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J. Santos

Publications and source records attributed to J. Santos.

At least 19 recordsLinked to original sources
arXiv2022

PlomBOX -- development of a low-cost CMOS device for environmental monitoring

This paper reports on the development of a novel CMOS device employing lead-sensing bacteria to assay lead in drinking water. The objective of the PlomBOX project is to develop a low-cost sensor (£10) which can expedite access to on-demand assay methods and thus help mitigate lead intake through contaminated drinking water. The project follows three development paths: a) Certain bacteria can fluoresce or change colour when in the presence of lead. A genetically modified strain of Escherichia coli sensitive to lead concentrations up to 10 ppb is being developed. This constitutes the biosensor that fluoresces in proportion to the presence of lead. b) Bacteria response is imaged using a microprocessor (ESP32) with a camera module. This constitutes the optical metrology component of the PlomBOX. c) Data acquisition and control of the PlomBOX is achieved through a Bluetooth connection with the PlomApp, a custom-developed mobile phone application. Data are sent from the PlomApp to a database where a bespoke automated analysis software provides a result of the lead concentration in a sample of water. This paper reports on the instrumentation challenges of developing the electronics for the PlomBOX and on the first prototype.

physics.ins-det↗
arXiv2021

Tolman-Ehrenfest-Klein Law in non-Riemannian geometries

Heat always flows from hotter to a colder temperature until thermal equilibrium be finally restored in agreement with the usual (zeroth, first and second) laws of thermodynamics. However, Tolman and Ehrenfest demonstrated that the relation between inertia and weight uniting all forms of energy in the framework of general relativity implies that the standard equilibrium condition is violated in order to maintain the validity of the first and second law of thermodynamics. Here we demonstrate that the thermal equilibrium condition for a static self-gravitating fluid, besides being violated, is also heavily dependent on the underlying spacetime geometry (whether Riemannian or non-Riemannian). As a particular example, a new equilibrium condition is deduced for a large class of Weyl and f(R) type gravity theories. Such results suggest that experiments based on the foundations of the heat theory (thermal sector) may also be used for confronting gravity theories and prospect the intrinsic geometric nature of the spacetime structure.

gr-qc↗
arXiv2020

Over-critical sharp-gradient plasma slab produced by the collision of laser-induced blast-waves in a gas jet; Application to high-energy proton acceleration

The generation of thin and high density plasma slabs at high repetition rate is a key issue for ultra-high intensity laser applications. We present a scheme to create such plasma slabs, based on the propagation and collision in a gas jet of two counter-propagating blast waves (BW). Each BW is launched by a sudden and local heating induced by a nanosecond laser beam that propagates along the side of the jet. The resulting cylindrical BW expands perpendicular to the beam. The shock front, bent by the gas jet density gradient, pushes and compresses the plasma toward the jet center. By using two parallel ns laser beams, this scheme enables to tailor independently two opposite sides of the jet, while avoiding the damage risks associated with counterpropagating laser beams. A parametric study is performed using two and three dimensional hydrodynamic, as well as kinetic simulations. The BWs bending combined with the collision in a stagnation regime increases the density by more than 10 times and generates a very thin (down to few microns), near to over-critical plasma slab with a high density contrast (> 100), and a lifetime of a few hundred picoseconds. Two dimensional particle-in-cell simulations are used to study the influence of plasma tailoring on proton acceleration by a high-intensity sub-picosecond laser pulse. Tailoring the plasma not only at the entrance but also the exit side of the ps-pulse enhances the proton beam collimation, increases significantly the number of high energy protons, as well as their maximum energy.

physics.plasm-ph↗
arXiv2019

Tailoring topological order and π-conjugation to engineer quasi-metallic polymers

Topological band theory provides a conceptual framework to predict or even engineer robust metallic states at the boundaries of topologically distinct phases. The bulk-boundary correspondence requires that a topological electronic phase transition between two insulators must proceed via closing of the electronic gap. Therefore, it can provide a conceptual solution to the instability of metallic phases in π-conjugated 1D polymers. In this work we predict and demonstrate that a clever design and on-surface synthesis of polymers consisting of 1D linearly bridged polyacene moieties, can position the resulting polymer near the topological transition from a trivial to a non-trivial quantum phase featuring a very narrow bandgap with in-gap zero-energy edge-states at the topologically non-trivial phase. We also reveal the fundamental connection between topological classes and electronic transformation of 1D π-conjugated polymers.

cond-mat.mes-hall↗
arXiv2018

When are the Hardy-Littlewood inequalities contractive?

The optimal constants of the $m$-linear Bohnenblust-Hille and Hardy-Littlewood inequalities are still not known despite its importance in several fields of Mathematics. For the Bohnenblust-Hille inequality and real scalars it is well-known that the optimal constants are not contractive. In this note, among other results, we show that if we consider sums over $M:=M(m)$ indexes with $M\log M=o(m)$, the optimal constants are contractive. For instance, we can consider% \[ M=\left\lfloor \frac{m}{\left( \log m\right) ^{1+\frac{1}{\log\log\log m}}% }\right\rfloor \] where $\lfloor x\rfloor:=\max\{n\in\mathbb{N}:n\leq x\}.$ In particular, if $\varepsilon>0$ and $M:=M(m)\leq m^{1-\varepsilon},$ then the Bohnenblust-Hille inequality restricted to sums over $M$ indexes is contractive.

math.FA↗
arXiv2016

Optimal Hardy--Littlewood inequalities uniformly bounded by a universal constant

The Hardy--Littlewood inequality for $m$-linear forms on $\ell _{p}$ spaces and $m<p\leq 2m$ asserts that \begin{equation*} \left( \sum_{j_{1},...,j_{m}=1}^{\infty }\left\vert T\left( e_{j_{1}},\ldots ,e_{j_{m}}\right) \right\vert ^{\frac{p}{p-m}}\right) ^{\frac{p-m}{p}}\leq 2^{\frac{m-1}{2}}\left\Vert T\right\Vert \end{equation*} for all continuous $m$-linear forms $T:\ell _{p}\times \cdots \times \ell _{p}\rightarrow \mathbb{R}$ or $\mathbb{C}.$ The case $m=2$ recovers a classical inequality proved by Hardy and Littlewood in 1934. As a consequence of the results of the present paper we show that the same inequality is valid with $2^{\frac{m-1}{2}}$ replaced by $2^{\frac{\left( m-1\right) \left( p-m\right) }{p}}$. In particular, for $m<p\leq m+1$ the optimal constants of the above inequality are uniformly bounded by $2.$

math.FA↗
arXiv2016

Estimating the index of summability of pairs of Banach spaces

Let $E_{1},...,E_{m},F$ be Banach spaces. The index of summability of $\left(E_{1}\times\cdots\times E_{m},F\right) $ is a kind of measure of how far the $m$-linear operators $T:E_{1}\times\cdots\times E_{m}\rightarrow F$ are from being multiple summing. In this paper we estimate the index of summability of several pairs of Banach spaces$.$

math.FA↗
arXiv2016

An index of summability for pairs of Banach spaces

We introduce the notion of index of summability for pairs of Banach spaces; for Banach spaces E; F, this index plays the role of a kind of measure of how the m-homogeneous polynomials from E to F are far from being absolutely summing. In some cases the optimal index of summability is computed.

math.FA↗
arXiv2015

The XXL Survey: I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme

We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to date and involving an international consortium of roughly 100 members. The XXL Survey covers two extragalactic areas of 25 deg2 each at a point-source sensitivity of ~ 5E-15 erg/sec/cm2 in the [0.5-2] keV band (completeness limit). The survey's main goals are to provide constraints on the dark energy equation of state from the space-time distribution of clusters of galaxies and to serve as a pathfinder for future, wide-area X-ray missions. We review science objectives, including cluster studies, AGN evolution, and large-scale structure, that are being conducted with the support of approximately 30 follow-up programmes. We describe the 542 XMM observations along with the associated multi-lambda and numerical simulation programmes. We give a detailed account of the X-ray processing steps and describe innovative tools being developed for the cosmological analysis. The paper provides a thorough evaluation of the X-ray data, including quality controls, photon statistics, exposure and background maps, and sky coverage. Source catalogue construction and multi-lambda associations are briefly described. This material will be the basis for the calculation of the cluster and AGN selection functions, critical elements of the cosmological and science analyses. The XXL multi-lambda data set will have a unique lasting legacy value for cosmological and extragalactic studies and will serve as a calibration resource for future dark energy studies with clusters and other X-ray selected sources. With the present article, we release the XMM XXL photon and smoothed images along with the corresponding exposure maps. The XMM XXL observation list (Table B.1) is available in electronic form at the CDS. The present paper is the first in a series reporting results of the XXL-XMM survey.

astro-ph.CO↗
arXiv2015

Absolutely summing multilinear operators via interpolation

We use an interpolative technique from \cite{abps} to introduce the notion of multiple $N$-separately summing operators. Our approach extends and unifies some recent results; for instance we recover the best known estimates of the multilinear Bohnenblust-Hille constants due to F. Bayart, D. Pellegrino and J. Seoane-Sepúlveda. More precisely, as a consequence of our main result, for $1\leq t<2$ and $m\in \mathbb{N}$ we prove that $$ \left(\sum_{i_{1},\dots,i_{m}=1}^{\infty}\left\vert U\left(e_{i_{1}},\dots,e_{i_{m}}\right) \right\vert^{\frac{2tm}{2+(m-1)t}}\right)^{\frac{2+(m-1)t}{2tm}} \leq \left[\prod_{j=2}^{m}Γ\left(2-\frac{2-t}{jt-2t+2}\right) ^{\frac{t(j-2)+2}{2t-2jt}}\right] \left\Vert U\right\Vert $$ for all complex $m$-linear forms $U:c_{0}\times \cdot \cdot \cdot \times c_{0}\rightarrow \mathbb{C}$.

math.FA↗
arXiv2012

Cosmology with Hu-Sawicki gravity in Palatini Formalism

Cosmological models based on f(R)-gravity may exhibit a natural acceleration mechanism without introducing a dark energy component. In this paper, we investigate cosmological consequences of the so-called Hu-Sawicki f(R)-gravity in the Palatini formalism. We derive theoretical constraints on the model parameters and perform a statistical analysis to determine the parametric space allowed by current observational data. We find that this class of models is indistinguishable from the standard ΛCDM model at the background level. Differently, from previous results in the metric approach, we show that these scenarios are able to produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

astro-ph.CO↗
arXiv2012

When is the Haar measure a Pietsch measure for nonlinear mappings?

We show that, as in the linear case, the normalized Haar measure on a compact topological group $G$ is a Pietsch measure for nonlinear summing mappings on closed translation invariant subspaces of $C(G)$. This answers a question posed to the authors by J. Diestel. We also show that our result applies to several well-studied classes of nonlinear summing mappings. In the final section some problems are proposed.

math.FA↗
arXiv2010

Cosmological Consequences of Exponential Gravity in Palatini Formalism

We investigate cosmological consequences of a class of exponential $f(R)$ gravity in the Palatini formalism. By using the current largest type Ia Supernova sample along with determinations of the cosmic expansion at intermediary and high-$z$ we impose tight constraints on the model parameters. Differently from other $f(R)$ models, we find solutions of transient acceleration, in which the large-scale modification of gravity will drive the Universe to a new decelerated era in the future. We also show that a viable cosmological history with the usual matter-dominated era followed by an accelerating phase is predicted for some intervals of model parameters.

astro-ph.CO↗
arXiv2010

Simulating the WFXT sky

We investigate the scientific impact of the Wide Field X-ray Telescope mission. We present simulated images and spectra of X-ray sources as observed from the three surveys planned for the nominal 5-year WFXT lifetime. The goal of these simulations is to provide WFXT images of the extragalactic sky in different energy bands based on accurate description of AGN populations, normal and star forming galaxies, groups and clusters of galaxies. The images are realized using a detailed PSF model, instrumental and physical backgrounds/foregrounds, accurate model of the effective area and the related vignetting effect. Thanks to this comprehensive modelization of the WFXT properties, the simulated images can be used to evaluate the flux limits for detection of point and extended sources, the effect of source confusion at very faint fluxes, and in general the efficiency of detection algorithms. We also simulate the spectra of the detected sources, in order to address specific science topics which are unique to WFXT. Among them, we focus on the characterization of the Intra Cluster Medium (ICM) of high-z clusters, and in particular on the measurement of the redshift from the ICM spectrum in order to build a cosmological sample of galaxy clusters. The end-to-end simulation procedure presented here, is a valuable tool in optimizing the mission design. Therefore, these simulations can be used to reliably characterize the WFXT discovery space and to verify the connection between mission requirements and scientific goals. Thanks to this effort, we can conclude on firm basis that an X-ray mission optimized for surveys like WFXT is necessary to bring X-ray astronomy at the level of the optical, IR, submm and radio wavebands as foreseen in the coming decade.

astro-ph.CO↗
arXiv2010

Cluster galaxies in XMMU J2235-2557: galaxy population properties in most massive environments at z~1.4

We present a multi-wavelength study of galaxy populations in the core of the massive, X-ray luminous cluster XMMU J2235 at z=1.39, based on VLT and HST optical and near-infrared photometry. Luminosity functions in the z, H, and Ks bands show a faint-end slope consistent with being flat, and a characteristic magnitude M* close to passive evolution predictions of M* of local massive clusters, with a formation redshift z>2. The color-magnitude and color-mass diagrams show evidence of a tight red sequence of massive galaxies, with overall old stellar populations, generally early-type morphology, typically showing early-type spectral features and rest-frame far-UV emission consistent with very low star formation rates (SFR<0.2Msun/yr). Star forming spectroscopic members, with SFRs of up to ~100Msun/yr, are all located at clustercentric distances >~250kpc, with the central cluster region already appearing effectively quenched. Massive galaxies in the core of this cluster appear to be in an advanced evolutionary stage in terms of both star formation and mass assembly. The high-mass end of the galaxy stellar mass function is essentially already in place, and the stellar mass fraction estimated within r500 (~1%, Kroupa IMF) is already similar to that of local massive clusters. On the other hand, morphological analysis of the massive red sequence galaxies suggests that they are smaller than similarly massive local early-types. While possibly affected by systematics and biases, this result might imply that, in spite of the overall early assembly of these sources, their evolution is not complete, and processes like minor (and likely dry) merging might still shape their structural properties to resemble those of their local counterparts, without substantially affecting their stellar mass or host stellar populations.[abridged]

astro-ph.CO↗
arXiv2010

Violation of causality in f(R) gravity

We examine the question as to whether the f(R) gravity theories, in both metric and in Palatini formalisms, permit space-times in which the causality is violated. We show that the field equations of these f(R) gravity theories admit solutions with violation of causality for a physically well-motivated perfect-fluid matter content.

astro-ph.CO↗
arXiv2010

Gödel-type universes in Palatini f(R) gravity

We examine the question as to whether the Palatini f(R) gravity theories permit space-times in which the causality is violated. We show that every perfect-fluid Gödel-type solution of Palatini f(R) gravity with density $ρ$ and pressure $p$ that satisfy the weak energy condition $ρ+p \geq 0$ is necessarily isometric to the Gödel geometry, demonstrating therefore that these theories present causal anomalies in the form of closed time-like curves. This result extends a theorem on Gödel-type models to the framework of Palatini f(R) gravity theory. We concretely examine the Gödel-type perfect-fluid solutions in specific $f(R) = R - α/R^{n}$ Palatini gravity theory, where the free parameters $α$ and $n$ have been recently constrained by a diverse set of observational data. We show that for positive matter density and for $α$ and $n$ within the interval permitted by the observational data, this theory does not admit the Gödel geometry as a perfect-fluid solution of its field equations. In this sense, this theory remedies the causal pathology in the form of closed time-like curves which is allowed in general relativity. We derive an expression for a critical radius $r_c$ (beyond which the causality is violated) for an arbitrary Palatini f(R) theory, thus making apparent that the violation of causality depends on the form of f(R) and on the matter content components. We also examine the violation of causality of Gödel-type by considering a single scalar field as the matter content. For this source we show that Palatini f(R) gravity gives rise to a unique Gödel-type solution with no violation of causality.

astro-ph.CO↗