82 research outputs found
Electronic structure and magnetism of FeVX (X = Si, Ga and Al) alloys by the KKR-CPA method
We present first principles charge- and spin-selfconsistent electronic
structure computations on the Heusler-type disordered alloys FeVX
for three different metalloids X=(Si, Ga and Al). In these calculations we use
the methodology based on the Korringa-Kohn- Rostoker formalism and the
coherent-potential approximation (KKR-CPA), generalized to treat disorder in
multi-component complex alloys.Comment: RevTeX, 17 pages, 15 figures, to appear in Phys. Rev. B on Nov. 15
199
Temperature dependent orbital degree of freedom in a bilayer manganite by magnetic Compton scattering
We have measured temperature-dependent magnetic Compton profiles (MCPs) from
a single crystal of LaSrMnO. The MCPs, which involved
the scattering of circularly polarized x-rays, are in general related to the
momentum density of all the unpaired spins in the system. Nevertheless, we show
that when the x-ray scattering vector lies along the [110] direction, the
number of magnetic electrons of a specific symmetry, i.e. -electrons of
symmetry, yield a distinct signature in the MCP, allowing us to
monitor substantial changes in the occupancy of the states over
the investigated temperature range of 5-200K. This study indicates that
magnetic Compton scattering can provide a powerful window on the properties of
specific magnetic electrons in complex materials.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Detection and imaging of the oxygen deficiency in single crystalline YBaCuO thin films using a positron beam
Single crystalline YBaCuO
(YBCO) thin films were grown by pulsed laser deposition (PLD) in order to probe
the oxygen deficiency using a mono-energetic positron beam. The sample
set covered a large range of (0.191<<0.791) yielding a
variation of the critical temperature between 25 and 90\,K. We
found a linear correlation between the Doppler broadening of the positron
electron annihilation line and determined by X-ray diffraction (XRD).
Both, the origin of the found correlation and the influence of metallic
vacancies, were examined with the aid of ab-initio calculations that allowed us
(i) to exclude the presence of Y vacancies and (ii) to ensure that positrons
still probe despite the potential presence of Ba or Cu vacancies. In
addition, by scanning with the positron beam the spatial variation of
could be analyzed. It was found to fluctuate with a standard deviation of up to
within a single YBCO film
Lindhard and RPA susceptibility computations in extended momentum space in electron doped cuprates
We present an approximation for efficient calculation of the Lindhard
susceptibility in a periodic system through the use of
simple products of real space functions and the fast Fourier transform (FFT).
The method is illustrated by providing results for the
electron doped cuprate NdCeCuO extended over several
Brillouin zones. These results are relevant for interpreting inelastic X-ray
scattering spectra from cuprates.Comment: 6 pages, 6 figures, accepted in Physical Review
Direct observation of localization in the minority-spin-band electrons of magnetite below the Verwey temperature
Two-dimensional spin-uncompensated momentum density distributions, s, were reconstructed in magnetite at 12K and 300K from
several measured directional magnetic Compton profiles. Mechanical de-twinning
was used to overcome severe twinning in the single crystal sample below the
Verwey transition. The reconstructed in the first
Brillouin zone changes from being negative at 300 K to positive at 12 K. This
result provides the first clear evidence that electrons with low momenta in the
minority spin bands in magnetite are localized below the Verwey transition
temperature.Comment: 13 pages, 4 figures, accepted in Physical Review
A High-Resolution Compton Scattering Study of the Electron Momentum Density in Al
We report high-resolution Compton profiles (CP's) of Al along the three
principal symmetry directions at a photon energy of 59.38 keV, together with
corresponding highly accurate theoretical profiles obtained within the
local-density approximation (LDA) based band-theory framework. A good accord
between theory and experiment is found with respect to the overall shapes of
the CP's, their first and second derivatives, as well as the anisotropies in
the CP's defined as differences between pairs of various CP's. There are
however discrepancies in that, in comparison to the LDA predictions, the
measured profiles are lower at low momenta, show a Fermi cutoff which is
broader, and display a tail which is higher at momenta above the Fermi
momentum. A number of simple model calculations are carried out in order to
gain insight into the nature of the underlying 3D momentum density in Al, and
the role of the Fermi surface in inducing fine structure in the CP's. The
present results when compared with those on Li show clearly that the size of
discrepancies between theoretical and experimental CP's is markedly smaller in
Al than in Li. This indicates that, with increasing electron density, the
conventional picture of the electron gas becomes more representative of the
momentum density and that shortcomings of the LDA framework in describing the
electron correlation effects become less important.Comment: 7 pages, 6 figures, regular articl
Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering
We have carried out first-principles calculations of the Compton scattering
spectra to demonstrate that the filling of the hole Fermi surface in
LaOFFeAs produces a distinct signature in the Fourier transformed
Compton spectrum when the momentum transfer vector lies along the [100]
direction. We thus show how the critical concentration , where hole Fermi
surface pieces are filled up and the superconductivity mediated by
antiferromagnetic spin fluctuations is expected to be suppressed, can be
obtained in a bulk-sensitive manner.Comment: 4 pages, 6 figures, accepted in Physical Review
Role of Oxygen Electrons in the Metal-Insulator Transition in the Magnetoresistive Oxide LaSrMnO Probed by Compton Scattering
We have studied the [100]-[110] anisotropy of the Compton profile in the
bilayer manganite. Quantitative agreement is found between theory and
experiment with respect to the anisotropy in the two metallic phases (i.e. the
low temperature ferromagnetic and the colossal magnetoresistant phase under a
magnetic field of 7 T). Robust signatures of the metal-insulator transition are
identified in the momentum density for the paramagnetic phase above the Curie
temperature. We interpret our results as providing direct evidence for the
transition from the metallic-like to the admixed ionic-covalent bonding
accompanying the magnetic transition. The number of electrons involved in this
phase transition is estimated from the area enclosed by the Compton profile
anisotropy differences. Our study demonstrates the sensitivity of the Compton
scattering technique for identifying the number and type of electrons involved
in the metal-insulator transition.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods
The effect of temperature controlled annealing on the confined valence
electron states in CdSe nanocrystal arrays, deposited as thin films, was
studied using two-dimensional angular correlation of annihilation radiation
(2D-ACAR). A reduction in the intensity by ~35% was observed in a feature of
the positron annihilation spectrum upon removal of the pyridine capping
molecules above 200 degrees Celsius in a vacuum. This reduction is explained by
an increased electronic interaction of the valence orbitals of neighboring
nanocrystals, induced by the formation of inorganic interfaces. Partial
evaporation of the nanoporous CdSe layer and additional sintering into a
polycrystalline thin film was observed at a relatively low temperature of ~486
degrees Celsius.Comment: The article has been accepted by Applied Physics Letters. After it is
published, it will be found at http://apl.aip.or
Bulk Fermi surface and momentum density in heavily doped LaSrCuO using high resolution Compton scattering and positron annihilation spectroscopies
We have observed the bulk Fermi surface (FS) in an overdoped (=0.3) single
crystal of LaSrCuO by using Compton scattering. A
two-dimensional (2D) momentum density reconstruction from measured Compton
profiles yields a clear FS signature in the third Brillouin zone along [100].
The quantitative agreement between density functional theory (DFT) calculations
and momentum density experiment suggests that Fermi-liquid physics is restored
in the overdoped regime. In particular the predicted FS topology is found to be
in good accord with the corresponding experimental data. We find similar
quantitative agreement between the measured 2D angular correlation of positron
annihilation radiation (2D-ACAR) spectra and the DFT based computations.
However, 2D-ACAR does not give such a clear signature of the FS in the extended
momentum space in either the theory or the experiment.Comment: 9 pages, 8 figure
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