84 research outputs found
The Electronic and Superconducting Properties of Oxygen-Ordered MgB2 compounds of the form Mg2B3Ox
Possible candidates for the Mg2B3Ox nanostructures observed in bulk of
polycrystalline MgB2 (Ref.1) have been studied using a combination of
Z-contrast imaging, electron energy loss spectroscopy (EELS) and
first-principles calculations. The electronic structures, phonon modes, and
electron phonon coupling parameters are calculated for two oxygen-ordered MgB2
compounds of composition Mg2B3O and Mg2B3O2, and compared with those of MgB2.
We find that the density of states for both Mg2B3Ox structures show very good
agreement with EELS, indicating that they are excellent candidates to explain
the observed coherent oxygen precipitates. Incorporation of oxygen reduces the
transition temperature and gives calculated TC values of 18.3 K and 1.6 K for
Mg2B3O and Mg2B3O2, respectively.Comment: Submitted to PR
Polarization anisotropy in the optical properties of silicon ellipsoids
A new real space quantum mechanical approach with local field effects
included is applied to the calculation of the optical properties of silicon
nanocrystals. Silicon ellipsoids are studied and the role of surface
polarization is discussed in details. In particular, surface polarization is
shown to be responsible for a strong optical anisotropy in silicon ellipsoids,
much more pronounced with respect to the case in which only quantum confinement
effects are considered. The static dielectric constant and the absorption
spectra are calculated, showing that the perpendicular and parallel components
have a very different dependence on the ellipsoid aspect ratio. Then, a
comparison with the classical dielectric model is performed, showing that the
model only works for large and regular structures, but it fails for thin
elongated ellipsoids.Comment: 5 pages, 4 figures, International Conference on NANO-Structures
Self-Assemblin
Observation of coherent oxide precipitates in polycrystalline MgB2
Here we describe the results of an atomic resolution study of oxygen
incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are
formed by ordered substitution of oxygen atoms onto boron lattice sites, while
the basic bulk MgB2 crystal structure and orientation is preserved. The
periodicity of the oxygen ordering is dictated by the oxygen concentration in
the precipitates and primarily occurs in the (010) plane. The presence of these
precipitates correlates well with an improved critical current density and
superconducting transition behavior, implying that they act as pinning centers.Comment: Submitted to Applied Physics Letters, 6 pages, 3 figure
Direct observation of nm-scale Mg- and B-oxide phases at grain boundaries in MgB2
Here we describe the results of an atomic resolution study of the structure
and composition of both the interior of the grains, and the grain boundaries in
polycrystalline MgB2. We find that there is no oxygen within the bulk of the
grains but significant oxygen enrichment at the grain boundaries. The majority
of grain boundaries contain BOx phases smaller than the coherence length, while
others contain larger areas of MgO sandwiched between BOx layers. Such results
naturally explain the differences in connectivity between the grains observed
by other techniques
Measuring the Hole State Anisotropy in MgB2 by Electron Energy-Loss Spectroscopy
We have examined polycrystalline MgB2 by electron energy loss spectroscopy
(EELS) and density of state calculations. In particular, we have studied two
different crystal orientations, [110] and [001] with respect to the incident
electron beam direction, and found significant changes in the near-edge
fine-structure of the B K-edge. Density functional theory suggests that the
pre-peak of the B K-edge core loss is composed of a mixture of pxy and pz hole
states and we will show that these contributions can be distinguished only with
an experimental energy resolution better than 0.5 eV. For conventional TEM/STEM
instruments with an energy resolution of ~1.0 eV the pre-peak still contains
valuable information about the local charge carrier concentration that can be
probed by core-loss EELS. By considering the scattering momentum transfer for
different crystal orientations, it is possible to analytically separate pxy and
pz components from of the experimental spectra With careful experiments and
analysis, EELS can be a unique tool measuring the superconducting properties of
MgB2, doped with various elements for improved transport properties on a
sub-nanometer scale.Comment: 26 Pages, 5 figures, 1 table. Submited to PR
Atomic-resolution visualization and doping effects of complex structures in intercalated bilayer graphene
Molecules intercalating two-dimensional materials form complex structures that have been characterized primarily by spatially averaged techniques. Here we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with FeCl3. In BLG, we discover two distinct intercalated structures that we identify as monolayer FeCl3 and monolayer FeCl2. The two structures are separated by atomically sharp boundaries and induce large free-carrier densities on the order of 1013cm−2 in the graphene layers. In FLG, we observe multiple FeCl3 layers stacked in a variety of possible configurations with respect to one another. Finally, we find that the microscope's electron beam can convert the FeCl3 monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties
Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4
An extensive theoretical study is performed for wide bandgap crystalline
oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and
Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}:
-quartz, - and -cristobalite and stishovite, for
GeO_{2}: -quartz, and rutile, for Al_{2}O_{3}: -phase, for
Si_{3}N_{4} and Ge_{3}N_{4}: - and -phases. This work
constitutes a comprehensive account of both electronic structure and the
elastic properties of these important insulating oxides and nitrides obtained
with high accuracy based on density functional theory within the local density
approximation. Two different norm-conserving \textit{ab initio}
pseudopotentials have been tested which agree in all respects with the only
exception arising for the elastic properties of rutile GeO_{2}. The agreement
with experimental values, when available, are seen to be highly satisfactory.
The uniformity and the well convergence of this approach enables an unbiased
assessment of important physical parameters within each material and among
different insulating oxide and nitrides. The computed static electric
susceptibilities are observed to display a strong correlation with their mass
densities. There is a marked discrepancy between the considered oxides and
nitrides with the latter having sudden increase of density of states away from
the respective band edges. This is expected to give rise to excessive carrier
scattering which can practically preclude bulk impact ionization process in
Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure
Flies (Calliphoridae, Muscidae) and Beetles (Silphidae) from Human Cadavers in Cali, Colombia
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