745 research outputs found
Influence of interface structure on electronic properties and Schottky barriers in Fe/GaAs magnetic junctions
The electronic and magnetic properties of Fe/GaAs(001) magnetic junctions are
investigated using first-principles density-functional calculations. Abrupt and
intermixed interfaces are considered, and the dependence of charge transfer,
magnetization profiles, Schottky barrier heights, and spin polarization of
densities of states on interface structure is studied. With As-termination, an
abrupt interface with Fe is favored, while Ga-terminated GaAs favors the
formation of an intermixed layer with Fe. The Schottky barrier heights are
particularly sensitive to the abruptness of the interface. A significant
density of states in the semiconducting gap arises from metal interface states.
These spin-dependent interface states lead to a significant minority spin
polarization of the density of states at the Fermi level that persists well
into the semiconductor, providing a channel for the tunneling of minority spins
through the Schottky barrier. These interface-induced gap states and their
dependence on atomic structure at the interface are discussed in connection
with potential spin-injection applications.Comment: 9 pages, 9 figures, to appear in PR
Innovations of the Beginning of the Sixth Millennium BC in the Northern Pontic Steppe
This study focuses on the pottery-bearing ("Neolithic") sites of the northern Azov Sea region. The vessels ornamented with comb imprints appeared there in the sixth millennium BC. In the light of a recent re-dating of the Rakushechny Yar site sequence, the sites of the northern Azov region appeared to be the earliest evidence for this innovation. The innovation in the ceramic assemblage is accompanied by an innovative lithic tool set. The latter included macro-blades and fan-shaped end-scrapers, which were previously unknown in the studied region. Their reanalysis (including new field work at the single-layer site of Chapaevka) helped formulate a hypothesis of maritime transmission of comb-ornamented ceramics in the Black and Azov Sea. This hypothesis will stimulate further discussions regarding the ways of Neolithization in Eastern Europe. It underlines the connections between Balkan "classic"Neolithic and pottery-bearing sites of the Ukrainian Steppe. The impressed ware from Makri and other mainland Greek sites is treated as the closest analogy to the finds of the northern Azov Sea region
Optical transitions and nature of Stokes shift in spherical CdS quantum dots
We study the structure of the energy spectra along with the character of the
states participating in optical transitions in colloidal CdS quantum dots (QDs)
using the {\sl ab initio} accuracy charge patching method combined with the
%pseudopotential based folded spectrum calculations of electronic structure of
thousand-atom nanostructures. In particular, attention is paid to the nature of
the large resonant Stokes shift observed in CdS quantum dots. We find that the
top of the valence band state is bright, in contrast with the results of
numerous {\bf kp} calculations, and determine the limits of
applicability of the {\bf kp} approach. The calculated electron-hole
exchange splitting suggests the spin-forbidden valence state may explain the
nature of the ``dark exciton'' in CdS quantum dots.Comment: 5 pages, 4 figure
The Birth of a Galaxy - III. Propelling reionisation with the faintest galaxies
Starlight from galaxies plays a pivotal role throughout the process of cosmic
reionisation. We present the statistics of dwarf galaxy properties at z > 7 in
haloes with masses up to 10^9 solar masses, using a cosmological radiation
hydrodynamics simulation that follows their buildup starting with their
Population III progenitors. We find that metal-enriched star formation is not
restricted to atomic cooling ( K) haloes, but can occur
in haloes down to masses ~10^6 solar masses, especially in neutral regions.
Even though these smallest galaxies only host up to 10^4 solar masses of stars,
they provide nearly 30 per cent of the ionising photon budget. We find that the
galaxy luminosity function flattens above M_UV ~ -12 with a number density that
is unchanged at z < 10. The fraction of ionising radiation escaping into the
intergalactic medium is inversely dependent on halo mass, decreasing from 50 to
5 per cent in the mass range . Using our galaxy
statistics in a semi-analytic reionisation model, we find a Thomson scattering
optical depth consistent with the latest Planck results, while still being
consistent with the UV emissivity constraints provided by Ly forest
observations at z = 4-6.Comment: 21 pages, 15 figures, 4 tables. Accepted in MNRA
Verifying the chronology of Ukrainian Neolithic
This paper addresses the earliest Neolithic phases in Ukraine: aspects relating to the chronology, methods and time of Neolithisation. In general, the earliest Neolithisation relates to the times of the Linear Pottery culture. Nevertheless, Ukraine has numerous eco-zones so the process could have been different in other zones. The authors emphasize that the key to getting closer to solving this problem is the chronology-re-dating and validation program. Hitherto dates of one culture or period of culture have been treated against each other. This article presents a small series of AMS from well-defined contexts of four sites, previously dated by the Kyiv radiocarbon facility. The results are compared with existing dates in order to establish the validity of existing chronologies for the Neolithic of Ukraine. Obtained results allowed the authors to conclude that the Kyiv dates should not be obligatorily treated as wrong. Furthermore, the authors reopened the discussion to establish what was the role of LBK and Azov-Dnieper culture communities in spreading farming and herding in Ukraine during the second half of the VIth millennium BC
Green luminescence in Mg-doped GaN
A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also grown in very Ga-rich conditions. The unique properties of the GL2 band allowed us to reliably identify it in different samples. The best candidate for the defect which causes the GL2 band is a nitrogen vacancy (VN). We propose that transitions of electrons from the conduction band to the +/2+ transition level of the VN defect are responsible for the GL2 band in high-resistivity undoped and Mg-doped GaN
Green luminescence in Mg-doped GaN
A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also grown in very Ga-rich conditions. The unique properties of the GL2 band allowed us to reliably identify it in different samples. The best candidate for the defect which causes the GL2 band is a nitrogen vacancy (VN). We propose that transitions of electrons from the conduction band to the +/2+ transition level of the VN defect are responsible for the GL2 band in high-resistivity undoped and Mg-doped GaN
- …