13,802 research outputs found
The boson-fermion model: An exact diagonalization study
The main features of a generic boson-fermion scenario for electron pairing in
a many-body correlated fermionic system are: i) a cross-over from a poor metal
to an insulator and finally a superconductor as the temperature decreases, ii)
the build-up of a finite amplitude of local electron pairing below a certain
temperature , followed by the onset of long-range phase correlations among
electron pairs below a second characteristic temperature , iii) the
opening of a pseudogap in the DOS of the electrons below , rendering these
electrons poorer and poorer quasi-particles as the temperature decreases, with
the electron transport becoming ensured by electron pairs rather than by
individual electrons. A number of these features have been so far obtained on
the basis of different many-body techniques, all of which have their built-in
shortcomings in the intermediate coupling regime, which is of interest here. In
order to substantiate these features, we investigate them on the basis of an
exact diagonalization study on rings up to eight sites. Particular emphasis has
been put on the possibility of having persistent currents in mesoscopic rings
tracking the change-over from single- to two-particle transport as the
temperature decreases and the superconducting state is approached.Comment: 7 pages, 8 figures; to be published in Phys. Rev.
The 2015 outburst of the accreting millisecond pulsar IGR J17511-3057 as seen by INTEGRAL, Swift and XMM-Newton
We report on INTEGRAL, Swift and XMM-Newton observations of IGR J17511-3057
performed during the outburst that occurred between March 23 and April 25,
2015. The source reached a peak flux of 0.7(2)E-9 erg/cm/s and decayed to
quiescence in approximately a month. The X-ray spectrum was dominated by a
power-law with photon index between 1.6 and 1.8, which we interpreted as
thermal Comptonization in an electron cloud with temperature > 20 keV . A broad
({\sigma} ~ 1 keV) emission line was detected at an energy (E =
6.9 keV) compatible with the K{\alpha} transition of ionized
Fe, suggesting an origin in the inner regions of the accretion disk. The
outburst flux and spectral properties shown during this outburst were
remarkably similar to those observed during the previous accretion event
detected from the source in 2009. Coherent pulsations at the pulsar spin period
were detected in the XMM-Newton and INTEGRAL data, at a frequency compatible
with the value observed in 2009. Assuming that the source spun up during the
2015 outburst at the same rate observed during the previous outburst, we derive
a conservative upper limit on the spin down rate during quiescence of 3.5E-15
Hz/s. Interpreting this value in terms of electromagnetic spin down yields an
upper limit of 3.6E26 G/cm to the pulsar magnetic dipole (assuming a
magnetic inclination angle of 30{\deg}). We also report on the detection of
five type-I X-ray bursts (three in the XMM-Newton data, two in the INTEGRAL
data), none of which indicated photospheric radius expansion.Comment: 10 pages, 7 figures, accepted for publication in A&
A lattice Boltzmann formulation to the analysis of radiative heat transfer problems in a participating medium
Chemical evolution of star clusters
I discuss the chemical evolution of star clusters, with emphasis on old
globular clusters, in relation to their formation histories. Globular clusters
clearly formed in a complex fashion, under markedly different conditions from
any younger clusters presently known. Those special conditions must be linked
to the early formation epoch of the Galaxy and must not have occurred since.
While a link to the formation of globular clusters in dwarf galaxies has been
suggested, present-day dwarf galaxies are not representative of the
gravitational potential wells within which the globular clusters formed.
Instead, a formation deep within the proto-Galaxy or within dark-matter
minihaloes might be favoured. Not all globular clusters may have formed and
evolved similarly. In particular, we may need to distinguish Galactic halo from
Galactic bulge clusters.Comment: 27 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 6 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
Corrections to the apparent value of the cosmological constant due to local inhomogeneities
Supernovae observations strongly support the presence of a cosmological
constant, but its value, which we will call apparent, is normally determined
assuming that the Universe can be accurately described by a homogeneous model.
Even in the presence of a cosmological constant we cannot exclude nevertheless
the presence of a small local inhomogeneity which could affect the apparent
value of the cosmological constant. Neglecting the presence of the
inhomogeneity can in fact introduce a systematic misinterpretation of
cosmological data, leading to the distinction between an apparent and true
value of the cosmological constant. We establish the theoretical framework to
calculate the corrections to the apparent value of the cosmological constant by
modeling the local inhomogeneity with a solution. Our assumption
to be at the center of a spherically symmetric inhomogeneous matter
distribution correspond to effectively calculate the monopole contribution of
the large scale inhomogeneities surrounding us, which we expect to be the
dominant one, because of other observations supporting a high level of isotropy
of the Universe around us.
By performing a local Taylor expansion we analyze the number of independent
degrees of freedom which determine the local shape of the inhomogeneity, and
consider the issue of central smoothness, showing how the same correction can
correspond to different inhomogeneity profiles. Contrary to previous attempts
to fit data using large void models our approach is quite general. The
correction to the apparent value of the cosmological constant is in fact
present for local inhomogeneities of any size, and should always be taken
appropriately into account both theoretically and observationally.Comment: 16 pages,new sections added analyzing central smoothness and accuracy
of the Taylor expansion approach, Accepted for publication by JCAP. An essay
based on this paper received honorable mention in the 2011 Essay Context of
the Gravity Research Foundatio
Discovery of the accretion-powered millisecond pulsar SWIFT J1756.9-2508 with a low-mass companion
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the
eighth known transient accretion-powered millisecond pulsar, SWIFT
J1756.9-2508, as part of routine observations with the Swift Burst Alert
Telescope hard X-ray transient monitor. The pulsar was subsequently observed by
both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer
Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an
orbital period of 54.7 minutes. The minimum companion mass is between 0.0067
and 0.0086 solar masses, depending on the mass of the neutron star, and the
upper limit on the mass is 0.030 solar masses (95% confidence level). Such a
low mass is inconsistent with brown dwarf models, and comparison with white
dwarf models suggests that the companion is a He-dominated donor whose thermal
cooling has been at least modestly slowed by irradiation from the accretion
flux. No X-ray bursts, dips, eclipses or quasi-periodic oscillations were
detected. The current outburst lasted approximately 13 days and no earlier
outbursts were found in archival data.Comment: 13 pages, 2 figures, accepted by Astrophysical Journal Letter
The Role of Volunteered Geographic Information towards 3D Property Cadastral Systems (2): A Purpose Driven Web Application
VGI has not proved to be readily suitable to replace well-established accurate methods and technologies such as those of full standard cadastral surveys. Even so, VGI potentialities as relevant source of geospatial data have been widely acknowledged. As such, some authors have defended that VGI may in fact play an important role such as at a local cadastral jurisdiction level towards local spatial data infrastructures. As far as property cadastre is concerned, the full extent 3D complexity inside a property is in many instances only known to their occupants, thus making crowd sourcing perhaps the only economically feasible approach for its capture. While the crowd cannot be expected to conduct a full cadastral survey, it may be possible to ask them to indicate at least the location of complex 3D situations and thus to facilitate local authorities’ understanding of the extent of some cadastral issues. As such, it was argued in our previous work that geoinformation from the crowd might in fact be taken into account as an interim step before a full surveyed 3D cadastre is eventually achieved. As such, possible room for VGI in the context of 3D cadastre was discussed, and a hierarchical framework of levels of data acquisition to be used at local cadastral jurisdiction level was proposed. Such framework is revisited in this paper.Given context above, this paper focuses primarily on two aspects. Firstly, to review technical requirements of the official cadastral process in Portugal in order to identify which sorts of cadastral data are likely to be acquirable/not acquirable through VGI. Secondly, to design and to implement the prototype of a web-based application (IGV3Dcad) envisaged for general public usage to flag different land and property ownership situations. Having information about the extent of the 2D/3D issue is also fundamental to making a decision as to whether a 3D cadastral approach is actually needed and hence to further invest resources in even more expensive 3D survey
Approximate analytical description of the nonaffine response of amorphous solids
An approximation scheme for model disordered solids is proposed that leads to
the fully analytical evaluation of the elastic constants under explicit account
of the inhomogeneity (nonaffinity) of the atomic displacements. The theory is
in quantitative agreement with simulations for central-force systems and
predicts the vanishing of the shear modulus at the isostatic point with the
linear law {\mu} ~ (z - 2d), where z is the coordination number. The vanishing
of rigidity at the isostatic point is shown to be a consequence of the
canceling out of positive affine and negative nonaffine terms
Non-relativistic ten-dimensional minimal supergravity
We construct a non-relativistic limit of ten-dimensional N=1 supergravity
from the point of view of the symmetries, the action, and the equations of
motion. This limit can only be realized in a supersymmetric way provided we
impose by hand a set of geometric constraints, invariant under all the
symmetries of the non-relativistic theory, that define a so-called `self-dual'
Dilatation-invariant String Newton-Cartan geometry. The non-relativistic action
exhibits three emerging symmetries: one local scale symmetry and two local
conformal supersymmetries. Due to these emerging symmetries the Poisson
equation for the Newton potential and two partner fermionic equations do not
follow from a variation of the non-relativistic action but, instead, are
obtained by a supersymmetry variation of the other equations of motion that do
follow from a variation of the non-relativistic action. We shortly discuss the
inclusion of the Yang-Mills sector that would lead to a non-relativistic
heterotic supergravity action.Comment: 40 pages, minor change
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