4,074 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..
Kurtosis in Large-Scale Structure as a Constraint on Non-Gaussian Initial Conditions
We calculate the kurtosis of a large-scale density field which has undergone
weakly non-linear gravitational evolution from arbitrary non-Gaussian initial
conditions. It is well known that the weakly evolved {\twelveit skewness} is
equal to its initial value plus the term induced by gravity, which scales with
the rms density fluctuation in precisely the same way as for Gaussian initial
conditions. As in the case of skewness, the evolved {\twelveit kurtosis} is
equal to its initial value plus the contribution induced by gravity. The
scaling of this induced contribution, however, turns out to be qualitatively
different for Gaussian versus non-Gaussian initial conditions. Therefore,
measurements of the kurtosis can serve as a powerful discriminating test
between the hypotheses of Gaussian and non-Gaussian nature of primordial
density fluctuations.Comment: uuencoded compressed tar file including postscript text (17 pages)
and 2 postscript figures, submitted to MNRA
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
The FIR/submm window on galaxy formation
Our view on the deep universe has been so far biased towards optically bright
galaxies. Now, the measurement of the Cosmic Infrared Background in FIRAS and
DIRBE residuals, and the observations of FIR/submm sources by the ISOPHOT and
SCUBA instruments begin unveiling the ``optically dark side'' of galaxy
formation. Though the origin of dust heating is still unsolved, it appears very
likely that a large fraction of the FIR/submm emission is due to
heavily-extinguished star formation. Consequently, the level of the CIRB
implies that about 2/3 of galaxy/star formation in the universe is hidden by
dust shrouds. In this review, we introduce a new modeling of galaxy formation
and evolution that provides us with specific predictions in FIR/submm
wavebands. These predictions are compared with the current status of the
observations. Finally, the capabilities of current and forthcoming instruments
for all-sky and deep surveys of FIR/submm sources are briefly described.Comment: 10 pages, Latex, 5 postscript figures, to appear in ``The Birth of
Galaxies'', 1999, B. Guiderdoni, F.R. Bouchet, T.X. Thuan & J. Tran Thanh Van
(eds), Editions Frontiere
Simulations of the Microwave Sky and of its ``Observations''
Here follows a preliminary report on the construction of fake millimeter and
sub-millimeter skies, as observed by virtual instruments, e.g. the COBRA/SAMBA
mission, using theoretical modeling and data extrapolations. Our goal is to
create maps as realistic as possible of the relevant physical contributions
which may contribute to the detected signals. This astrophysical modeling is
followed by simulations of the measurement process itself by a given
instrumental configuration. This will enable a precise determination of what
can and cannot be achieved with a particular experimental configuration, and
provide a feedback on how to improve the overall design. It is a key step on
the way to define procedures for the separation of the different physical
processes in the future observed maps. Note that this tool will also prove
useful in preparing and analyzing current (\eg\ balloon borne) Microwave
Background experiments. Keywords: Cosmology -- Microwave Background
Anisotropies.Comment: 6 pages of uuencoded compressed postscript (1.2 Mb uncompressed), to
appear in the proceedings of the meeting "Far Infrared and Sub-millimeter
Space Missions in the Next Decade'', Paris, France, Eds. M. Sauvage, Space
Science Revie
Stability criteria of the Vlasov equation and quasi-stationary states of the HMF model
We perform a detailed study of the relaxation towards equilibrium in the
Hamiltonian Mean-Field (HMF) model, a prototype for long-range interactions in
-particle dynamics. In particular, we point out the role played by the
infinity of stationary states of the associated Vlasov dynamics. In this
context, we derive a new general criterion for the stability of any spatially
homogeneous distribution, and compare its analytical predictions with numerical
simulations of the Hamiltonian, finite , dynamics. We then propose and
verify numerically a scenario for the relaxation process, relying on the Vlasov
equation. When starting from a non stationary or a Vlasov unstable stationary
initial state, the system shows initially a rapid convergence towards a stable
stationary state of the Vlasov equation via non stationary states: we
characterize numerically this dynamical instability in the finite system by
introducing appropriate indicators. This first step of the evolution towards
Boltzmann-Gibbs equilibrium is followed by a slow quasi-stationary process,
that proceeds through different stable stationary states of the Vlasov
equation. If the finite system is initialized in a Vlasov stable homogenous
state, it remains trapped in a quasi-stationary state for times that increase
with the nontrivial power law . Single particle momentum distributions
in such a quasi-stationary regime do not have power-law tails, and hence cannot
be fitted by the -exponential distributions derived from Tsallis statistics.Comment: To appear in Physica
Large parallel cosmic string simulations: New results on loop production
Using a new parallel computing technique, we have run the largest cosmic
string simulations ever performed. Our results confirm the existence of a long
transient period where a non-scaling distribution of small loops is produced at
lengths depending on the initial correlation scale. As time passes, this
initial population gives way to the true scaling regime, where loops of size
approximately equal to one-twentieth the horizon distance become a significant
component. We observe similar behavior in matter and radiation eras, as well as
in flat space. In the matter era, the scaling population of large loops becomes
the dominant component; we expect this to eventually happen in the other eras
as well.Comment: 23 pages, 10 figures, 2 tables. V2: combine 3 figures, add 1 table,
better discussion + citation of prev. wor
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
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