1,515 research outputs found
Flowing with Eight Supersymmetries in M-Theory and F-theory
We consider holographic RG flow solutions with eight supersymmetries and
study the geometry transverse to the brane. For both M2-branes and for
D3-branes in F-theory this leads to an eight-manifold with only a four-form
flux. In both settings there is a natural four-dimensional hyper-Kahler slice
that appears on the Coulomb branch. In the IIB theory this hyper-Kahler
manifold encodes the Seiberg-Witten coupling over the Coulomb branch of a U(1)
probe theory. We focus primarily upon a new flow solution in M-theory. This
solution is first obtained using gauged supergravity and then lifted to eleven
dimensions. In this new solution, the brane probes have an Eguchi-Hanson moduli
space with the M2-branes spread over the non-trivial 2-sphere. It is also shown
that the new solution is valid for a class of orbifold theories. We discuss how
the hyper-Kahler structure on the slice extends to some form of G-structure in
the eight-manifold, and describe how this can be computed.Comment: 29 pages, 1 figure, harvma
Diseño optimizado y caracterización de hormigones autocompactantes reforzados con fibra polimérica de alto módulo
Los hormigones autocompactantes suponen una alternativa de ejecución fácil y rápida, que garantiza una puesta en obra de calidad y una mejora de las prestaciones mecánicas a igualdad de contenido de cemento. Por otro lado, los hormigones reforzados con fibra de carácter estructural son cada vez más demandados ya que, en el caso de contar con el tipo y proporción de fibra adecuado, permiten la sustitución total o parcial del armado convencional de acero. Este estudio persigue el diseño optimizado de hormigones autocompactantes reforzados con fibra polimérica de alto módulo para la ejecución de obras de rehabilitación, como las llevadas a cabo en las bóvedas de la catedral de San Cristóbal de la Laguna, de tal manera que, manteniendo la caracterÃstica de autocompactabilidad y sin perjuicio de la durabilidad, dicho hormigón presente un comportamiento post-fisuración que cumpla con las especificaciones del Anejo 14 de la EHE-08, en lo que se refiere a los valores mÃnimos de resistencia caracterÃstica residual a tracción por flexión fR,1,k y fR,3,k , de tal forma que la fibra alcance la consideración de estructural, lo que supondrÃa una alternativa técnica al refuerzo con fibra de acero que permitirÃa la sustitución parcial o total de la armadura de acero
Growth Route Toward III-V Multispectral Solar Cells on Silicon
To date, high efficiency multijunction solar cells have been developed on Ge
or GaAs substrates for space applications, and terrestrial applications are
hampered by high fabrication costs. In order to reduce this cost, we propose a
breakthrough technique of III-V compound heteroepitaxy on Si substrates without
generation of defects critical to PV applications. With this technique we
expect to achieve perfect integration of heterogeneous Ga1-xInxAs
micro-crystals on Si substrates. In this paper, we show that this is the case
for x=0. GaAs crystals were grown by Epitaxial Lateral Overgrowth on Si (100)
wafers covered with a thin SiO2 nanostructured layer. The cristallographic
structure of these crystals is analysed by MEB and TEM imaging. Micro-Raman and
Micro-Photomuminescence spectra of GaAs crystals grown with different
conditions are compared with those of a reference GaAs wafer in order to have
more insight on eventual local strains and their cristallinity. This work aims
at developping building blocks to further develop a GaAs/Si tandem demonstrator
with a potential conversion efficiency of 29.6% under AM1.5G spectrum without
concentration, as inferred from our realistic modeling. This paper shows that
Epitaxial Lateral Overgrowth has a very interesting potential to develop
multijunction solar cells on silicon approaching the today 30.3% world record
of a GaInP/GaAs tandem cell under the same illumination conditions, but on a
costlier substrate than silicon.Comment: Preprint of the 28th EUPVSEC proceedings, September 2013, Paris,
France. (5 pages
Polypropylene fiber-reinforced self-compacting concrete for the reconstruction of the cathedral of La Laguna, Canary Island, Spain
High performance materials are needed for the reconstruction of such a singular building as a cathedral, since in addition to special mechanical properties, high self compact ability, high durability and high surface quality, are specified. Because of the project’s specifications, the use of polypropylene fiber-reinforced, self-compacting concrete was selected by the engineering office. The low quality of local materials and the lack of experience in applying macro polypropylene fiber for structural reinforcement with these components materials required the development of a pretesting program. To optimize the mix design, performance was evaluated following technical, economical and constructability criteria. Since the addition of fibers reduces concrete self-compactability, many trials were run to determine the optimal mix proportions. The variables introduced were paste volume; the aggregate skeleton of two or three fractions plus limestone filler; fiber type and dosage. Two mix designs were selected from the preliminary results. The first one was used as reference for self-compactability and mechanical properties. The second one was an optimized mix with a reduction in cement content of 20 kg/m3and fiber dosage of 1 kg/m3. For these mix designs, extended testing was carried out to measure the compression and flexural strength, modulus of elasticity, toughness, and water permeability resistanc
Vortex Strings and Four-Dimensional Gauge Dynamics
We study the low-energy quantum dynamics of vortex strings in the Higgs phase
of N=2 supersymmetric QCD. The exact BPS spectrum of the stretched string is
shown to coincide with the BPS spectrum of the four-dimensional parent gauge
theory. Perturbative string excitations correspond to bound W-bosons and quarks
while the monopoles appear as kinks on the vortex string. This provides a
physical explanation for an observation by N. Dorey relating the quantum
spectra of theories in two and four dimensions.Comment: 23 pages, 1 figure. v2: Two extra appendices included: one on the
brane construction, the other describing the potential on the vortex moduli
space. Two figures added. Typos corrected and references added. v3: BPS
nature of quarks correcte
Activity as the Main Explanation of Light Variations of <i>o</i> And — Towards a Rotational Modulation Model
Observations carried out for 5 years on o And show that 65 to 85% of its light variations can be described by a double wave: A sin (2πt/P1 + ϕ1) + B sin (2πt/P2 + ϕ2) + C with P1 ≈ 1.6 d = 2P2. When determined independently, P1 and P2 are always found in a 2:1 ratio (within 1%), while they can vary together by as much as 4%. The peak to peak amplitudes of this double-wave fit lay between 40 and 140 mmag (and can even be reduced to less than 10 mmag - our 1987 observations). The rest of the light variations do not show any permanent period or behavior, although a ∼ 2.3 d. (i.e. ≈ 3P1/2) period is frequently detected. Sometimes a marginal ∼ 6 d. period or time constant has been detected.In spite of the quality of our photometric data, the precision on the periods and amplitudes obtained over a few nights is never increased by longer observations: our phase diagrams show significant irregular displacements around the average double-wave analytical solution if we include longer data strings (Fig. 1). This phenomenon was already apparent in our 1992 study (Sareyan et al., 1998): the star shows real irregular behaviour superimposed onto its double-wave "mean" light curve; these changes may show up as a progressive, or sometimes abrupt, modification of the shape of the double-wave light curve (Fig. 1)
A Singular Conformal Spacetime
The infinite cosmological "constant" limit of the de Sitter solutions to
Einstein's equation is studied. The corresponding spacetime is a singular,
four-dimensional cone-space, transitive under proper conformal transformations,
which constitutes a new example of maximally-symmetric spacetime. Grounded on
its geometric and thermodynamic properties, some speculations are made in
connection with the primordial universe.Comment: RevTeX4, 10 pages, 1 eps figure. Presentation changes, including a
new title; section II.E, on the thermodynamic properties of the de Sitter
horizon, completely revised. Version to be published in Journal of Geometry
and Physic
Long range effects on the optical model of 6He around the Coulomb barrier
We present an optical model (OM) analysis of the elastic scattering data of
the reactions 6He+27Al and 6He+208Pb at incident energies around the Coulomb
barrier. The bare part of the optical potential is constructed microscopically
by means of a double folding procedure, using the Sao Paulo prescription
without any renormalization. This bare interaction is supplemented with a
Coulomb dipole polarization (CDP) potential, which takes into account the
effect of the dipole Coulomb interaction. For this CDP potential, we use an
analytical formula derived from the semiclassical theory of Coulomb excitation.
The rest of the optical potential is parametrized in terms of Woods-Saxon
shapes. In the 6He+208Pb case, the analysis confirms the presence of long range
components, in agreement with previous works. Four-body Continuum-Discretized
Coupled-Channels calculations have been performed in order to better understand
the features of the optical potentials found in the OM analysis. This study
searches to elucidate some aspects of the optical potential of weakly bound
systems, such as the dispersion relation and the long range (attractive and
absorptive) mechanisms.Comment: Accepted in Nucl. Phys. A; 26 pages, 8 figures, 6 tables
Galaxy rotation curves: the effect of j x B force
Using the Galaxy as an example, we study the effect of j x B force on the
rotational curves of gas and plasma in galaxies. Acceptable model for the
galactic magnetic field and plausible physical parameters are used to fit the
flat rotational curve for gas and plasma based on the observed baryonic
(visible) matter distribution and j x B force term in the static MHD equation
of motion. We also study the effects of varied strength of the magnetic field,
its pitch angle and length scale on the rotational curves. We show that j x B
force does not play an important role on the plasma dynamics in the
intermediate range of distances 6-12 kpc from the centre, whilst the effect is
sizable for larger r (r > 15 kpc), where it is the most crucial.Comment: Accepted for publication in Astrophysics & Space Science (final
printed version, typos in proofs corrected
Comparative plastome genomics and phylogenomics of Brachypodium: flowering time signatures, introgression and recombination in recently diverged ecotypes
Few pan-genomic studies have been conducted in plants, and none of them have focused on the intraspecific diversity and evolution of their plastid genomes. We address this issue in Brachypodium distachyon and its close relatives B. stacei and B. hybridum, for which a large genomic data set has been compiled. We analyze inter- and intraspecific plastid comparative genomics and phylogenomic relationships within a family-wide framework. Major indel differences were detected between Brachypodium plastomes. Within B. distachyon, we detected two main lineages, a mostly Extremely Delayed Flowering (EDF+) clade and a mostly Spanish (S+) – Turkish (T+) clade, plus nine chloroplast capture and two plastid DNA (ptDNA) introgression and micro-recombination events. Early Oligocene (30.9 million yr ago (Ma)) and Late Miocene (10.1 Ma) divergence times were inferred for the respective stem and crown nodes of Brachypodium and a very recent Mid-Pleistocene (0.9 Ma) time for the B. distachyon split. Flowering time variation is a main factor driving rapid intraspecific divergence in B. distachyon, although it is counterbalanced by repeated introgression between previously isolated lineages. Swapping of plastomes between the three different genomic groups, EDF+, T+, S+, probably resulted from random backcrossing followed by stabilization through selection pressure
- …