34 research outputs found
Spin moment over 10-300 K and delocalization of magnetic electrons above the Verwey transition in magnetite
In order to probe the magnetic ground state, we have carried out temperature
dependent magnetic Compton scattering experiments on an oriented single crystal
of magnetite (FeO), together with the corresponding first-principles
band theory computations to gain insight into the measurements. An accurate
value of the magnetic moment associated with unpaired spins is obtained
directly over the temperature range of 10-300K. is found to be
non-integral and to display an anomalous behavior with the direction of the
external magnetic field near the Verwey transition. These results reveal how
the magnetic properties enter the Verwey energy scale via spin-orbit coupling
and the geometrical frustration of the spinel structure, even though the Curie
temperature of magnetite is in excess of 800 K. The anisotropy of the magnetic
Compton profiles increases through the Verwey temperature and indicates
that magnetic electrons in the ground state of magnetite become delocalized on
Fe B-sites above .Comment: 5 pages, 5 figures, to appear in Journal of Physics and Chemistry of
Solid
A Novel 2D Folding Technique for Enhancing Fermi Surface Signatures in the Momentum Density: Application to Compton Scattering Data from an Al-3at%Li Disordered Alloy
We present a novel technique for enhancing Fermi surface (FS) signatures in
the 2D distribution obtained after the 3D momentum density in a crystal is
projected along a specific direction in momentum space. These results are
useful for investigating fermiology via high resolution Compton scattering and
positron annihilation spectroscopies. We focus on the particular case of the
(110) projection in an fcc crystal where the standard approach based on the use
of the Lock-Crisp-West (LCW) folding theorem fails to give a clear FS image due
to the strong overlap with FS images obtained through projection from higher
Brillouin zones. We show how these superposed FS images can be disentangled by
using a selected set of reciprocal lattice vectors in the folding process. The
applicability of our partial folding scheme is illustrated by considering
Compton spectra from an Al-3at%Li disordered alloy single crystal. For this
purpose, high resolution Compton profiles along nine directions in the (110)
plane were measured. Corresponding highly accurate theoretical profiles in
Al-3at%Li were computed within the local density approximation (LDA)-based
Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA)
first-principles framework. A good level of overall accord between theory and
experiment is obtained, some expected discrepancies reflecting electron
correlation effects notwithstanding, and the partial folding scheme is shown to
yield a clear FS image in the (110) plane in Al-3%Li.Comment: 24 pages, 8 figures, to appear in Phys. Rev.
A Fermi Surface study of BaKBiO
We present all electron computations of the 3D Fermi surfaces (FS's) in
BaKBiO for a number of different compositions based on the
selfconsistent Korringa-Kohn-Rostoker coherent-potential-approximation
(KKR-CPA) approach for incorporating the effects of Ba/K substitution. By
assuming a simple cubic structure throughout the composition range, the
evolution of the nesting and other features of the FS of the underlying
pristine phase is correlated with the onset of various structural transitions
with K doping. A parameterized scheme for obtaining an accurate 3D map of the
FS in BaKBiO for an arbitrary doping level is developed. We
remark on the puzzling differences between the phase diagrams of
BaKBiO and BaPbBiO by comparing aspects
of their electronic structures and those of the end compounds BaBiO,
KBiO and BaPbO. Our theoretically predicted FS's in the cubic phase are
relevant for analyzing high-resolution Compton scattering and
positron-annihilation experiments sensitive to the electron momentum density,
and are thus amenable to substantial experimental verification.Comment: 12 pages, 7 figures, to appear in Phys. Rev.
Positron annihilation in metals: A contribution to the analysis of angular correlation measurements, with an application to copper
Applied Science
Microcavities in Semiconductor Materials
Positron beam and helium desorption techniques have been applied to different materials, in particular semiconductor materials, to determine the presence of defects. The positron technique yields values of the positron diffusion length and values of the Doppler broadening parameters. In principle, defect concentrations can be derived and an indication can be obtained about the nature of the defect. Results are presented which show that cavities can be easily detected. It is also demonstrated that gas accumulated in the cavities reduces the observed differences between the defected and the defect-free material. Large cavities were detected in solar cell hydrogenated amorphous silicon and low temperature deposited amorphous silicon. It was found that layers deposited under irradiation with low energy ions (ion assisted deposition) did not show evidence of microcavities. Desorption techniques were successfully employed to detect cavities in silicon