144 research outputs found
D-branes, Orientifolds and K-theory
The complete D-brane spectrum in ZZ[squared] orientifolds is computed. Stable non-BPS D-branes with both integral and torsion charges are found. The relation to K-theory is discussed and a new K-theory relevant to orientifolds is suggested
Transformación de arsénico por bacterias aisladas de sedimentos enriquecidos con el metaloide
Las bacterias presentes en ambientes contaminados con arsénico son capaces de resistir esta presión ambiental y adaptarse mediante mecanismos de resistencia, lo que involucra, entre otros, la oxidación y/o reducción del metaloide. El objetivo del presente trabajo fue investigar las propiedades redox en bacterias tolerantes a arsénico, aisladas de sedimentos naturales. Los sedimentos se obtuvieron del río Camarones(I región, Chile). Las muestras se incubaron en medio mineral durante 7 días, a temperatura ambiente. Diluciones apropiadas fueron sembradas en agar R2A se incubaron a 25 ºC, hasta 7 días. Los niveles de tolerancia para cada metal se determinaron mediante dilución seriada en placa. La propiedad de oxidar o reducir arsénico se investigó mediante nitrato de plata. Los resultados muestran la presencia de bacterias resistentes a arsenito y arseniato, con niveles de tolerancia > 8mM. El 36,84% de las cepas aisladas de sedimentos ricos en arsénico presentaron actividades reductoras(>50%). En cambio, en bacterias aisladas de sedimentoscon bajas concentraciones del metaloide, la actividad reductora varió entre un 10% y 25%. Se puede concluir que la presencia de arsénico selecciona bacterias con la capacidad de tolerar concentraciones altas de arsénico a través de la transformación de As(V) en As(III)
CFT/CFT interpolating RG flows and the holographic c-function
We consider holographic RG flows which interpolate between non-trivial
ultra-violet (UV) and infra-red (IR) conformal fixed points. We study the
``superpotentials'' which characterize different flows and discuss their
expansions near the fixed points. Then we focus on the holographic -function
as defined from the two-point function of the stress-energy tensor. We point
out that the equation for the metric fluctuations in the background flow is
equivalent to a scattering problem and we use this to obtain an expression for
the -function in terms of the associated Jost functions. We propose two
explicit models that realize UV-IR flows. In the first example we consider a
thin wall separating two AdS spaces with different radii, while in the second
one the UV region is replaced with an asymptotically AdS space. We find that
the holographic -function obeys the expected properties. In particular it
reduces to the correct -- nonzero -- central charge in the IR limit.Comment: 20 pages, discussion at the end of sec. 3 and sec. 4.1 change
Constructing Self-Dual Strings
We present an ADHMN-like construction which generates self-dual string
solutions to the effective M5-brane worldvolume theory from solutions to the
Basu-Harvey equation. Our construction finds a natural interpretation in terms
of gerbes, which we develop in some detail. We also comment on a possible
extension to stacks of multiple M5-branes.Comment: 1+19 pages, presentation improved, minor corrections, published
versio
Supersymmetric 3-branes on smooth ALE manifolds with flux
We construct a new family of classical BPS solutions of type IIB supergravity
describing 3-branes transverse to a 6-dimensional space with topology R^2*ALE.
They are characterized by a non-trivial flux of the supergravity 2-forms
through the homology 2-cycles of a generic smooth ALE manifold. Our solutions
have two Killing spinors and thus preserve N=2 supersymmetry. They are
expressed in terms of a quasi harmonic function H (the ``warp factor''), whose
properties we study in the case of the simplest ALE, namely the Eguchi-Hanson
manifold. The equation for H is identified as an instance of the confluent Heun
equation. We write explicit power series solutions and solve the recurrence
relation for the coefficients, discussing also the relevant asymptotic
expansions. While, as in all such N=2 solutions, supergravity breaks down near
the brane, the smoothing out of the vacuum geometry has the effect that the
warp factor is regular in a region near the cycle. We interpret the behavior of
the warp factor as describing a three-brane charge ``smeared'' over the cycle
and consider the asymptotic form of the geometry in that region, showing that
conformal invariance is broken even when the complex type IIB 3-form field
strength is assumed to vanish. We conclude with a discussion of the basic
features of the gauge theory dual.Comment: latex, 36 pages (26 + appendix). Some statements on the gauge dual
made more precise. Final version to appear on NP
Magnetized Particle Capture Cross Section for Braneworld Black Hole
Capture cross section of magnetized particle (with nonzero magnetic moment)
by braneworld black hole in uniform magnetic field is considered. The magnetic
moment of particle was chosen as it was done by \citet{rs99} and for the
simplicity particle with zero electric charge is chosen. It is shown that the
spin of particle as well as the brane parameter are to sustain the stability of
particles circularly orbiting around the black hole in braneworld i.e. spin of
particles and brane parameter try to prevent the capture by black hole.Comment: 7 pages, 4 figures, Accepted for publication in Astrophysics & Space
Scienc
RG flows from Spin(7), CY 4-fold and HK manifolds to AdS, Penrose limits and pp waves
We obtain explicit realizations of holographic renormalization group (RG)
flows from M-theory, from E^{2,1} \times Spin(7) at UV to AdS_4 \times
\tilde{S^7} (squashed S^7) at IR, from E^{2,1} \times CY4 at UV to AdS_4 \times
Q^{1,1,1} at IR, and from E^{2,1} \times HK (hyperKahler) at UV to AdS_4 \times
N^{0,1,0} at IR. The dual type IIA string theory configurations correspond to
D2-D6 brane systems where D6 branes wrap supersymmetric four-cycles. We also
study the Penrose limits and obtain the pp-wave backgrounds for the above
configurations. Besides, we study some examples of non-supersymmetric and
supersymmetric flows in five-dimensional gauge theories.Comment: 42 pages, 6 eps figures, typos and misprints correcte
Renormalization Group Flows from D=3, N=2 Matter Coupled Gauged Supergravities
We study holographic RG flows of N=2 matter couple AdS_3 supergravities which
admit both compact and non-compact sigma manifolds. For the compact case the
supersymmetric domain wall solution interpolates between a conformal IR region
and flat spacetime and this corresponds to a deformation of the CFT by an
irrelevant operator. When it is non-compact, the solution can be interpreted as
a flow between an UV fixed point and a non-conformal(singular) IR region. This
is an exact example of a deformation flow when the singularity is physical. We
also find a non-supersymmetric deformation flow when the scalar potential has a
second AdS vacua. The ratio of the central charges is rational for certain
values of the size of the sigma model. Next, we analyze the spectrum of a
massless scalar on our background by transforming the problem into Schroedinger
form. The spectrum is continuous for the compact model, yet it can be both
continuous (with or without mass gap) and discrete otherwise. Finally, 2-point
functions are computed for two examples whose quantum mechanical potentials are
of Calogero type.Comment: 15 pages, 5 figures, Latex; v2 minor improvements, references added;
v3 a footnote added, typos correcte
Electromagnetic Fields of Slowly Rotating Compact Magnetized Stars in Braneworld
We study the structure of electromagnetic field of slowly rotating magnetized
star in a Randall-Sundrum II type braneworld. The star is modeled as a sphere
consisting of perfect highly magnetized fluid with infinite conductivity and
frozen-in dipolar magnetic field. Maxwell's equations for the external magnetic
field of the star in the braneworld are analytically solved in approximation of
small distance from the surface of the star. We have also found numerical
solution for the electric field outside the rotating magnetized neutron star in
the braneworld in dependence on brane tension. The influence of brane tension
on the electromagnetic energy losses of the rotating magnetized star is
underlined. Obtained "brane" corrections are shown to be relevant and have
non-negligible values. In comparison with astrophysical observations on pulsars
spindown data they may provide an evidence for the brane tension and, thus,
serve as a test for the braneworld model of the Universe.Comment: 11 pages, 5 figure
The antiferromagnetic phi4 Model, I. The Mean-field Solution
Certain higher dimensional operators of the lagrangian may render the vacuum
inhomogeneous. A rather rich phase structure of the phi4 scalar model in four
dimensions is presented by means of the mean-field approximation. One finds
para- ferro- ferri- and antiferromagnetic phases and
commensurate-incommensurate transitions. There are several particles described
by the same quantum field in a manner similar to the species doubling of the
lattice fermions. It is pointed out that chiral bosons can be introduced in the
lattice regularized theory.Comment: To appear in Phys. Rev.
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