204 research outputs found
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Interface bonding of a ferromagnetic/semiconductor junction : a photoemission study of Fe/ZnSe(001)
We have probed the interface of a ferromagnetic/semiconductor (FM/SC)
heterojunction by a combined high resolution photoemission spectroscopy and
x-ray photoelectron diffraction study. Fe/ZnSe(001) is considered as an example
of a very low reactivity interface system and it expected to constitute large
Tunnel Magnetoresistance devices. We focus on the interface atomic environment,
on the microscopic processes of the interface formation and on the iron
valence-band. We show that the Fe contact with ZnSe induces a chemical
conversion of the ZnSe outermost atomic layers. The main driving force that
induces this rearrangement is the requirement for a stable Fe-Se bonding at the
interface and a Se monolayer that floats at the Fe growth front. The released
Zn atoms are incorporated in substitution in the Fe lattice position. This
formation process is independent of the ZnSe surface termination (Zn or Se).
The Fe valence-band evolution indicates that the d-states at the Fermi level
show up even at submonolayer Fe coverage but that the Fe bulk character is only
recovered above 10 monolayers. Indeed, the Fe 1-band states,
theoretically predicted to dominate the tunneling conductance of Fe/ZnSe/Fe
junctions, are strongly modified at the FM/SC interface.Comment: 23 pages, 5 figures, submitted to Physical review
An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials
Here we present an integrated ultra-high vacuum apparatus \u2013 named MBE-Cluster \u2013 dedicated to the growth
and in situ structural, spectroscopic and magnetic characterization of complex materials. Molecular Beam
Epitaxy (MBE) growth of metal oxides, e.g. manganites, and deposition of patterned metallic layers can be
fabricated and in situ characterized by reflection high-energy electron diffraction (RHEED), low-energy
electron diffraction (LEED) - Auger Electron Spectroscopy, X-ray photoemission spectroscopy (PES) and
azimuthal longitudinal magneto-optic Kerr effect (MOKE). The temperature can be controlled in the range
from 5 to 580 K, with the possibility of application of magnetic fields H up to \ub17 kOe and electric fields E for
voltages up to \ub1500 V. The MBE-Cluster operates for in-house research as well as user facility in combination
with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator (HHG) facility for timeresolved
spectroscopy
Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet
Polarization dependent x-ray absorption spectroscopy was used to study the
magnetic ground state and the orbital occupation in bulk-phase VI van der
Waals crystals below and above the ferromagnetic and structural transitions.
X-ray natural linear dichroism and X-ray magnetic circular dichroism spectra
acquired at the V edges are compared against multiplet cluster
calculations within the frame of the ligand field theory to quantify the
intra-atomic electronic interactions at play and evaluate the effects of
symmetry reduction occurring in a trigonally distorted VI unit. We observed
a non zero linear dichroism proving the presence of an anisotropic charge
density distribution around the V ion due to the unbalanced
hybridization between the Vanadium and the ligand states. Such hybridization
acts as an effective trigonal crystal field, slightly lifting the degeneracy of
the ground state. However, the energy splitting associated to the
distortion underestimates the experimental band gap, suggesting that the
insulating ground state is stabilized by Mott correlation effects rather than
via a Jahn-Teller mechanism. Our results clarify the role of the distortion in
VI and establish a benchmark for the study of the spectroscopic properties
of other van der Waals halides, including emerging 2D materials with mono and
few-layers thickness, whose fundamental properties might be altered by reduced
dimensions and interface proximity
Identification of hidden orbital contributions in the La_{0.65} Sr_{0.35} MnO_{3} valence band
Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La_{0.65} Sr_{0.35} MnO_{3} (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3d and O 2p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5p, Mn 4s, O 2s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general
Valence band electronic structure of V2O3: identification of V and O bands
We present a comprehensive study of the photon energy dependence of the
valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The
analysis of our experimental results, covering an extended photon energy range
(20-6000 eV) and combined with GW calculations, allow us to identify the nature
of the orbitals contributing to the total spectral weight at different binding
energies, and in particular to locate the V 4s at about 8 eV binding energy.
From this comparative analysis we can conclude that the intensity of the
quasiparticle photoemission peak, observed close to the Fermi level in the
paramagnetic metallic phase upon increasing photon energy, does not have a
significant correlation with the intensity variation of the O 2p and V 3d
yield, thus confirming that bulk sensitivity is an essential requirement for
the detection of this coherent low energy excitation
A reaction cell for ambient pressure soft x-ray absorption spectroscopy
We present a new experimental setup for performing X-ray Absorption Spectroscopy (XAS) in the soft X-ray range at ambient pressure. The ambient pressure XAS setup is fully compatible with the ultra high vacuum environment of a synchrotron radiation spectroscopy beamline end station by means of ultrathin Si3N4 membranes acting as windows for the X-ray beam and seal of the atmospheric sample environment. The XAS detection is performed in total electron yield (TEY) mode by probing the drain current from the sample with a picoammeter. The high signal/noise ratio achievable in the TEY mode, combined with a continuous scanning of the X-ray energies, makes it possible recording XAS spectra in a few seconds. The first results show the performance of this setup to record fast XAS spectra from sample surfaces exposed at atmospheric pressure, even in the case of highly insulating samples. The use of a permanent magnet inside the reaction cell enables the measurement of X-ray magnetic circular dichroism at ambient pressure
TOPLHA: an accurate and efficient numerical tool for analysis and design of LH antennas
This paper presents a self-consistent, integral-equation approach for the analysis of plasma-facing lower hybrid (LH) launchers; the geometry of the waveguide grill structure can be completely arbitrary, including the non-planar mouth of the grill. This work is based on the theoretical approach and code implementation of the TOPICA code, of which it shares the modular structure and constitutes the extension into the LH range. Code results are validated against the literature results and simulations from similar code
Electronic structure of In₂O₃ and Sn-doped In₂O₃ by hard x-ray photoemission spectroscopy
The valence and core levels of In₂O₃ and Sn-doped In₂O₃ have been studied by hard x-ray photoemission spectroscopy (hν=6000 eV) and by conventional Al Kα (hν=1486.6 eV) x-ray photoemission spectroscopy. The experimental spectra are compared with density-functional theory calculations. It is shown that structure deriving from electronic levels with significant In or Sn 5s character is selectively enhanced under 6000 eV excitation. This allows us to infer that conduction band states in Sn-doped samples and states at the bottom of the valence band both contain a pronounced In 5s contribution. The In 3d core line measured at hν=1486.6 eV for both undoped and Sn-doped In₂O₃ display an asymmetric lineshape, and may be fitted with two components associated with screened and unscreened final states. The In 3d core line spectra excited at hν=6000 eV for the Sn-doped samples display pronounced shoulders and demand a fit with two components. The In 3d core line spectrum for the undoped sample can also be fitted with two components, although the relative intensity of the component associated with the screened final state is low, compared to excitation at 1486.6 eV. These results are consistent with a high concentration of carriers confined close to the surface of nominally undoped In₂O₃. This conclusion is in accord with the fact that a conduction band feature observed for undoped In₂O₃ in Al Kα x-ray photoemission is much weaker than expected in hard x-ray photoemission
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