45,754 research outputs found
The mass of the dark matter particle from theory and observations
We combine observed properties of galaxies as the core density and radius
with the theoretical linear evolution of density fluctuations computed from
first principles since the end of inflation till today. The halo radius r_0 is
computed in terms of cosmological parameters. The theoretical density profiles
rho(r)/rho(0) have an universal shape as a function of r/r_0 which reproduces
the observations. We show that the linear approximation to the Boltzmann-Vlasov
equation is valid for very large galaxies and correctly provides universal
quantities which are common to all galaxies, as the surface density and density
profile. By matching the theoretically computed surface density to its observed
value we obtain (i) the decreasing of the phase-space density during the MD era
(ii) the mass of the dark matter particle which turns to be between 1 and 2 keV
and the decoupling temperature T_d which turns to be above 100 GeV (iii) the
core vs. cusp discrimination: keV dark matter particles produce cored density
profiles while wimps (m \sim 100 GeV, T_d \sim 5 GeV) produce cusped profiles
at scales about 0.003 pc. These results are independent of the particle model
and vary very little with the statistics of the dark matter particle.
Non-universal galaxy quantities (which need to include non-linear effects as
mergers and baryons) are reproduced in the linear approximation up to a factor
of order one for the halo radius r_0, galaxy mass M_{gal}, halo central density
rho_{0} and velocity dispersion sqrt{{\bar {v^2}}_{halo}} in the limiting case
of large galaxies (both r_0 and M_{gal} large). This shows the power of the
linear approximation scheme: although it cannot capture the whole content of
the structure formation, it correctly provides universal quantities which as
well as the main non-universal galaxy properties.Comment: 17 pages, 15 figures, improved and expanded version to appear in New
Astronom
Unpolarized states and hidden polarization
We capitalize on a multipolar expansion of the polarisation density matrix,
in which multipoles appear as successive moments of the Stokes variables. When
all the multipoles up to a given order vanish, we can properly say that the
state is th-order unpolarized, as it lacks of polarization information to
that order. First-order unpolarized states coincide with the corresponding
classical ones, whereas unpolarized to any order tally with the quantum notion
of fully invariant states. In between these two extreme cases, there is a rich
variety of situations that are explored here. The existence of \textit{hidden}
polarisation emerges in a natural way in this context.Comment: 7 pages, 3 eps-color figures. Submitted to PRA. Comments welcome
Assessment of the U and Co magnetic moments in UCoGe by X-ray magnetic circular dichroism
The ferromagnetic superconductor UCoGe has been investigated by high field
X-ray magnetic circular dichroism (XMCD) at the U-M and Co/Ge-K edges.
The analysis of the branching ratio and XMCD at the U-M edges reveals
that the U-5 electrons count is close to 3. The orbital ()
and spin () moments of U at 2.1K and 17T (H//c) have been
determined. Their ratio () suggests a significant delocalization of
the 5 electron states. The similar field dependences of the local U/Co and
the macroscopic magnetization indicate that the Co moment is induced by the U
moment. The XMCD at the Co/Ge-K edges reveal the presence of small Co-4 and
Ge-4 orbital moments parallel to the macroscopic magnetization. In addition,
the Co-3 moment is estimated to be at most of the order of 0.1 at
17T. Our results rule out the possibility of an unusual polarisability of the U
and Co moments as well as their antiparallel coupling. We conclude that the
magnetism which mediates the superconductivity in UCoGe is driven by U.Comment: 4 figures + supplementary materia
Extremal states for photon number and quadratures as gauges for nonclassicality
Rotated quadratures carry the phase-dependent information of the
electromagnetic field, so they are somehow conjugate to the photon number. We
analyze this noncanonical pair, finding an exact uncertatinty relation, as well
as a couple of weaker inequalities obtained by relaxing some restrictions of
the problem. We also find the intelligent states saturating that relation and
complete their characterization by considering extra constraints on the
second-order moments of the variables involved. Using these moments, we
construct performance measures tailored to diagnose photon-added and
Schr\"odinger catlike states, among others.Comment: 6 pages, 4 color figures. Comments welcome
An X-ray study of the SNR G344.7-0.1 and the central object CXOU J170357.8-414302
Aims. We report results of an X-ray study of the supernova remnant (SNR)
G344.7-0.1 and the point-like X-ray source located at the geometrical center of
the SNR radio structure. Methods. The morphology and spectral properties of the
remnant and the central X-ray point-like source were studied using data from
the XMM-Newton and Chandra satellites. Archival radio data and infrared Spitzer
observations at 8 and 24 m were used to compare and study its multi-band
properties at different wavelengths. Results. The XMM-Newton and Chandra
observations reveal that the overall X-ray emission of G344.7-0.1 is extended
and correlates very well with regions of bright radio and infrared emission.
The X-ray spectrum is dominated by prominent atomic emission lines. These
characteristics suggest that the X-ray emission originated in a thin thermal
plasma, whose radiation is represented well by a plane-parallel shock plasma
model (PSHOCK). Our study favors the scenario in which G344.7-0.1 is a 6 x 10^3
year old SNR expanding in a medium with a high density gradient and is most
likely encountering a molecular cloud on the western side. In addition, we
report the discovery of a soft point-like X-ray source located at the
geometrical center of the radio SNR structure. The object presents some
characteristics of the so-called compact central objects (CCO). However, its
neutral hydrogen absorption column (N_{H}) is inconsistent with that of the
SNR. Coincident with the position of the source, we found infrared and optical
objects with typical early-K star characteristics. The X-ray source may be a
foreground star or the CCO associated with the SNR. If this latter possibility
were confirmed, the point-like source would be the farthest CCO detected so far
and the eighth member of the new population of isolated and weakly magnetized
neutron stars.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics. Higher resolution figures can be seen on A&
Coulomb displacement energies, energy differenced and neutron skins
A Fock space representation of the monopole part of the Coulomb potential is
presented. Quantum effects show through a small orbital term in . Once
it is averaged out, the classical electrostatic energy emerges as an
essentially exact expression, which makes it possible to eliminate the
Nolen-Schiffer anomaly, and to estimate neutron skins and the evolution of
radii along yrast states of mirror nuclei. The energy differences of the latter
are quantitatively reproduced by the monopole term and a schematic multipole
one.Comment: 4 pages, 3 figures, Revte
Multiscale approaches to high efficiency photovoltaics
While renewable energies are achieving parity around the globe, efforts to
reach higher solar cell efficiencies becomes ever more difficult as they
approach the limiting efficiency. The so-called third generation concepts
attempt to break this limit through a combination of novel physical processes
and new materials and concepts in organic and inorganic systems. Some examples
of semi-empirical modelling in the field are reviewed, in particular for
multispectral solar cells on silicon (french ANR project MULTISOLSI). Their
achievements are outlined, and the limits of these approaches shown. This
introduces the main topic of this contribution, which is the use of multiscale
experimental and theoretical techniques to go beyond the semi-empirical
understanding of these systems. This approach has already led to great advances
at modelling which have led to modelling software which is widely known. Yet a
survey of the topic reveals a fragmentation of efforts across disciplines,
firstly, such as organic and inorganic fields, but also between the high
efficiency concepts such as hot carrier cells and intermediate band concepts.
We show how this obstacle to the resolution of practical research obstacles may
be lifted by inter-disciplinary cooperation across length scales, and across
experimental and theoretical fields, and finally across materials systems. We
present a European COST Action MultiscaleSolar kicking off in early 2015 which
brings together experimental and theoretical partners in order to develop
multiscale research in organic and inorganic materials. The goal of this
defragmentation and interdisciplinary collaboration is to develop understanding
across length scales which will enable the full potential of third generation
concepts to be evaluated in practise, for societal and industrial applications.Comment: Draft paper accompanying a plenary presentation to the World
Renewable Energy Conference WREC 2015, June 2015, Bucharest. In press (IOP
Controlling the Spin Polarization of the Electron Current in a Semimagnetic Resonant-Tunneling Diode
The spin filtering effect of the electron current in a double-barrier
resonant-tunneling diode (RTD) consisting of ZnMnSe semimagnetic layers has
been studied theoretically. The influence of the distribution of the magnesium
ions on the coefficient of the spin polarization of the electron current has
been investigated. The dependence of the spin filtering degree of the electron
current on the external magnetic field and the bias voltage has been obtained.
The effect of the total spin polarization of the electron current has been
predicted. This effect is characterized by total suppression of the spin-up
component of electron current, that takes place when the Fermi level coincides
with the lowest Landau level for spin-up electrons in the RTD semimagnetic
emitter
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