13,910 research outputs found
Mott-Hubbard quantum criticality in paramagnetic CMR pyrochlores
We present a correlated {\it ab initio} description of the paramagnetic phase
of TlMnO, employing a combined local density approximation (LDA)
with multiorbital dynamical mean field theory (DMFT) treatment. We show that
the insulating state observed in this colossal magnetoresistance (CMR)
pyrochlore is determined by strong Mn intra- and inter-orbital local
electron-electron interactions. Hybridization effects are reinforced by the
correlation-induced spectral weight transfer. Our result coincides with optical
conductivity measurements, whose low energy features are remarkably accounted
for by our theory. Based on this agreement, we study the disorder-driven
insulator-metal transition of doped compounds, showing the proximity of
TlMnO to quantum phase transitions, in agreement with recent
measurements.Comment: 4 pages, 4 figure
Normal state magnetotransport properties of -FeSe superconductors
We present -FeSe magnetotransport data, and describe them
theoretically. Using a simplified microscopic model with two correlated
effective orbitals, we determined the normal state electrical conductivity and
Hall coefficient, using Kubo formalism. With model parameters relevant for
Fe-chalcogenides, we describe the observed effect of the structural transition
on the ab-plane electrical resistivity, as well as on the magnetoresistance.
Temperature-dependent Hall coefficient data were measured at 16 Tesla, and
their theoretical description improves upon inclusion of moderate electron
correlations. We confirm the effect of the structural transition on the
electronic structure, finding deformation-induced band splittings comparable to
those reported in angle-resolved photoemission.Comment: 6 pages, 5 figure
An insight into Capella (alpha Aurigae): from the extent of core overshoot to its evolutionary history
The binary star alpha Aurigae (otherwise known as Capella) is extremely
important to understand the core hydrogen and helium burning phases of the
stars, as the primary star is likely evolving through the core helium burning
phase, and the masses of the two components are 2.5 Msun and 2.6 Msun, which
fall into a mass range for which the extention of the core overshoot during the
main sequence phase is uncertain. We aim at deriving the extent of the core
overshoot experienced during the core burning phases and testing the efficiency
of the convective transport of energy in the external envelope, by comparing
results from stellar evolution modelling with the results from the
observations. We consider evolutionary tracks calculated on purpose for the
present work, for the primary and secondary star of Capella. We determine the
extent of the extra-mixing from the core during the main sequence evolution and
the age of the system, by requiring that the effective temperatures and surface
gravities of the model stars reproduce those derived from the observations at
the same epoch. We further check consistency between the observed and predicted
surface chemistry of the stars. Consistency between results from stellar
evolution modelling and the observations of Capella is found when extra-mixing
from the core is assumed, the extent of the extra-mixed zone being of the order
of 0.25 H_P. The age of the system is estimated to be 710 Myr. These results
allow to nicely reproduce the observed surface chemistry, particularly the
recent determination of the 12C/13C ratio based on LBT (Large Binocular
Telescope) and VATT (Vatican Advanced Technology Telescope) observation
The Lithium Depletion Boundary and the Age of the Young Open Cluster IC~2391
We have obtained new photometry and intermediate resolution ( \AA\ ) spectra of 19 of these objects
(14.9 17.5) in order to confirm cluster membership. We
identify 15 of our targets as likely cluster members based on their
photometry, spectral types, radial velocity, and H emission strengths.
Higher S/N spectra were obtained for 8 of these probable cluster members in
order to measure the strength of the lithium 6708 \AA\ doublet and thus obtain
an estimate of the cluster's age. One of these 8 stars has a definite lithium
detection and two other (fainter) stars have possible lithium detections. A
color-magnitude diagram for our program objects shows that the lithium
depletion boundary in IC~2391 is at =16.2. Using recent theoretical model
predictions, we derive an age for IC~2391 of 535 Myr. While this is
considerably older than the age most commonly attributed for this cluster
(35 Myr) this result for IC~2391 is comparable those recently derived for
the Pleiades and Alpha Persei clusters and can be explained by new models for
high mass stars that incorporate a modest amount of convective core
overshooting.Comment: ApJ Letters, acccepte
The early evolution of Globular Clusters: the case of NGC 2808
Enhancement and spread of helium among globular cluster stars have been
recently suggested as a way to explain the horizontal branch blue tails, in
those clusters which show a primordial spread in the abundances of CNO and
other elements involved in advanced CNO burning (D'Antona et al. 2002). In this
paper we examine the implications of the hypothesis that, in many globular
clusters, stars were born in two separate events: an initial burst (first
generation), which gives origin to probably all high and intermediate mass
stars and to a fraction of the cluster stars observed today, and a second,
prolonged star formation phase (second generation) in which stars form directly
from the ejecta of the intermediate mass stars of the first generation. In
particular, we consider in detail the morphology of the horizontal branch in
NGC 2808 and argue that it unveils the early cluster evolution, from the birth
of the first star generation to the end of the second phase of star formation.
This framework provides a feasible interpretation for the still unexplained
dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the
RR Lyr region to the gap in the helium content between the red clump, whose
stars are considered to belong to the first stellar generation and have
primordial helium, and the blue side of the horizontal branch, whose minimum
helium content reflects the helium abundance in the smallest mass
(~4Msun)contributing to the second stellar generation. This scenario provides
constraints on the required Initial Mass Function, in a way that a great deal
of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa
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