1,175 research outputs found
AdS Black Holes from Duality in Gauged Supergravity
We study and utilize duality transformations in a particular STU-model of
four dimensional gauged supergravity. This model is a truncation of the de
Wit-Nicolai N=8 theory and as such has a lift to eleven-dimensional
supergravity on the seven-sphere. Our duality group is and while it
can be applied to any solution of this theory, we consider the known
asymptotically AdS, supersymmetric black holes and focus on duality
transformations which preserve supersymmetry. For static black holes we
generalize the supersymmetric solutions of Cacciatori and Klemm from three
magnetic charges to include two additional electric charges and argue that this
is co-dimension one in the full space of supersymmetric static black holes in
the STU-model. These new static black holes have nontrivial profiles for
axions. For rotating black holes, we generalize the known two-parameter
supersymmetric solution to include an additional parameter which represents
scalar hair. When lifted to M-theory, these black holes correspond to the near
horizon geometry of a stack of BPS rotating M2-branes, spinning on an
which is fibered non-trivially over a Riemann surface.Comment: 21 page
Pressure screening and fluctuations at the bottom of a granular column
We report sets of precise and reproducible measurements on the static
pressure at the bottom of a granular column. We make a quantitative analysis of
the pressure saturation when the column height is increased. We evidence a
great sensitivity of the measurements with the global packing fraction and the
eventual presence of shear bands at the boundaries. We also show the limit of
the classical Janssen model and discuss these experimental results under the
scope of recently proposed theoretical frameworks.Comment: 17 pages, Latex, 8 eps figures, to appear in the European Physical
Journal B (1999
Dressing the Electron Star in a Holographic Superconductor
We construct new asymptotically AdS_4 solutions dual to 2+1 CFTs at finite
density and zero temperature by combining the ingredients of the electron star
and the holographic superconductor. The solutions, which we call "compact
electron stars", contain both a fermionic fluid and charged scalar hair in the
bulk. We show that the new solutions are thermodynamically favoured in the
region of parameter space where they exist. Along the boundary of this region,
we find evidence for a continuous phase transition between the holographic
superconductor and the compact star solution.Comment: 31 pages, 10 figures; added reference
Slow crack growth : models and experiments
The properties of slow crack growth in brittle materials are analyzed both
theoretically and experimentally. We propose a model based on a thermally
activated rupture process. Considering a 2D spring network submitted to an
external load and to thermal noise, we show that a preexisting crack in the
network may slowly grow because of stress fluctuations. An analytical solution
is found for the evolution of the crack length as a function of time, the time
to rupture and the statistics of the crack jumps. These theoretical predictions
are verified by studying experimentally the subcritical growth of a single
crack in thin sheets of paper. A good agreement between the theoretical
predictions and the experimental results is found. In particular, our model
suggests that the statistical stress fluctuations trigger rupture events at a
nanometric scale corresponding to the diameter of cellulose microfibrils.Comment: to be published in EPJ (European Physical Journal
Polarized solutions and Fermi surfaces in holographic Bose-Fermi systems
We use holography to study the ground state of a system with interacting
bosonic and fermionic degrees of freedom at finite density. The gravitational
model consists of Einstein-Maxwell gravity coupled to a perfect fluid of
charged fermions and to a charged scalar field which interact through a
current-current interaction. When the scalar field is non-trivial, in addition
to compact electron stars, the screening of the fermion electric charge by the
scalar condensate allows the formation of solutions where the fermion fluid is
made of antiparticles, as well as solutions with coexisting, separated regions
of particle-like and antiparticle-like fermion fluids. We show that, when the
latter solutions exist, they are thermodynamically favored. By computing the
two-point Green function of the boundary fermionic operator we show that, in
addition to the charged scalar condensate, the dual field theory state exhibits
electron-like and/or hole-like Fermi surfaces. Compared to fluid-only
solutions, the presence of the scalar condensate destroys the Fermi surfaces
with lowest Fermi momenta. We interpret this as a signal of the onset of
superconductivity.Comment: 46 pages, 17 figure
Les monnaies antiques des fouilles d'Annecy (1971 – 2001)
Synthèse rapide des découvertes de monnaies antiques à Annecy entre 1971 et 2001
Un denier celtibèrique à Annecy
Découverte, en juillet 1909, d'un denier des Celtibères à Annecy
Asymptotic network models of subwavelength metamaterials formed by closely packed photonic and phononic crystals
We demonstrate that photonic and phononic crystals consisting of closely
spaced inclusions constitute a versatile class of subwavelength metamaterials.
Intuitively, the voids and narrow gaps that characterise the crystal form an
interconnected network of Helmholtz-like resonators. We use this intuition to
argue that these continuous photonic (phononic) crystals are in fact
asymptotically equivalent, at low frequencies, to discrete capacitor-inductor
(mass-spring) networks whose lumped parameters we derive explicitly. The
crystals are tantamount to metamaterials as their entire acoustic branch, or
branches when the discrete analogue is polyatomic, is squeezed into a
subwavelength regime where the ratio of wavelength to period scales like the
ratio of period to gap width raised to the power 1/4; at yet larger wavelengths
we accordingly find a comparably large effective refractive index. The fully
analytical dispersion relations predicted by the discrete models yield
dispersion curves that agree with those from finite-element simulations of the
continuous crystals. The insight gained from the network approach is used to
show that, surprisingly, the continuum created by a closely packed hexagonal
lattice of cylinders is represented by a discrete honeycomb lattice. The
analogy is utilised to show that the hexagonal continuum lattice has a
Dirac-point degeneracy that is lifted in a controlled manner by specifying the
area of a symmetry-breaking defect
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