3,409 research outputs found
Interacting Dark Sector and Precision Cosmology
We consider a recently proposed model in which dark matter interacts with a
thermal background of dark radiation. Dark radiation consists of relativistic
degrees of freedom which allow larger values of the expansion rate of the
universe today to be consistent with CMB data (-problem). Scattering
between dark matter and radiation suppresses the matter power spectrum at small
scales and can explain the apparent discrepancies between CDM
predictions of the matter power spectrum and direct measurements of Large Scale
Structure LSS (-problem). We go beyond previous work in two ways: 1.
we enlarge the parameter space of our previous model and allow for an arbitrary
fraction of the dark matter to be interacting and 2. we update the data sets
used in our fits, most importantly we include LSS data with full -dependence
to explore the sensitivity of current data to the shape of the matter power
spectrum. We find that LSS data prefer models with overall suppressed matter
clustering due to dark matter - dark radiation interactions over CDM
at 3-4 . However recent weak lensing measurements of the power spectrum
are not yet precise enough to clearly distinguish two limits of the model with
different predicted shapes for the linear matter power spectrum. In two
Appendices we give a derivation of the coupled dark matter and dark radiation
perturbation equations from the Boltzmann equation in order to clarify a
confusion in the recent literature, and we derive analytic approximations to
the solutions of the perturbation equations in the two physically interesting
limits of all dark matter weakly interacting or a small fraction of dark matter
strongly interacting.Comment: 29 pages + 2 Appendices; published versio
Development of a FAst Compton TELescope (FACTEL)
This dissertation describes the development of a FAst Compton TELescope (FACTEL) instrument. It is designed to be the prototype of a larger Advanced Scintillators COmpton Telescope (ASCOT) aimed for general astronomical observations in the medium energy gamma-ray range between 500 keV and 50 MeV. This dissertation presents the instrument and the observation results from the successful 2011 balloon campaign which took place on September 23rd and 24th at Fort Sumner, New Mexico (Flight 624N). The instrument was at float altitude for twenty-six hours at an average 36 km altitude. The FACTEL prototype achieved a 1-ns Time-of-flight resolution between the two detectors layers of the instrument
Origin of the orbital and spin orderings in rare-earth titanates
Rare-earth titanates RTiO are Mott insulators displaying a rich physical
behavior, featuring most notably orbital and spin orders in their ground state.
The origin of their ferromagnetic to antiferromagnetic transition as a function
of the size of the rare-earth however remains debated. Here we show on the
basis of symmetry analysis and first-principles calculations that although
rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller
distortion is negligible and irrelevant for the description of the ground state
properties. At the same time, we demonstrate that the combination of two
antipolar motions produces an effective Jahn-Teller-like motion which is the
key of the varying spin-orbital orders appearing in titanates. Thus, titanates
are prototypical examples illustrating how a subtle interplay between several
lattice distortions commonly appearing in perovskites can produce orbital
orderings and insulating phases irrespective of proper Jahn-Teller motions.Comment: Accepted in Physical Review
Origin of band gaps in 3d perovskite oxides
With their broad range of magnetic, electronic and structural properties,
transition metal perovskite oxides ABO3 have long served as a platform for
testing condensed matter theories. In particular, their insulating character -
found in most compounds - is often ascribed to dynamical electronic
correlations through the celebrated Mott-Hubbard mechanism where gaping arises
from a uniform, symmetry-preserving electron repulsion mechanism. However,
structural distortions are ubiquitous in perovskites and their relevance with
respect to dynamical correlations in producing this rich array of properties
remains an open question. Here, we address the origin of band gap opening in
the whole family of 3d perovskite oxides. We show that a single-determinant
mean-field approach such as density functional theory (DFT) successfully
describes the structural, magnetic and electronic properties of the whole
series, at low and high temperatures. We find that insulation occurs via
energy-lowering crystal symmetry reduction (octahedral rotations, Jahn-Teller
and bond disproportionation effects), as well as intrinsic electronic
instabilities, all lifting orbital degeneracies. Our work therefore suggests
that whereas ABO3 oxides may be complicated, they are not necessarily strongly
correlated. It also opens the way towards systematic investigations of doping
and defect physics in perovskites, essential for the full realization of
oxide-based electronics
Mott gapping in 3d ABO3 perovskites without Mott-Hubbard interelectronic U
The existence of band gaps in Mott insulators such as perovskite oxides with
partially filled 3d shells has been traditionally explained in terms of strong,
dynamic inter-electronic repulsion codified by the on-site repulsion energy U
in the Hubbard Hamiltonian. The success of the "DFT+U approach" where an
empirical on-site potential term U is added to the exchange-and correlation
Density Functional Theory (DFT) raised questions on whether U in DFT+U
represents interelectronic correlation in the same way as it does in the
Hubbard Hamiltonian, and if empiricism in selecting U can be avoided. Here we
illustrate that ab-initio DFT without any U is able to predict gapping trends
and structural symmetry breaking (octahedra rotations, Jahn-Teller modes, bond
disproportionation) for all ABO3 3d perovskites from titanates to nickelates in
both spin-ordered and spin disordered paramagnetic phases. We describe the
paramagnetic phases as a supercell where individual sites can have different
local environments thereby allowing DFT to develop finite moments on different
sites as long as the total cell has zero moment. We use a recently developed
exchange and correlation functional ("SCAN") that is sanctioned by the usual
single-determinant, mean-field DFT paradigm with static correlations, but has a
more precise rendering of self-interaction cancelation. Our results suggest
that strong dynamic electronic correlations are not playing a universal role in
gapping of 3d ABO3 Mott insulators, and opens the way for future applications
of DFT for studying a plethora of complexity effects that depend on the
existence of gaps, such as doping, defects, and band alignment in ABO3 oxides
First-principles study of electron and hole doping in perovskite nickelates
Rare-earth nickelates RNiO (R=Lu-Pr, Y) show a striking
metal-insulator transition in their bulk phase whose temperature can be tuned
by the rare-earth radius. These compounds are also the parent phases of the
newly identified infinite layer RNiO2 superconductors. Although intensive
theoretical works have been devoted to understand the origin of the
metal-insulator transition in the bulk, there have only been a few studies on
the role of hole and electron doping by rare-earth substitutions in RNiO
materials. Using first-principles calculations based on density functional
theory (DFT) we study the effect of hole and electron doping in a prototypical
nickelate SmNiO3. We perform calculations without Hubbard-like U potential on
Ni 3d levels but with a meta-GGA better amending self-interaction errors. We
find that at low doping, polarons form with intermediate localized states in
the band gap resulting in a semiconducting behavior. At larger doping, the
intermediate states spread more and more in the band gap until they merge
either with the valence (hole doping) or the conduction (electron doping) band,
ultimately resulting in a metallic state at 25% of R cation substitution. These
results are reminiscent of experimental data available in the literature and
demonstrate that DFT simulations without any empirical parameter are qualified
for studying doping effects in correlated oxides and to explore the mechanisms
underlying the superconducting phase of rare-earth nickelates
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