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 $L$. 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