental study of the evolution of InAs/GaAs quantum dots partially capped with GaAs, as an annealing process transforms them first into quantum rings and later into holes penetrating the whole cap layer. Shape, composition, and optical emission were monitored as a function of annealing time by means of atomic force microscopy, x-ray photoemission microscopy, and photoluminescence, respectively. Our results show a progressive dissolution of the original dot, with the outdiffused material forming a second wetting layer on the planar region surrounding the dot. For the longest annealing time, in a situation close to thermodynamic equilibrium, no residual dot material is left in the holes, and an In-rich layer covers uniformly the surface. Our findings were examined by taking into account the surface chemical potential previously written for Stranski-Krastanov islands partially covered with a cap layer. We show that the energy gain due to the formation of the second wetting layer is the driving force for the shape and composition evolution, but its analytical expression should be modified, with respect to previous formulations, by taking into account the observed material interdiffusion.

We report measurements of the electronic structure and surface morphology of exfoliated graphene on an insulating substrate using angle-resolved photoemission and low energy electron diffraction. Our results show that although exfoliated graphene is microscopically corrugated, the valence band retains a massless fermionic dispersion, with a Fermi velocity of ~10^6 m/s. We observe a close relationship between the morphology and electronic structure, which suggests that controlling the interaction between graphene and the supporting substrate is essential for graphene device applications., Comment: 10 pages of text, 4 JPEG figures

It is shown that the pulses which develop in the NO+H 2 reaction on an alkali promoted Rh(110) surface reaction can transport alkali metal. This leads to the accumulation of a substantial alkali-metal concentration in the collision area of pulse trains. Realistic simulations revealed that the effect is due to the strong energetic interactions of the alkali metal with coadsorbates, i.e., the attractive interaction with coadsorbed oxygen and the effectively repulsive interaction with coadsorbed nitrogen.

A stress-induced stripe phase of submonolayer Pd on W(110) is observed by low-energy electron microscopy. The temperature dependence of the pattern is explained by the change both in the boundary free energy and elastic relaxation energy due to the increasing boundary width. The stripes are shown to disorder when the correlation length of the condensed phase becomes comparable to its period, while the condensate to lattice-gas transition takes place at a higher temperature, as revealed by low-energy electron diffraction.

We demonstrate for the first time that seeded harmonic generation on electron storage rings can produce coherent optical pulses in the vacuum ultraviolet spectral range. The experiment is performed at Elettra, where coherent pulses are generated at 132 nm, with a duration of about 100 fs. The light source has a repetition rate of 1 kHz and adjustable polarization; it is very bright, with a peak power several orders of magnitude above that of spontaneous synchrotron radiation. Owing to high stability, the source is used in a test photoemission electron microscopy experiment. We anticipate that seeded harmonic generation on storage rings can lead to unprecedented developments in time-resolved femtosecond spectroscopy and microscopy.

Electron-stimulated desorption of bridge-bonded oxygen atoms on the $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface at elevated temperature is used to produce an oxygen-deficient $(1\ifmmode\times\else\texttimes\fi{}2)$ reconstruction. Low energy electron diffraction, x-ray photoelectron spectroscopy, and first-principles calculations show that Au adsorbs on this surface with the substrate periodicity in strongly bonded monomer, dimer, and trimer rows with metallic character. In addition, the experimental evidence combined with the theoretical results points to a missing-row type of structure for the initial $(1\ifmmode\times\else\texttimes\fi{}2)$ surface.

We have studied the initial stages of the heteroepitaxy of Mg on W(110) in situ, by means of complementary microscopic and spectromicroscopic techniques. In the submonolayer coverage regime the Mg film undergoes a series of structural transitions which are clearly distinguished in the diffraction pattern. Monitoring the Mg $2p$ core-level permits the identification of the different energy shifts associated with each superstructure. During deposition of the second Mg monolayer at moderate temperatures, the growth of single atomic height stripes is observed. The Mg $2p$ core level of the first (interface) and second monolayer are remarkably separated. The density, length, and width of the stripes, stabilized by the anisotropic misfit between the W(110) and Mg(0001) lattices, can be tuned by modifying the growth parameters.