3,791 research outputs found
Ab-initio Gutzwiller method: first application to Plutonium
Except for small molecules, it is impossible to solve many electrons systems
without imposing severe approximations. If the configuration interaction
approaches (CI) or Coupled Clusters techniques \cite{FuldeBook} are applicable
for molecules, their generalization for solids is difficult. For materials with
a kinetic energy greater than the Coulomb interaction, calculations based on
the density functional theory (DFT), associated with the local density
approximation (LDA) \cite{Hohenberg64, Kohn65} give satisfying qualitative and
quantitative results to describe ground state properties. These solids have
weakly correlated electrons presenting extended states, like materials or
covalent solids. The application of this approximation to systems where the
wave functions are more localized ( or -states) as transition metals
oxides, heavy fermions, rare earths or actinides is more questionable and can
even lead to unphysical results : for example, insulating FeO and CoO are
predicted to be metalic by the DFT-LDA..
On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL
The 511 keV positron annihilation emission remains a mysterious component of
the high energy emission of our Galaxy. Its study was one of the key scientific
objective of the SPI spectrometer on-board the INTEGRAL satellite. In fact, a
lot of observing time has been dedicated to the Galactic disk with a particular
emphasis on the central region. A crucial issue in such an analysis concerns
the reduction technique used to treat this huge quantity of data, and more
particularly the background modeling. Our method, after validation through a
variety of tests, is based on detector pattern determination per ~6 month
periods, together with a normalisation variable on a few hour timescale. The
Galactic bulge is detected at a level of ~70 sigma allowing more detailed
investigations. The main result is that the bulge morphology can be modelled
with two axisymmetric Gaussians of 3.2 deg. and 11.8 deg. FWHM and respective
fluxes of 2.5 and 5.4 x 10^-4 photons/(cm^2.s^1). We found a possible shift of
the bulge centre towards negative longitude at l=-0.6 +/- 0.2 degrees. In
addition to the bulge, a more extended structure is detected significantly with
flux ranging from 1.7 to 2.9 x10^-3 photons/(cm^2.s^1) depending on its assumed
geometry (pure disk or disk plus halo). The disk emission is also found to be
symmetric within the limits of the statistical errors.Comment: This paper has 12 pages and 14 figures. Accepted for publication by
the Astrophysical Journa
Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: Grain size and doping effects
The hot compaction of nanocrystalline TiO2 anatase powders is investigated using dilatometry. The constant rate of heating (CRH) method is applied to determine effective activation energies of the processes involved during sintering. Grain size and doping effects are studied, using dopant cations of different radius and charge: Zn2+, Al3+, Si4+, Nb5+. The results are interpreted by a mechanism including superplastic deformation and boundary diffusion. The former is predominant for small particles and low temperature, whereas the latter is more important for larger particles and higher temperature. Dopant effects on densification kinetics are discussed in view of defect chemistry
Separation of foregrounds from cosmic microwave background observations with the MAP satellite
Simulated observations of a 10\dg \times 10\dg field by the Microwave
Anisotropy Probe (MAP) are analysed in order to separate cosmic microwave
background (CMB) emission from foreground contaminants and instrumental noise
and thereby determine how accurately the CMB emission can be recovered. The
simulations include emission from the CMB, the kinetic and thermal
Sunyaev-Zel'dovich (SZ) effects from galaxy clusters, as well as Galactic dust,
free-free and synchrotron. We find that, even in the presence of these
contaminating foregrounds, the CMB map is reconstructed with an rms accuracy of
about 20 K per 12.6 arcmin pixel, which represents a substantial
improvement as compared to the individual temperature sensitivities of the raw
data channels. We also find, for the single 10\dg \times 10\dg field, that
the CMB power spectrum is accurately recovered for \ell \la 600.Comment: 7 pages, 7 figures, MNRAS submitte
Phase space gaps and ergodicity breaking in systems with long range interactions
We study a generalized isotropic XY-model which includes both two-spin and
four-spin mean-field interactions. This model can be solved in the
microcanonical ensemble. It is shown that in certain parameter regions the
model exhibits gaps in the magnetization at fixed energy, resulting in
ergodicity breaking. This phenomenon has previously been reported in
anisotropic and discrete spin models. The entropy of the model is calculated
and the microcanonical phase diagram is derived, showing the existence of first
order phase transitions from the ferromagnetic to a paramagnetic disordered
phase. It is found that ergodicity breaking takes place both in the
ferromagnetic and the paramagnetic phases. As a consequence, the system can
exhibit a stable ferromagnetic phase within the paramagnetic region, and
conversely a disordered phase within the magnetically ordered region
A review of the deep-water volute genus <i>Calliotectum</i> (Gastropoda: Volutidae)
Calliotectum Dall, 1890, until now a monotypic deep-water volute genus from the Eastern Pacific, is shown to be a senior synonym of Teramachia Kuroda, 1931 from the Western Pacific. Pakaurangia Finlay, 1926 (originally Thiaridae; Miocene of New Zealand) and Butonius Martin. 1933 (originally Fusinidae; Neogene of Indonesia) are new synonyms. Calliotectum has a fossil record in the Neogene of the Pacific region (Okinawa, Indonesia, New Zealand and Ecuador), with a total of 5 species. All fossil records are from deep-water facies. Seven Recent species of Calliotectum are recognised, all from deep water in tropical latitudes. Three species occur in South-East Asia and the Eastern Indian Ocean, at 200-1660 m depth. Of these, C. tibiaeforme is treated as a polytypic species, with C. johnsoni and C. dupreyae considered to be geographical forms. Calliotectum piersonorum sp. nov. and C. egregium sp. nov. are described from the South-West Pacific at 450-1060 m depth. Single species occur each in the East Pacific and in the Caribbean
Ground-based follow up of IRAS galaxies
Optical, near infrared, radio continuum and HI observations were undertaken of the galaxies identified with IRAS sources in a few fields roughly of the size of a sky survey plate. Results are presented from two fields at galactic latitude +27 and +43 deg over a total area of 100 sq. deg. These regions contained 115 IRAS point sources, out of which 26 were identified with stars and 81 with faint galaxies, 10 of which were difficult to recognize on the Schmidt plates. Spectroscopy was obtained with the ESO telescopes at a resolution of about 10 A. The vast majority of galaxies have low excitation spectra dominated by low ionization lines. The spectra are typical of HII region type galaxies, however of much lower excitation that other starbursts galaxies. The importance of the reddening as determined from the H alpha/H beta ratio is stressed: the visual absorption A sub v ranges from 2 to 6 magnitudes and as a consequence the corrected L sub IR/L sub B ratios are considerably reduced if those reddenings apply to the whole galaxy
Ensemble inequivalence, bicritical points and azeotropy for generalized Fofonoff flows
We present a theoretical description for the equilibrium states of a large
class of models of two-dimensional and geophysical flows, in arbitrary domains.
We account for the existence of ensemble inequivalence and negative specific
heat in those models, for the first time using explicit computations. We give
exact theoretical computation of a criteria to determine phase transition
location and type. Strikingly, this criteria does not depend on the model, but
only on the domain geometry. We report the first example of bicritical points
and second order azeotropy in the context of systems with long range
interactions.Comment: 4 pages, submitted to Phys. Rev. Let
On the Minimum Degree up to Local Complementation: Bounds and Complexity
The local minimum degree of a graph is the minimum degree reached by means of
a series of local complementations. In this paper, we investigate on this
quantity which plays an important role in quantum computation and quantum error
correcting codes. First, we show that the local minimum degree of the Paley
graph of order p is greater than sqrt{p} - 3/2, which is, up to our knowledge,
the highest known bound on an explicit family of graphs. Probabilistic methods
allows us to derive the existence of an infinite number of graphs whose local
minimum degree is linear in their order with constant 0.189 for graphs in
general and 0.110 for bipartite graphs. As regards the computational complexity
of the decision problem associated with the local minimum degree, we show that
it is NP-complete and that there exists no k-approximation algorithm for this
problem for any constant k unless P = NP.Comment: 11 page
All sky CMB map from cosmic strings integrated Sachs-Wolfe effect
By actively distorting the Cosmic Microwave Background (CMB) over our past
light cone, cosmic strings are unavoidable sources of non-Gaussianity.
Developing optimal estimators able to disambiguate a string signal from the
primordial type of non-Gaussianity requires calibration over synthetic full sky
CMB maps, which till now had been numerically unachievable at the resolution of
modern experiments. In this paper, we provide the first high resolution full
sky CMB map of the temperature anisotropies induced by a network of cosmic
strings since the recombination. The map has about 200 million sub-arcminute
pixels in the healpix format which is the standard in use for CMB analyses
(Nside=4096). This premiere required about 800,000 cpu hours; it has been
generated by using a massively parallel ray tracing method piercing through a
thousands of state of art Nambu-Goto cosmic string numerical simulations which
pave the comoving volume between the observer and the last scattering surface.
We explicitly show how this map corrects previous results derived in the flat
sky approximation, while remaining completely compatible at the smallest
scales.Comment: 8 pages, 4 figures, uses RevTeX. References added, matches published
versio
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