321 research outputs found
High energy photoelectron diffraction: model calculations and future possibilities
We discuss the theoretical modelling of x-ray photoelectron diffraction (XPD)
with hard x-ray excitation at up to 20 keV, using the dynamical theory of
electron diffraction to illustrate the characteristic aspects of diffraction
patterns resulting from such localized emission sources in a multi-layer
crystal.
We show via dynamical calculations for diamond, Si, and Fe that the dynamical
theory well predicts available current data for lower energies around 1 keV,
and that the patterns for energies above about 1 keV are dominated by Kikuchi
bands which are created by the dynamical scattering of electrons from lattice
planes. The origin of the fine structure in such bands is discussed from the
point of view of atomic positions in the unit cell. The profiles and positions
of the element-specific photoelectron Kikuchi bands are found to be sensitive
to lattice distortions (e.g. a 1% tetragonal distortion) and the position of
impurities or dopants with respect to lattice sites. We also compare the
dynamical calculations to results from a cluster model that is more often used
to describe lower-energy XPD. We conclude that hard XPD (HXPD) should be
capable of providing unique bulk-sensitive structural information for a wide
variety of complex materials in future experiments.Comment: 29 pages, 13 figure
Interface Engineering to Create a Strong Spin Filter Contact to Silicon
Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on
silicon is a perfect route to enrich silicon nanotechnology with spin filter
functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a
heteroepitaxial integration without significant contaminations of the interface
with Eu silicides and Si oxides.
We present a solution to this long-standing problem by applying two
complementary passivation techniques for the reactive EuO/Si interface:
() an hydrogen-Si passivation and () the
application of oxygen-protective Eu monolayers --- without using any additional
buffer layers.
By careful chemical depth profiling of the oxide-semiconductor interface via
hard x-ray photoemission spectroscopy, we show how to systematically minimize
both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how
to ultimately interface-engineer chemically clean, heteroepitaxial and
ferromagnetic EuO/Si in order to create a strong spin filter contact to
silicon.Comment: 11 pages of scientific paper, 10 high-resolution color figures.
Supplemental information on the thermodynamic problem available (PDF).
High-resolution abstract graphic available (PNG). Original research (2016
Temperature-Dependent X-Ray Absorption Spectroscopy of Colossal Magnetoresistive Perovskites
The temperature dependence of the O K-edge pre-edge structure in the x-ray
absorption spectra of the perovskites La(1-x)A(x)MnO(3), (A = Ca, Sr; x = 0.3,
0.4) reveals a correlation between the disappearance of the splitting in the
pre-edge region and the presence of Jahn-Teller distortions. The different
magnitudes of the distortions for different compounds is proposed to explain
some dissimilarity in the line shape of the spectra taken above the Curie
temperature.Comment: To appear in Phys. Rev. B, 5 pages, 3 figure
Direct Observation of High-Temperature Polaronic Behavior In Colossal Magnetoresistive Manganites
The temperature dependence of the electronic and atomic structure of the
colossal magnetoresistive oxides (x = 0.3, 0.4) has
been studied using core and valence level photoemission, x-ray absorption and
emission, and extended x-ray absorption fine structure spectroscopy. A dramatic
and reversible change of the electronic structure is observed on crossing the
Curie temperature, including charge localization and spin moment increase of
Mn, together with Jahn-Teller distortions, both signatures of polaron
formation. Our data are also consistent with a phase-separation scenario.Comment: 5 pages, 4 figures, revte
Multiatom resonant photoemission: Theory and systematics
A first-principles calculation of the recently discovered interatomic multiatom resonant photoemission (MARPE) effect is presented. In this phenomenon, core photoelectron intensities are enhanced when the photon energy is tuned to a core-level absorption edge of nonidentical neighboring atoms, thus enabling direct determination of near-neighbor atomic identities. Both the multiatom character of MARPE and retardation effects in the photon and electron interactions in the resonant channel are shown to be
crucial. Measured peak-intensity enhancements of 40% in MnO and spectral shapes similar to the corresponding x-ray absorption profiles are well reproduced by this theory.This work was supported by the U.S. Department of Energy, under Contract No. DE-AC03-76SF00098, the University of the Basque Country, and the Spanish Ministerio de Educación y Cultura (Fulbright Grant No. FU-98-22726216).Peer reviewe
Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films
X-ray photoelectron diffraction is used to directly probe the intra-cell
polar atomic distortion and tetragonality associated with ferroelectricity in
ultrathin epitaxial PbTiO3 films. Our measurements, combined with ab-initio
calculations, unambiguously demonstrate non-centro-symmetry in films a few unit
cells thick, imply that films as thin as 3 unit cells still preserve a
ferroelectric polar distortion, and also show that there is no thick
paraelectric dead layer at the surface
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