3,551 research outputs found
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
Metastable Vacua in Brane Worlds
We analyze vacuum decay in brane world setups, where a free scalar field in
five dimensions has a localized potential admitting metastable vacua. We study
in particular the bounce solution and its properties in flat and warped spaces.
In the latter case, placing into a deeply warped region the term in the
potential that lifts the vacuum degeneracy, can increase indefinitely the
lifetime of the false vacuum. We discuss the application to metastable vacua in
supersymmetric brane-world constructions.Comment: LaTeX, 24 page
Implementation of an experimental platform for the social internet of things
The convergence of the Internet of Things (IoT) technologies with the social networking concepts has led to a new paradigm called the Social Internet of Things (SIoT), where the objects mimic the human behavior and create their own relationships based on the rules set by their owner. This is aimed at simplifying the complexity in handling the communications between billions of objects to the benefits of the humans. Whereas several IoT platforms are already available, the SIoT paradigm has represented only a field for pure research and simulations, until now. The aim of this paper is to present our implementation of a SIoT platform. We begin by analyzing the major IoT implementations, pointing out their common characteristics that could be re-used for our goal. We then discuss the major extensions we had to introduce on the existing platforms to introduce the functionalities of the SIoT. We also present the major functionalities of the proposed system: how to register a new social object to the platform, how the system manages the creation of new relationships, and how the devices create groups of members with similar characteristics. We conclude with the description of possible simple application scenarios
The confining trailing string
We extend the holographic trailing string picture of a heavy quark to the
case of a bulk geometry dual to a confining gauge theory. We compute the
classical trailing confining string solution for a static as well as a
uniformly moving quark. The trailing string is infinitely extended and
approaches a confining horizon, situated at a critical value of the radial
coordinate, along one of the space-time directions, breaking boundary
rotational invariance. We compute the equations for the fluctuations around the
classical solutions, which are used to obtain boundary force correlators
controlling the Langevin dynamics of the quark. The imaginary part of the
correlators has a non-trivial low-frequency limit, which gives rise to a
viscous friction coefficient induced by the confining vacuum. The vacuum
correlators are used to define finite-temperature dressed Langevin correlators
with an appropriate high-frequency behavior.Comment: 63 pages plus appendices, 19 figures; version accepted for
publication in JHE
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
Scalar-tensor theories, trace anomalies and the QCD-frame
We consider the quantum effects of matter fields in scalar-tensor theories
and clarify the role of trace anomaly when switching between conformally
related `frames'. We exploit the property that the couplings between the scalar
and the gauge fields are not frame-invariant in order to define a `QCD-frame',
where the scalar is not coupled to the gluons. We show that this frame is a
natural generalization of the `Jordan frame' in the case of non-metric theories
and that it is particularly convenient for gravitational phenomenology: test
bodies have trajectories that are as close as possible to geodesics with
respect to such a metric and equivalence principle violations are directly
proportional to the scalar coupling parameters written in this frame. We show
how RG flow and decoupling work in metric and non-metric theories. RG-running
commutes with the operation of switching between frames at different scales.
When only matter loops are considered, our analysis confirms that metricity is
stable under radiative corrections and shows that approximate metricity is
natural in a technical sense.Comment: 10 pages. Minor changes to the main text, appendix added. To appear
on PR
An integration scheme for reaction-diffusion models
A detailed description and validation of a recently developed integration
scheme is here reported for one- and two-dimensional reaction-diffusion models.
As paradigmatic examples of this class of partial differential equations the
complex Ginzburg-Landau and the Fitzhugh-Nagumo equations have been analyzed.
The novel algorithm has precision and stability comparable to those of
pseudo-spectral codes, but it is more convenient to employ for systems with
quite large linear extention . As for finite-difference methods, the
implementation of the present scheme requires only information about the local
enviroment and this allows to treat also system with very complicated boundary
conditions.Comment: 14 page, Latex - 4 EPS Figs - Submitted to Int. J. Mod. Phys.
Grammars of Urban Space: A. Perret’s Project for Place de l’Hôtel de Ville in Le Havre
This study proposes to recognize that ‘idea of city’ at the foundation of the A. Perret’s project for Le Havre. This experience seems to be defined as a paradigm in the relationship between building typology, urban morphology and physical geography: the aggregation of residential collective buildings defines those monumental places through which the city represents itself in front of those great forms of physical geography in which the project recognizes the identity of the locus.
This foundational relationship seems however to be achieved through the establishment of an appropriate grammar of urban form, through wich the ‘internal’ space of the city defines a significant relation with the ‘external’ spaces of the nature and with the great forms of physical geography.
In the Place de l'Hôtel de Ville this grammar manifests itself in the most eloquent expression. In this place the city, thanks to the extraordinary expansion of the open space, defines its relation with the cliff, which in this way is introjected in the urban form. On the other hand, the definition of those spaces more contracts, such as the courts of the residential collective building, seems to affirm the value of the circumscribed space. The research of A. Perret is therefore oriented to define the correct syntactic relationships between spaces of different signification: the analytical definition of the parts that make up the edge of the void and the typological variation of these builings makes intelligible the relations between the circumscribed spaces of residences and the dilated size of public places. In this way the urban space assumes the urbanity and the condition of finiteness of the historical city, but defines a significant relationship with the outdoor spaces of nature, thus acquiring the ‘dimension’ of the contemporary city
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