8,908 research outputs found
The brightest pure-H ultracool white dwarf
We report the identification of LSR J0745+2627 in the United Kingdom InfraRed
Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool
white dwarf with kinematics and age compatible with the thick-disk/halo
population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high
reduced proper motion value in the J band (H_J=21.87). We show how the
infrared-reduced proper motion diagram is useful for selecting a sample of cool
white dwarfs with low contamination. LSR J0745+2627 is also detected in the
Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer
(WISE). We have spectroscopically confirmed this object as a cool white dwarf
using X-Shooter on the Very Large Telescope. A detailed analysis of its
spectral energy distribution reveals that its atmosphere is compatible with a
pure-H composition model with an effective temperature of 3880+-90 K. This
object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever
identified. We have constrained the distance (24-45 pc), space velocities and
age considering different surface gravities. The results obtained suggest that
LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the
closest ultracool white dwarfs.Comment: 5 pages, 7 figures, accepted for publication in A&A Letter
Einstein gravity as a 3D conformally invariant theory
We give an alternative description of the physical content of general
relativity that does not require a Lorentz invariant spacetime. Instead, we
find that gravity admits a dual description in terms of a theory where local
size is irrelevant. The dual theory is invariant under foliation preserving
3-diffeomorphisms and 3D conformal transformations that preserve the 3-volume
(for the spatially compact case). Locally, this symmetry is identical to that
of Horava-Lifshitz gravity in the high energy limit but our theory is
equivalent to Einstein gravity. Specifically, we find that the solutions of
general relativity, in a gauge where the spatial hypersurfaces have constant
mean extrinsic curvature, can be mapped to solutions of a particular gauge
fixing of the dual theory. Moreover, this duality is not accidental. We provide
a general geometric picture for our procedure that allows us to trade foliation
invariance for conformal invariance. The dual theory provides a new proposal
for the theory space of quantum gravity.Comment: 27 pages. Published version (minor changes and corrections
Scalar-Kinetic Branes
This work tries to find out thick brane solutions in braneworld scenarios
described by a real scalar field in the presence of a scalar-kinetic term
with a single extra dimension, where
stands for the standard kinetic term and
. We mainly consider bent branes, namely de Sitter and Anti-de
Sitter four-dimensional slices. The solutions of a flat brane are obtained when
taking the four-dimensional cosmological constant .
When the parameter , these solutions turn to those of the standard
scenario. The localization and spectrum of graviton on these branes are also
analyzed.Comment: 10 pages, no figures, accepted by EP
Excitons in quasi-one dimensional organics: Strong correlation approximation
An exciton theory for quasi-one dimensional organic materials is developed in
the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short range
extended Hubbard interactions. Within a strong electron-electron correlation
approximation, the exciton properties are extensively studied. Using scattering
theory, we analytically obtain the exciton energy and wavefunction and derive a
criterion for the existence of a exciton. We also systematically
investigate the effect of impurities on the coherent motion of an exciton. The
coherence is measured by a suitably defined electron-hole correlation function.
It is shown that, for impurities with an on-site potential, a crossover
behavior will occur if the impurity strength is comparable to the bandwidth of
the exciton, corresponding to exciton localization. For a charged impurity with
a spatially extended potential, in addition to localization the exciton will
dissociate into an uncorrelated electron-hole pair when the impurity is
sufficiently strong to overcome the Coulomb interaction which binds the
electron-hole pair. Interchain coupling effects are also discussed by
considering two polymer chains coupled through nearest-neighbor interchain
hopping and interchain Coulomb interaction . Within the
matrix scattering formalism, for every center-of-mass momentum, we find two
poles determined only by , which correspond to the interchain
excitons. Finally, the exciton state is used to study the charge transfer from
a polymer chain to an adjacent dopant molecule.Comment: 24 pages, 23 eps figures, pdf file of the paper availabl
Scattering in one-dimensional heterostructures described by the Dirac equation
We consider electronic transport accross one-dimensional heterostructures
described by the Dirac equation. We discuss the cases where both the velocity
and the mass are position dependent. We show how to generalize the Dirac
Hamiltonian in order to obtain a Hermitian problem for spatial dependent
velocity. We solve exactly the case where the position dependence of both
velocity and mass is linear. In the case of velocity profiles, it is shown that
there is no backscattering of Dirac electrons. In the case of the mass profile
backscattering exists. In this case, it is shown that the linear mass profile
induces less backscattering than the abrupt step-like profile. Our results are
a first step to the study of similar problems in graphene
Abrupt Onset of Second Energy Gap at Superconducting Transition of Underdoped Bi2212
The superconducting gap - an energy scale tied to the superconducting
phenomena-opens on the Fermi surface at the superconducting transition
temperature (TC) in conventional BCS superconductors. Quite differently, in
underdoped high-TC superconducting cuprates, a pseudogap, whose relation to the
superconducting gap remains a mystery, develops well above TC. Whether the
pseudogap is a distinct phenomenon or the incoherent continuation of the
superconducting gap above TC is one of the central questions in high-TC
research. While some experimental evidence suggests they are distinct, this
issue is still under intense debate. A crucial piece of evidence to firmly
establish this two-gap picture is still missing: a direct and unambiguous
observation of a single-particle gap tied to the superconducting transition as
function of temperature. Here we report the discovery of such an energy gap in
underdoped Bi2212 in the momentum space region overlooked in previous
measurements. Near the diagonal of Cu-O bond direction (nodal direction), we
found a gap which opens at TC and exhibits a canonical (BCS-like) temperature
dependence accompanied by the appearance of the so-called Bogoliubov
quasiparticles, a classical signature of superconductivity. This is in sharp
contrast to the pseudogap near the Cu-O bond direction (antinodal region)
measured in earlier experiments. The emerging two-gap phenomenon points to a
picture of richer quantum configurations in high temperature superconductors.Comment: 16 pages, 4 figures, authors' version Corrected typos in the abstrac
Ab-initio prediction of the electronic and optical excitations in polythiophene: isolated chains versus bulk polymer
We calculate the electronic and optical excitations of polythiophene using
the GW approximation for the electronic self-energy, and include excitonic
effects by solving the electron-hole Bethe-Salpeter equation. Two different
situations are studied: excitations on isolated chains and excitations on
chains in crystalline polythiophene. The dielectric tensor for the crystalline
situation is obtained by modeling the polymer chains as polarizable line
objects, with a long-wavelength polarizability tensor obtained from the
ab-initio polarizability function of the isolated chain. With this model
dielectric tensor we construct a screened interaction for the crystalline case,
including both intra- and interchain screening. In the crystalline situation
both the quasi-particle band gap and the exciton binding energies are
drastically reduced in comparison with the isolated chain. However, the optical
gap is hardly affected. We expect this result to be relevant for conjugated
polymers in general.Comment: 15 pages including 4 figures; to appear in Phys. Rev. B, 6/15/200
A cartilage tissue engineering approach combining starch-polycaprolactone fibre mesh scaffolds with bovine articular chondrocytes
In the present work we originally tested the suitability
of corn starch-polycaprolactone (SPCL) scaffolds for
pursuing a cartilage tissue engineering approach. Bovine articular
chondrocytes were seeded on SPCL scaffolds under
dynamic conditions using spinner flasks (total of 4 scaffolds
per spinner flask using cell suspensions of 0.5×106 cells/ml)
and cultured under orbital agitation for a total of 6 weeks.
Poly(glycolic acid) (PGA) non-woven scaffolds and bovine
native articular cartilage were used as standard controls for
the conducted experiments. PGA is a kind of standard in
tissue engineering approaches and it was used as a control
in that sense. The tissue engineered constructs were characterized
at different time periods by scanning electron microscopy
(SEM), hematoxylin-eosin (H&E) and toluidine
blue stainings, immunolocalisation of collagen types I and II,
and dimethylmethylene blue (DMB) assay for glycosaminoglycans
(GAG) quantification assay. SEM results for SPCL
constructs showed that the chondrocytes presented normal
morphological features, with extensive cells presence at the
surface of the support structures, and penetrating the scaffolds
pores. These observations were further corroborated
by H&E staining. Toluidine blue and immunohistochemistry
exhibited extracellular matrix deposition throughout the 3D structure. Glycosaminoglycans, and collagen types I and II
were detected. However, stronger staining for collagen type
II was observed when compared to collagen type I. The PGA
constructs presented similar features toSPCLat the end of the
6 weeks. PGA constructs exhibited higher amounts of matrix
glycosaminoglycans when compared to the SPCL scaffolds.
However, we also observed a lack of tissue in the central
area of the PGA scaffolds. Reasons for these occurrences
may include inefficient cells penetration, necrosis due to high
cell densities, or necrosis related with acidic by-products
degradation. Such situation was not detected in the SPCL
scaffolds, indicating the much better biocompatibility of the
starch based scaffolds
Angle-resolved photoemission study and first principles calculation of the electronic structure of GaTe
The electronic band structure of GaTe has been calculated by numerical atomic
orbitals density-functional theory, in the local density approximation. In
addition, the valence-band dispersion along various directions of the GaTe
Brillouin zone has been determined experimentally by angle-resolved
photoelectron spectroscopy. Along these directions, the calculated valence-band
structure is in good concordance with the valence-band dispersion obtained by
these measurements. It has been established that GaTe is a direct-gap
semiconductor with the band gap located at the Z point, that is, at Brillouin
zone border in the direction perpendicular to the layers. The valence-band
maximum shows a marked \textit{p}-like behavior, with a pronounced anion
contribution. The conduction band minimum arises from states with a comparable
\textit{s}- \textit{p}-cation and \textit{p}-anion orbital contribution.
Spin-orbit interaction appears to specially alter dispersion and binding energy
of states of the topmost valence bands lying at . By spin-orbit, it is
favored hybridization of the topmost \textit{p}-valence band with deeper
and flatter \textit{p}-\textit{p} bands and the valence-band minimum at
is raised towards the Fermi level since it appears to be determined by
the shifted up \textit{p}-\textit{p} bands.Comment: 7 text pages, 6 eps figures, submitted to PR
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