1,935 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
Recurrence relation for relativistic atomic matrix elements
Recurrence formulae for arbitrary hydrogenic radial matrix elements are
obtained in the Dirac form of relativistic quantum mechanics. Our approach is
inspired on the relativistic extension of the second hypervirial method that
has been succesfully employed to deduce an analogous relationship in non
relativistic quantum mechanics. We obtain first the relativistic extension of
the second hypervirial and then the relativistic recurrence relation.
Furthermore, we use such relation to deduce relativistic versions of the
Pasternack-Sternheimer rule and of the virial theorem.Comment: 10 pages, no figure
Vela X-1 as a laboratory for accretion in High-Mass X-ray Binaries
Vela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s
accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib
supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a
prototype of wind-accreting HMXB and it has been used as a baseline in
different theoretical or modelling studies.
We discuss the observational properties of the system and the use of the
observational data as laboratory to test recent developments in modelling the
accretion process in High-Mass X-ray Binaries (e.g., Sander et al. 2018; El
Mellah et al. 2018), which range from detailed descriptions of the wind
acceleration to modelling of the structure of the flow of matter close to the
neutron star and its variations.Comment: 4 pages, 2 figures, proceedings of the 12th INTEGRAL conference
"INTEGRAL looks AHEAD to Multimessenger astronomy" in Geneva (Switzerland) on
11-15 February 201
Relativistically extended Blanchard recurrence relation for hydrogenic matrix elements
General recurrence relations for arbitrary non-diagonal, radial hydrogenic
matrix elements are derived in Dirac relativistic quantum mechanics. Our
approach is based on a generalization of the second hypervirial method
previously employed in the non-relativistic Schr\"odinger case. A relativistic
version of the Pasternack-Sternheimer relation is thence obtained in the
diagonal (i.e. total angular momentum and parity the same) case, from such
relation an expression for the relativistic virial theorem is deduced. To
contribute to the utility of the relations, explicit expressions for the radial
matrix elements of functions of the form and
---where is a Dirac matrix--- are presented.Comment: 21 pages, to be published in J. Phys. B: At. Mol. Opt. Phys. in Apri
INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302
In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has
been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are
X-ray binary systems with a compact companion orbiting a supergiant star which
show very short and bright outbursts in a series of activity periods
overimposed on longer quiescent periods. Only very recently the first attempts
to model the behaviour of these sources have been published, some of them
within the framework of accretion from clumpy stellar winds.Our goal is to
analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of
the clumpy structure of the supergiant wind. We have used INTEGRAL and RXTE/PCA
observations in order to obtain broad band (1-200 keV) spectra and light curves
of XTE J1739-302 and investigate its X-ray spectrum and temporal variability.
We have found that XTE J1739-302 follows a much more complex behaviour than
expected. Far from presenting a regular variability pattern, XTE J1739-302
shows periods of high, intermediate, and low flaring activity.Comment: 9 pages, 7 figures, accepted for publication in A&
Dripping Faucet Dynamics Clarified by an Improved Mass-Spring Model
An improved mass-spring model for a dripping faucet is presented. The model
is constructed based on the numerical results which we recently obtained from
fluid dynamical calculations. Both the fluid dynamical calculations and the
present mass-spring model exhibit a variety of complex behavior including
transition to chaos in good agreement with experiments. Further, the
mass-spring model reveals fundamental dynamics inherent in the dripping faucet
system.Comment: 17 pages, 17 figure
Gravitational perturbations of Schwarzschild spacetime at null infinity and the hyperboloidal initial value problem
We study gravitational perturbations of Schwarzschild spacetime by solving a
hyperboloidal initial value problem for the Bardeen-Press equation.
Compactification along hyperboloidal surfaces in a scri-fixing gauge allows us
to have access to the gravitational waveform at null infinity in a general
setup. We argue that this hyperboloidal approach leads to a more accurate and
efficient calculation of the radiation signal than the common approach where a
timelike outer boundary is introduced. The method can be generalized to study
perturbations of Kerr spacetime using the Teukolsky equation.Comment: 14 pages, 9 figure
Relativistic quantum mechanics of a Dirac oscillator
The Dirac oscillator is an exactly soluble model recently introduced in the
context of many particle models in relativistic quantum mechanics. The model
has been also considered as an interaction term for modelling quark confinement
in quantum chromodynamics. These considerations should be enough for
demonstrating that the Dirac oscillator can be an excellent example in
relativistic quantum mechanics. In this paper we offer a solution to the
problem and discuss some of its properties. We also discuss a physical picture
for the Dirac oscillator's non-standard interaction, showing how it arises on
describing the behaviour of a neutral particle carrying an anomalous magnetic
moment and moving inside an uniformly charged sphere.Comment: 19 pages, 1 figur
Automated detection of microaneurysms by using region growing and fuzzy artmap neural network
Objective: To assess whether the methodological changes of this new algorithm improves
the results of a previously presented strategy.
Methods: We enhance the image and filter out the green channel of the digital color retinog-
raphy. Multitolerance thresholding was applied to obtain candidate points and make a seed
growing region by varying intensities. We took 15 characteristics from each region to train a
fuzzy Artmap neural network using 42 retinal photographs. This network was then applied
in the study of 11 good quality retinal photographs included in the diabetic retinopathy early
detection screening program, with initial stages of retinopathy, obtained with the Topcon
NW200 non-mydriatic retinal camera.
Results: Two experienced ophthalmologists detected 52 microaneurysms in 11 images. The
algorithm detected 39 microaneurysms and 3752 more regions, confirming 38 microa-
neurysm and 135 false positives. The sensitivity is improved compared to the previous
algorithm, from 60.53% to 73.08%. False positives have dropped from 41.8 to 12.27 per image.
Conclusions: The new algorithm is better than the previous one, but there is still room for
improvement, especially in the initial determination of seed
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