74 research outputs found
Spectromicroscopy of pulses transporting alkali metal in a surface reaction
The NO + H2 reaction on a potassium promoted Rh(110) surface is shown to sustain the formation of spatio-temporal periodic patterns leading to mass transport phenomena. The excitation of pulses and the mass transport mechanism are studied in the 10-7 and 10-6 mbar pressure range, with the potassium coverage varying between K = 0.05 and K = 0.12 ML. Using spectroscopic photoemission and spectroscopic low energy electron microscopy (SPELEEM) as well as related microprobe diffraction techniques, we show that the excitation mechanism comprises a cyclic structural transformation: K + O-coadsorbate → (2 × 1)-N → c(2 × 4)-2O,N → K + O coadsorbate. Laterally resolved spectroscopy demonstrates that potassium is accumulated in front of the nitrogen pulses, suggesting that adsorbed nitrogen acts as a diffusion barrier for potassium. © 2013 The Owner Societies
Quantitative analysis of shadow X-ray Magnetic Circular Dichroism Photo-Emission Electron Microscopy
Shadow X-ray Magnetic Circular Dichroism Photo-Emission Electron Microscopy
(XMCD-PEEM) is a recent technique, in which the photon intensity in the shadow
of an object lying on a surface, may be used to gather information about the
three-dimensional magnetization texture inside the object. Our purpose here is
to lay the basis of a quantitative analysis of this technique. We first discuss
the principle and implementation of a method to simulate the contrast expected
from an arbitrary micromagnetic state. Text book examples and successful
comparison with experiments are then given. Instrumental settings are finally
discussed, having an impact on the contrast and spatial resolution : photon
energy, microscope extraction voltage and plane of focus, microscope background
level, electric-field related distortion of three-dimensional objects, Fresnel
diffraction or photon scattering
Making ARPES Measurements on Corrugated Monolayer Crystals: Suspended Exfoliated Single-Crystal Graphene
Free-standing exfoliated monolayer graphene is an ultra-thin flexible
membrane, which exhibits out of plane deformation or corrugation. In this
paper, a technique is described to measure the band structure of such
free-standing graphene by angle-resolved photoemission. Our results show that
photoelectron coherence is limited by the crystal corrugation. However, by
combining surface morphology measurements of the graphene roughness with
angle-resolved photoemission, energy dependent quasiparticle lifetime and
bandstructure measurements can be extracted. Our measurements rely on our
development of an analytical formulation for relating the crystal corrugation
to the photoemission linewidth. Our ARPES measurements show that, despite
significant deviation from planarity of the crystal, the electronic structure
of exfoliated suspended graphene is nearly that of ideal, undoped graphene; we
measure the Dirac point to be within 25 meV of . Further, we show that
suspended graphene behaves as a marginal Fermi-liquid, with a quasiparticle
lifetime which scales as ; comparison with other graphene and
graphite data is discussed
Absence of Dirac cones in monolayer silicene and multilayer Si films on Ag(111)
Monolayer silicene and multilayer silicon films on Ag(111) have been the subject of many investigations within the last few years. For both systems, photoemission data have been interpreted in terms of linearly dispersing bands giving rise to the characteristic Dirac cone features, similar to graphene. Here we demonstrate, on the basis of angle-resolved valence band and core level photoemission data that this assignment is not correct. The bands previously attributed to states with Dirac fermion character are shown to derive from Ag(111) interface and bulk states in the silicene monolayer and from the well-known Ag-View the MathML source(3×3)R30°-Si(111) structure in Si multilayers. These results question the validity of the claim that graphene-like silicene and silicene multilayers are in fact formed on Ag(111)
Domain-wall depinning assisted by pure spin currents
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.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Let
Pnictogens Allotropy and Phase Transformation during van der Waals Growth
Pnictogens have multiple allotropic forms resulting from their ns2 np3
valence electronic configuration, making them the only elemental materials to
crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout
the group. Light group VA elements are found in the layered orthorhombic A17
phase such as black phosphorus, and can transition to the layered rhombohedral
A7 phase at high pressure. On the other hand, bulk heavier elements are only
stable in the A7 phase. Herein, we demonstrate that these two phases not only
co-exist during the vdW growth of antimony on weakly interacting surfaces, but
also undertake a spontaneous transformation from the A17 phase to the
thermodynamically stable A7 phase. This metastability of the A17 phase is
revealed by real-time studies unraveling its thickness-driven transition to the
A7 phase and the concomitant evolution of its electronic properties. At a
critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle
transition from AB to AA stacked alpha-antimonene followed by a gradual
relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of
this intermediate phase is found to be determined by surface self-passivation
and the associated competition between A7- and A17-like bonding in the bulk.
These results highlight the critical role of the atomic structure and
interfacial interactions in shaping the stability and electronic
characteristics of vdW layered materials, thus enabling a new degree of freedom
to engineer their properties using scalable processes
Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures
Magnetic skyrmions are chiral spin structures with a whirling configuration.
Their topological properties, nanometer size and the fact that they can be
moved by small current densities have opened a new paradigm for the
manipulation of magnetisation at the nanoscale. To date, chiral skyrmion
structures have been experimentally demonstrated only in bulk materials and in
epitaxial ultrathin films and under external magnetic field or at low
temperature. Here, we report on the observation of stable skyrmions in
sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero
applied magnetic field. We use high lateral resolution X-ray magnetic circular
dichroism microscopy to image their chiral N\'eel internal structure which we
explain as due to the large strength of the Dzyaloshinskii-Moriya interaction
as revealed by spin wave spectroscopy measurements. Our results are
substantiated by micromagnetic simulations and numerical models, which allow
the identification of the physical mechanisms governing the size and stability
of the skyrmions.Comment: Submitted version. Extended version to appear in Nature
Nanotechnolog
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