643 research outputs found
Theoretical study of the mechanism of dry oxidation of 4H-SiC
Possible defect structures, arising from the interaction of O-2 molecules with an ideal portion of the SiC/SiO2 interface, have been investigated systematically using density functional theory. Based on the calculated total energies and assuming thermal quasiequilibrium during oxidation, the most likely routes leading to complete oxidation have been determined. The defect structures produced along these routes will remain at the interface in significant concentration when stopping the oxidation process. The results obtained for their properties are well supported by experimental findings about the SiC/SiO2 interface. It is found that carbon-carbon bonds can explain most of the observed interface states but not the high density near the conduction band of 4H-SiC
Coherent control of the cooperative branching ratio for nuclear x-ray pumping
Coherent control of nuclear pumping in a three level system driven by x-ray
light is investigated. In single nuclei, the pumping performance is determined
by the branching ratio of the excited state populated by the x-ray pulse. Our
results are based on the observation that in ensembles of nuclei, cooperative
excitation and decay leads to a greatly modified nuclear dynamics, which we
characterize by a time-dependent cooperative branching ratio. We discuss
prospects of steering the x-ray pumping by coherently controlling the
cooperative decay. First, we study an ideal case with purely superradiant decay
and perfect control of the cooperative emission. A numerical analysis of x-ray
pumping in nuclear forward scattering with coherent control of the cooperative
decay via externally applied magnetic fields is presented. Next, we provide an
extended survey of nuclei suitable for our scheme, and propose
proof-of-principle implementations already possible with typical M\"ossbauer
nuclear systems such as . Finally, we discuss the application
of such control techniques to the population or depletion of long-lived nuclear
states.Comment: 11 pages, 8 figures; updated to the published versio
Defects in SiO2 as the possible origin of near interface traps in the SiC∕SiO2 system: A systematic theoretical study
A systematic study of the level positions of intrinsic and carbon defects in SiO2 is presented, based on density functional calculations with a hybrid functional in an alpha-quartz supercell. The results are analyzed from the point of view of the near interface traps (NIT), observed in both SiC/SiO2 and Si/SiO2 systems, and assumed to have their origins in the oxide. It is shown that the vacancies and the oxygen interstitial can be excluded as the origin of such NIT, while the silicon interstitial and carbon dimers give rise to gap levels in the energy range inferred from experiments. The properties of these defects are discussed in light of the knowledge about the SiC/SiO2 interface
Spectrum of Light in a Quantum Fluctuating Periodic Structure
We address the general problem of the excitation spectrum for light coupled
to scatterers having quantum fluctuating positions around the sites of a
periodic lattice. In addition to providing an imaginary part to the spectrum,
we show that these quantum fluctuations affect the real part of the spectrum,
in a way that we determine analytically. Our predictions may be observed with
ultracold atoms in an optical lattice, on a J=0->J'=1 narrow atomic transition.
As a side result, we resolve a controversy for the occurrence of a spectral gap
in a fcc lattice.Comment: 4 pages, 1 figure; introduction, conclusion and title change
Single-Photon Entanglement in the keV Regime via Coherent Control of Nuclear Forward Scattering
Generation of single-photon entanglement is discussed in nuclear forward
scattering. Using successive switchings of the direction of the nuclear
hyperfine magnetic field, the coherent scattering of photons on nuclei is
controlled such that two signal pulses are generated out of one initial pump
pulse. The two time-resolved correlated signal pulses have different
polarizations and energy in the keV regime. Spatial separation of the entangled
field modes and extraction of the signal from the background can be achieved
with the help of state-of-the-art x-ray polarizers and piezoelectric fast
steering mirrors.Comment: minor changes, updated to the final version; 4 pages, 2 figure
Intrinsic electron trapping in amorphous oxide
We demonstrate that electron trapping at intrinsic precursor sites is endemic in non-glass-forming amorphous oxide films. The energy distributions of trapped electron states in ultra-pure prototype amorphous (a)-HfO2 insulator obtained from exhaustive photo-depopulation experiments demonstrate electron states in the energy range of 2–3 eV below the oxide conduction band. These energy distributions are compared to the results of density functional calculations of a-HfO2 models of realistic density. The experimental results can be explained by the presence of intrinsic charge trapping sites formed by under-coordinated Hf cations and elongated Hf–O bonds in a-HfO2. These charge trapping states can capture up to two electrons, forming polarons and bi-polarons. The corresponding trapping sites are different from the dangling-bond type defects responsible for trapping in glass-forming oxides, such as SiO2, in that the traps are formed without bonds being broken. Furthermore, introduction of hydrogen causes formation of somewhat energetically deeper electron traps when a proton is immobilized next to the trapped electron bi-polaron. The proposed novel mechanism of intrinsic charge trapping in a-HfO2 represents a new paradigm for charge trapping in a broad class of non-glass-forming amorphous insulators
QSOs Associated with Messier 82
The starburst / AGN galaxy M82 was studied by Dahlem, Weaver and Heckman
using X-ray data from ROSAT and ASCA, as part of their X-ray survey of edge-on
starburst galaxies. They found seventeen unresolved hard-X-ray sources around
M82, in addition to its strong nuclear source, and other X-rays within the main
body of M82. We have measured optical point sources at these positions, and
have obtained redshifts of six candidates at the Keck I 10-m telescope, using
the low-resolution imaging spectrograph (LRIS). All six are highly compact
optical and X-ray objects with redshifts ranging from 0.111 to 1.086. They all
show emission lines. The three with the highest redshifts are clearly QSOs. The
others with lower redshifts may either be QSOs or compact emission-line
galaxies. In addition to these six there are nine QSOs lying very close to M82
which were discovered many years ago. There is no difference between optical
spectra of these latter QSOs, only two of which are known to be X-ray sources,
and the X-ray emitting QSOs. The redshifts of all fifteen range between 0.111
and 2.05. The large number of QSOs and their apparent association with ejected
matter from M82 suggest that they are physically associated with the galaxy,
and have large intrinsic redshift components. If this is correct, the absolute
magnitudes lie in the range -8 < M_v < -10. Also we speculate that the luminous
variable X-ray source which has been detected by Chandra in the main body of
M82 some 9 arcseconds from the center is another QSO in the process of ejection
from the nucleus, and propose some observational tests of this hypothesis.Comment: 16 pages aastex, 3 eps figures, accepted for publication on Ap
Observations of Stellar Objects at a Shell Boundary in the Star-Forming Complex in the Galaxy IC1613
The single region of ongoing star formation in the galaxy IC 1613 has been
observed in order to reveal the nature of compact emission-line objects at the
edges of two shells in the complex, identified earlier in H-alpha line images.
The continuum images show these compact objects to be stars. Detailed
spectroscopic observations of these stars and the surrounding nebulae were
carried out with an integral field spectrograph MPFS mounted on the 6m
telescope of the Special Astrophysical Observatory. The resulting stellar
spectra were used to determine the spectral types and luminosity classes of the
objects. An Of star we identified is the only object of this spectral type in
IC 1613. The results of optical observations of the multi-shell complex are
compared to 21cm radio observations. The shells harboring the stars at their
boundaries constitute the most active part of the star-forming region. There is
evidence that shocks have played an important role in the formation of the
shells.Comment: 10 pages, 5 PS and 1 color JPEG figur
Detection of an intergalactic meteor particle with the 6-m telescope
On July 28, 2006 the 6-m telescope of the Special Astrophysical Observatory
of the Russian Academy of Sciences recorded the spectrum of a faint meteor. We
confidently identify the lines of FeI and MgI, OI, NI and molecular-nitrogen
N_2 bands. The entry velocity of the meteor body into the Earth's atmosphere
estimated from radial velocity is equal to 300 km/s. The body was several tens
of a millimeter in size, like chondrules in carbon chondrites. The radiant of
the meteor trajectory coincides with the sky position of the apex of the motion
of the Solar system toward the centroid of the Local Group of galaxies.
Observations of faint sporadic meteors with FAVOR TV CCD camera confirmed the
radiant at a higher than 96% confidence level. We conclude that this meteor
particle is likely to be of extragalactic origin. The following important
questions remain open: (1) How metal-rich dust particles came to be in the
extragalactic space? (2) Why are the sizes of extragalactic particles larger by
two orders of magnitude (and their masses greater by six orders of magnitude)
than common interstellar dust grains in our Galaxy? (3) If extragalactic dust
surrounds galaxies in the form of dust (or gas-and-dust) aureoles, can such
formations now be observed using other observational techniques (IR
observations aboard Spitzer satellite, etc.)? (4) If inhomogeneous
extragalactic dust medium with the parameters mentioned above actually exists,
does it show up in the form of irregularities on the cosmic microwave
background (WMAP etc.)?Comment: 9 pages, 6 EPS figure
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