232 research outputs found
Electronic structure of PrCaMnO near the Fermi level studied by ultraviolet photoelectron and x-ray absorption spectroscopy
We have investigated the temperature-dependent changes in the near-
occupied and unoccupied states of PrCaMnO which shows the
presence of ferromagnetic and antiferromagnetic phases. The
temperature-dependent changes in the charge and orbital degrees of freedom and
associated changes in the Mn 3 - O 2 hybridization result in varied O
2 contributions to the valence band. A quantitative estimate of the charge
transfer energy () shows a larger value compared to the earlier
reported estimates. The charge localization causing the large is
discussed in terms of different models including the electronic phase
separation.Comment: 19 pages, 7 figures, To be published in Phy. Rev.
On the Kramers-Kronig transform with logarithmic kernel for the reflection phase in the Drude model
We use the Kramers-Kronig transform (KKT) with logarithmic kernel to obtain
the reflection phase and, subsequently, the complex refractive index of a bulk
mirror from reflectance. However, there remains some confusion regarding the
formulation for this analysis. Assuming the damped Drude model for the
dielectric constant and the oblique incidence case, we calculate the additional
terms: phase at zero frequency and Blashke factor and we propose a reformulated
KKT within this model. Absolute reflectance in the s-polarization case of a
gold film is measured between 40 and 350 eV for various glancing angles using
synchrotron radiation and its complex refractive index is deduced using the
reformulated KKT that we propose. The results are discussed with respect to the
data available in the literature.Comment: 18 pages, piblished in j. Mod. Opt. 57, 1504 (2010
Substrate induced proximity effect in superconducting niobium nanofilms
Structural and superconducting properties of high quality Niobium nanofilms
with different thicknesses are investigated on silicon oxide and sapphire
substrates. The role played by the different substrates and the superconducting
properties of the Nb films are discussed based on the defectivity of the films
and on the presence of an interfacial oxide layer between the Nb film and the
substrate. The X-ray absorption spectroscopy is employed to uncover the
structure of the interfacial layer. We show that this interfacial layer leads
to a strong proximity effect, specially in films deposited on a SiO
substrate, altering the superconducting properties of the Nb films. Our results
establish that the critical temperature is determined by an interplay between
quantum-size effects, due to the reduction of the Nb film thicknesses, and
proximity effects
Determination of the magnetization profile of Co/Mg periodic multilayers by magneto-optic Kerr effect and X-ray magnetic resonant reflectivity
The resonant magnetic reflectivity of Co/Mg multilayers around the Co L2,3
absorption edge is simulated then measured on a specifically designed sample.
The dichroic signal is obtained when making the difference between the two
reflectivities measured with the magnetic field applied in two opposite
directions parallel to the sample surface. The simulations show that the
existence of magnetic dead layers at the interfaces between the Co and Mg
layers leads to an important increase of the dichroic signal measured in the
vicinity of the third Bragg peak that otherwise should be negligible. The
measurements are in agreement with the model introducing 0.25 nm thick dead
layers. This is attributed to the Co atoms in contact with the Mg layers and
thus we conclude that the Co-Mg interfaces are abrupt from the magnetic point
of view.Comment: 8 page
GaAs(111)A and B in hydrazine sulfide solutions : extreme polarity dependence of surface adsorption processes
Chemical bonds formed by hydrazine-sulfide treatment of GaAs(111) were
studied by synchrotron photoemission spectroscopy. At the B surface, the top
arsenic atoms are replaced by nitrogen atoms, while GaAs(111)A is covered by
sulfur, also bonded to underlying gallium, despite the sulfide molar
concentration being 103 times smaller than that of the hydrazine. This extreme
dependence on surface polarity is explained by competitive adsorption processes
of HS- and OH- anions and of hydrazine molecules, on Ga- adsorption sites,
which have distinct configurations on the A and B surfaces
Electronic properties and orbital-filling mechanism in Rb-intercalated copper phthalocyanine
The evolution of the electronic properties of a thin film of copper phthalocyanine deposited on Al(100) and progressively intercalated with rubidium atoms was followed by photoemission and X-ray absorption spectroscopies. Electron donation from the Rb atoms to the C32H16N8Cu molecules results in the lifting of the degeneracy in the e(g) ligand-derived molecular orbital and the lowering of the molecular symmetry. For Rb similar to 2C32H16N8Cu, spectral evidence indicates that both donated electrons reside in the first split-off e(g)-derived level, thus creating an electronic inequivalence between the C atoms in the benzene rings. For higher Rb concentrations, a reduction of the Cu oxidation state is observed, together with a new Cu-derived state in valence-band photoemission spectra, testifying to the filling of the b(1g) orbital. Thus, even though b(1g) is the lowest unoccupied orbital of the neutral molecule, in the film, the Cu-derived b(1g)-derived states are occupied only after a partial filling of the e.-derived band has taken place. Despite the fact that the eg-derived spectral weight becomes larger as the rubidium content in the RbxC32H16N8Cu compound increases, no spectral density was observed at the Fermi level, showing that the film remains insulating for all of the investigated stoichiometries
A Nexafs Study of Nitric Oxide Layers Adsorbed from a nitrite Solution onto a Pt(111) Surface
NO molecules adsorbed on a Pt(111) surface from dipping in an acidic nitrite
solution are studied by near edge X-ray absorption fine structure spectroscopy
(NEXAFS), X-ray photoelectron spectroscopy (XPS), low energy electron
diffraction (LEED) and scanning tunnelling microscopy (STM) techniques. LEED
patterns and STM images show that no long range ordered structures are formed
after NO adsorption on a Pt(111) surface. Although the total NO coverage is
very low, spectroscopic features in N K-edge and O K-edge absorption spectra
have been singled out and related to the different species induced by this
preparation method. From these measurements it is concluded that the NO
molecule is adsorbed trough the N atom in an upright conformation. The maximum
saturation coverage is about 0.3 monolayers, and although nitric oxide is the
major component, nitrite and nitrogen species are slightly co-adsorbed on the
surface. The results obtained from this study are compared with those
previously reported in the literature for NO adsorbed on Pt(111) under UHV
conditions
Measuring magnetic profiles at manganite surfaces with monolayer resolution
The performance of manganite-based magnetic tunnel junctions (MTJs) has
suffered from reduced magnetization present at the junction interfaces that is
ultimately responsible for the spin polarization of injected currents; this
behavior has been attributed to a magnetic "dead layer" that typically extends
a few unit cells into the manganite. X-ray magnetic scattering in resonant
conditions (XRMS) is one of the most innovative and effective techniques to
extract surface or interfacial magnetization profiles with subnanometer
resolution, and has only recently been applied to oxide heterostructures. Here
we present our approach to characterizing the surface and interfacial
magnetization of such heterostructures using the XRMS technique, conducted at
the BEAR beamline (Elettra synchrotron, Trieste). Measurements were carried out
in specular reflectivity geometry, switching the left/right elliptical
polarization of light as well the magnetization direction in the scattering
plane. Spectra were collected across the Mn L2,3 edge for at least four
different grazing angles in order to better analyse the interference phenomena.
The resulting reflectivity spectra have been carefully fit to obtain the
magnetization profiles, minimizing the number of free parameters as much as
possible. Optical constants of the samples (real and imaginary part of the
refractive index) in the interested frequency range are obtained through
absorption measurements in two magnetization states and subsequent
Kramers-Kronig transformation, allowing quantitative fits of the magnetization
profile at different temperatures. We apply this method to the study of
air-exposed surfaces of epitaxial La2/3Sr1/3MnO3 (001) films grown on SrTiO3
(001) substrates.Comment: 11 pages + 3 figures; accepted to JMMM (2009
Investigation of the thermal stability of Mg/Co periodic multilayers for EUV applications
We present the results of the characterization of Mg/Co periodic multilayers
and their thermal stability for the EUV range. The annealing study is performed
up to a temperature of 400\degree C. Images obtained by scanning transmission
electron microscopy and electron energy loss spectroscopy clearly show the good
quality of the multilayer structure. The measurements of the EUV reflectivity
around 25 nm (~49 eV) indicate that the reflectivity decreases when the
annealing temperature increases above 300\degreeC. X-ray emission spectroscopy
is performed to determine the chemical state of the Mg atoms within the Mg/Co
multilayer. Nuclear magnetic resonance used to determine the chemical state of
the Co atoms and scanning electron microscopy images of cross sections of the
Mg/Co multilayers reveal changes in the morphology of the stack from an
annealing temperature of 305\degreee;C. This explains the observed reflectivity
loss.Comment: Published in Applied Physics A: Materials Science \& Processing
Published at
http://www.springerlink.com.chimie.gate.inist.fr/content/6v396j6m56771r61/ 21
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