1,147 research outputs found
Anti-de Sitter branes with Neveu-Schwarz and Ramond-Ramond backgrounds
We review some facts about AdS2xS2 branes in AdS3xS3 with a Neveu-Schwarz
background, and consider the case of Ramond-Ramond backgrounds. We compute the
spectrum of quadratic fluctuations in the low-energy approximation and discuss
the open-string geometry.Comment: 8 pages, uses JHEP3.cl
Geroch group for Einstein spaces and holographic integrability
We review how Geroch's reduction method is extended from Ricci-flat to
Einstein spacetimes. The Ehlers-Geroch SL(2,R) group is still present in the
three-dimensional sigma-model that captures the dynamics, but only a subgroup
of it is solution-generating. Holography provides an alternative
three-dimensional perspective to integrability properties of Einstein's
equations in asymptotically anti-de Sitter spacetimes. These properties emerge
as conditions on the boundary data (metric and energy-momentum tensor) ensuring
that the hydrodynamic derivative expansion be resummed into an exact
four-dimensional Einstein geometry. The conditions at hand are invariant under
a set of transformations dubbed holographic U-duality group. The latter fills
the gap left by the Ehlers-Geroch group in Einstein spaces, and allows for
solution-generating maps mixing e.g. the mass and the nut charge.Comment: v1: 1+24 pages, Latex, imbrication with arXiv:1403.6511 in sections 2
and 3. arXiv admin note: text overlap with arXiv:1510.0645
On the Heterotic Effective Action at One-Loop, Gauge Couplings and the Gravitational Sector
We present in detail the procedure for calculating the heterotic one-loop
effective action. We focus on gravitational and gauge couplings. We show that
the two-derivative couplings of the gravitational sector are not renormalized
at one loop when the ground state is supersymmetric. Arguments are presented
that this non-renormalization theorem persists to all orders in perturbation
theory. We also derive the full one-loop correction to the gauge coupling. For
a class of ground states, namely those that are obtained by toroidal
compactification to four dimensions of generic six-dimensional models, we
give an explicit formula for the gauge-group independent thresholds, and show
that these are equal within the whole family.Comment: LateX, 17pp. A minor correction mad
A reconfigurable optical header recognition system for optical packet routing applications
We demonstrate a reconfigurable all-optical packet processing system. The key device is a code-reconfigurable header decoder based on a fiber Bragg grating. The performance of the system is tested for different packet headers, and error-free operation is confirmed
Rapidly reconfigurable optical phase encoder-decoders based on fiber Bragg gratings
We demonstrate the capacity for fast dynamic reconfiguration of optical code-division multiple access (OCDMA) phase en/decoders based on fiber Bragg gratings and a thermal phase-tuning technique. The tuning time between two different phase codes is measured to be less than 2 s. An OCDMA system using tunable-phase decoders is compared with a system using fixed-phase decoders and, although the system using fixed-phase decoders exhibits a shorter output autocorrelation pulsewidth and lower sidelobes, the system using tunable-phase decoders has advantages of flexibility and a more relaxed requirement on the input pulsewidth
Holographic perfect fluidity, Cotton energy-momentum duality and transport properties
We investigate background metrics for 2+1-dimensional holographic theories
where the equilibrium solution behaves as a perfect fluid, and admits thus a
thermodynamic description. We introduce stationary perfect-Cotton geometries,
where the Cotton--York tensor takes the form of the energy--momentum tensor of
a perfect fluid, i.e. they are of Petrov type D_t. Fluids in equilibrium in
such boundary geometries have non-trivial vorticity. The corresponding bulk can
be exactly reconstructed to obtain 3+1-dimensional stationary black-hole
solutions with no naked singularities for appropriate values of the black-hole
mass. It follows that an infinite number of transport coefficients vanish for
holographic fluids. Our results imply an intimate relationship between
black-hole uniqueness and holographic perfect equilibrium. They also point
towards a Cotton/energy--momentum tensor duality constraining the fluid
vorticity, as an intriguing boundary manifestation of the bulk mass/nut
duality.Comment: V3: 1+39 pages, JHEP versio
Anti-de-Sitter D-branes
We study D-branes of the {SL(2,\bR)} WZW model, and of its discrete
orbifolds. Gluing the currents by group automorphisms leads to three types of
D-branes: two-dimensional hyperbolic planes (), de Sitter branes (dS),
or anti-de-Sitter branes (AdS). We explain that the dS branes are
unphysical, because the electric field on their worldvolume is supercritical.
By combining {SL(2,\bR)} and SU(2), we exhibit a class of supersymmetric
AdS three-brane worldvolumes, and consider their possible
embeddings in the near-horizon region of stringy black holes. We point out the
intriguing difference between the induced and the effective geometries of these
D-branes, and speculate on its possible significance.Comment: 34 pages, 6 eps figures, latex, uses JHEP.cls. Final version to
appear in JHEP. Minor corrections, and references adde
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