Results for "Efthymios Sofos"

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Uniformly counting rational points on conicsMay 02 2013Jun 24 2014We provide an asymptotic estimate for the number of rational points of bounded height on a non-singular conic over the rationals. The estimate is uniform in the coefficients of the underlying quadratic form.
Serre's problem on the density of isotropic fibres in conic bundlesFeb 09 2016May 05 2016Let $\pi:X\to \mathbb{P}^1_{\mathbb{Q}}$ be a non-singular conic bundle over $\mathbb{Q}$ having $n$ non-split fibres and denote by $N(\pi,B)$ the cardinality of the fibres of Weil height at most $B$ that possess a rational point. Serre showed in $1990$ ... More
Rational Points on the Fermat Cubic SurfaceFeb 03 2014We prove a lower bound that agrees with Manin's prediction for the number of rational points of bounded height on the Fermat cubic surface. As an application we provide a simple counterexample to Manin's conjecture over the rationals.
Counting rational points on smooth cubic surfacesSep 22 2014May 20 2016We prove that any smooth cubic surface defined over any number field satisfies the lower bound predicted by Manin's conjecture possibly after an extension of small degree.
Sarnak's saturation problem for complete intersectionsMay 25 2017Aug 24 2018We study almost prime solutions of systems of Diophantine equations in the Birch setting. Previous work shows that there exist integer solutions of size B with each component having no prime divisors below $B^{1/u}$, where $u=c_0n^{3/2}$, $n$ is the number ... More
Generalised divisor sums of binary forms over number fieldsSep 13 2016Sep 16 2016Estimating averages of Dirichlet convolutions $1 \ast \chi$, for some real Dirichlet character $\chi$ of fixed modulus, over the sparse set of values of binary forms defined over $\mathbb{Z}$ has been the focus of extensive investigations in recent years, ... More
An Erdős-Kac law for local solubility in families of varietiesNov 22 2017Aug 12 2018We study probability distributions arising from local obstructions to the existence of $p$-adic points in families of varieties. In certain cases we show that an Erd\H{o}s-Kac type law holds.
Uniformly counting rational points on conicsMay 02 2013Mar 10 2018We provide an asymptotic estimate for the number of rational points of bounded height on a non-singular conic over the rationals. The estimate is uniform in the coefficients of the underlying quadratic form.
4-ranks and the general model for statistics of ray class groups of imaginary quadratic number fieldsOct 20 2017We extend the Cohen-Lenstra heuristics to the setting of ray class groups of imaginary quadratic number fields, viewed as exact sequences of Galois modules. By asymptotically estimating the mixed moments governing the distribution of a cohomology map, ... More
On the diffraction measure of $k$-free integersJul 10 2019We show that the scaling of the total diffraction intensity for the set of $k$-free integers near the origin is asymptotically approximated by a power law. This considerably improves on recent work by Baake and Coons whose main result provides upper and ... More
On the diffraction measure of $k$-free integersJul 10 2019Jul 11 2019We show that the scaling of the total diffraction intensity for the set of $k$-free integers near the origin is asymptotically approximated by a power law. This considerably improves on recent work by Baake and Coons whose main result provides upper and ... More
The density of fibres with a rational point for a fibration over hypersurfaces of low degreeApr 16 2018We prove asymptotics for the proportion of fibres with a rational point in a conic bundle fibration. The basis of the fibration is a general hypersurface of low degree.
On the distribution of the density of maximal order elements in general linear groupsAug 26 2013Apr 19 2014In this paper we consider the density of maximal order elements in $\mathrm{GL}_n(q)$. Fixing any of the rank $n$ of the group, the characteristic $p$ or the degree $r$ of the extension of the underlying field $\mathbb{F}_q$ of size $q=p^r$, we compute ... More
The density of fibres with a rational point for a fibration over hypersurfaces of low degreeApr 16 2018Feb 27 2019We prove asymptotics for the proportion of fibres with a rational point in a conic bundle fibration. The basis of the fibration is a general hypersurface of low degree.
Rational points and prime values of polynomials in moderately many variablesJan 09 2018Sep 21 2018We derive the Hasse principle and weak approximation for pencils of certain varieties in the spirit of work by Colliot-Th\'el\`ene,Sansuc and Harpaz-Skorobogatov-Wittenberg. Our varieties are defined through polynomials in many variables and part of our ... More
Rational points of bounded height on general conic bundle surfacesSep 14 2016Jul 06 2018A conjecture of Manin predicts the asymptotic distribution of rational points of bounded height on Fano varieties. In this paper we use conic bundles to obtain correct lower bounds or a wide class of surfaces over number fields for which the conjecture ... More
Rational points of bounded height on general conic bundle surfacesSep 14 2016A conjecture of Manin predicts the asymptotic distribution of rational points of bounded height on Fano varieties. In this paper we use conic bundles to obtain correct lower bounds or a wide class of surfaces over number fields for which the conjecture ... More
Unbounded twists of Hooley's $Δ$-functionSep 28 2016We prove tight estimates for averages of the twisted Hooley $\Delta$-function over arbitrary number fields.
Twists of Hooley's $Δ$-function over number fieldsSep 28 2016Mar 26 2018We prove tight estimates for averages of the twisted Hooley $\Delta$-function over arbitrary number fields.
An alternative and united proof of a double inequality for bounding the arithmetic-geometric meanFeb 15 2009In the paper, we provide an alternative and united proof of a double inequality for bounding the arithmetic-geometric mean.
Counting rational points on quartic del Pezzo surfaces with a rational conicSep 28 2016Upper and lower bounds, of the expected order of magnitude, are obtained for the number of rational points of bounded height on any quartic del Pezzo surface over $\mathbb{Q}$ that contains a conic defined over $\mathbb{Q}$.
Counting rational points on quartic del Pezzo surfaces with a rational conicSep 28 2016May 23 2018Upper and lower bounds, of the expected order of magnitude, are obtained for the number of rational points of bounded height on any quartic del Pezzo surface over $\mathbb{Q}$ that contains a conic defined over $\mathbb{Q}$.
Dispersive Cylindrical Cloaks under Non-Monochromatic IlluminationMay 18 2009Oct 01 2009Transformation-based cylindrical cloaks and concentrators are illuminated with non-monochromatic waves and unusual effects are observed with interesting potential applications. The transient responses of the devices are studied numerically with the Finite-Difference ... More
Averages of arithmetic functions over principal idealsJun 14 2017Mar 26 2018For a general class of non-negative functions defined on integral ideals of number fields, upper bounds are established for their average over the values of certain principal ideals that are associated to irreducible binary forms with integer coefficients. ... More
Whither probabilistic security management for real-time operation of power systems ?Feb 17 2016This paper investigates the stakes of introducing probabilistic approaches for the management of power system's security. In real-time operation, the aim is to arbitrate in a rational way between preventive and corrective control, while taking into account ... More
Projection-based reduced order models for a cut finite element method in parametrized domainsJan 12 2019This work presents a reduced order modelling technique built on a high fidelity embedded mesh finite element method. Such methods, and in particular the CutFEM method, are attractive in the generation of projection-based reduced order models thanks to ... More
Towards quantifying the role of exact exchange in predictions of transition metal complex propertiesJul 08 2015We estimate the prediction sensitivity with respect to Hartree-Fock exchange in approximate density functionals for representative Fe(II) and Fe(III) octahedral complexes. Based on the observation that the range of parameters spanned by the most widely-employed ... More
A Style Transfer Approach to Source SeparationMay 01 2019May 09 2019Training neural networks for source separation involves presenting a mixture recording at the input of the network and updating network parameters in order to produce an output that resembles the clean source. Consequently, supervised source separation ... More
Ground-Plane Quasi-Cloaking for Free SpaceFeb 10 2009Ground-plane cloak designs are presented, which minimize scattering of electromagnetic radiation from metallic objects in the visible spectrum. It is showed that simplified ground-plane cloaks made from only a few blocks of all-dielectric isotropic materials, ... More
Linear optical properties of solids within the full-potential linearized augmented planewave methodFeb 20 2004We present a scheme for the calculation of linear optical properties by the all-electron full-potential linearized augmented planewave (LAPW) method. A summary of the theoretical background for the derivation of the dielectric tensor within the random-phase ... More
Impurity State and Variable Range Hopping Conduction in GrapheneAug 24 2012Dec 21 2012The variable range hopping theory, as formulated for exponentially localized impurity states, does not necessarily apply in the case of graphene with covalently attached impurities. We analyze the localization of impurity states in graphene using the ... More
Diffusion and Transport Coefficients in Synthetic OpalsOct 22 1999Opals are structures composed of the closed packing of spheres in the size range of nano-to-micro meter. They are sintered to create small necks at the points of contact. We have solved the diffusion problem in such structures. The relation between the ... More
Integrating Recurrence Dynamics for Speech Emotion RecognitionNov 09 2018We investigate the performance of features that can capture nonlinear recurrence dynamics embedded in the speech signal for the task of Speech Emotion Recognition (SER). Reconstruction of the phase space of each speech frame and the computation of its ... More
Dynamic Effective Medium Theory for Two-Dimensional Non-Magnetic Metamaterial Lattices using Multipole ExpansionDec 07 2014Apr 11 2015We present a formulation for deriving effective medium properties of infinitely periodic two-dimensional metamaterial lattice structures beyond the static and quasi-static limits. We utilize the multipole expansions, where the polarization currents associated ... More
Manipulating the Loss in Electromagnetic Cloaks for Perfect Wave AbsorptionNov 26 2008Jan 23 2009We examine several ways to manipulate the loss in electromagnetic cloaks, based on transformation electromagnetics. It is found that, by utilizing inherent electric and magnetic losses of metamaterials, perfect wave absorption can be achieved based on ... More
Continual Learning of New Sound Classes using Generative ReplayJun 03 2019Continual learning consists in incrementally training a model on a sequence of datasets and testing on the union of all datasets. In this paper, we examine continual learning for the problem of sound classification, in which we wish to refine already ... More
Multilayer Thermionic Refrigerator and GeneratorJan 19 1998A new method of refrigeration is proposed. Cooling is obtained by thermionic emission of electrons over periodic barriers in a multilayer geometry. These could be either Schottky barriers between metals and semiconductors or else barriers in a semiconductor ... More
Studies on small charge packet transport in high-resistivity fully-depleted CCDsJun 26 2019In this work, we will present a physical model and measurements of the transport of small charge packets in the bulk of thick high resistivity CCD before being collected by the pixel potential wells. A new technique to measure the lateral spread of the ... More
Electronic structure of the pyrochlore metals Cd2Os2O7 and Cd2Re2O7Aug 14 2001First principles density functional calculations within the local spin density approximation (LSDA) and the generalized gradient approximation (GGA) are reported for pyrochlore Cd$_2$Os$_2$O$_7$ and Cd$_{2}$Re$_{2}$O$_7$. The transition metal $t_{2g}$ ... More
A Reduced Basis approach for PDEs on parametrized geometries based on the Shifted Boundary Finite Element Method and application to a Stokes FlowJul 20 2018Jan 10 2019We propose a model order reduction technique integrating the Shifted Boundary Method (SBM) with a POD-Galerkin strategy. This approach allows to treat more complex parametrized domains in an efficient and straightforward way. The impact of the proposed ... More
Metal-insulator transition in the double perovskitesMay 29 2001We construct an effective Hamiltonian for the motion of electrons among the transition metal ions of ordered double perovskites like Sr2FeMoO6. in which strong intra-atomic Coulomb repulsion U is present in only one of the inequivalent transition metal ... More
The influence of additives in the stoichiometry of hybrid lead halide perovskitesJul 19 2018We investigate the employment of carefully selected solvent additives in the processing of a commercial perovskite precursor ink and analyze their impact on the performance of organometal trihalide perovskite photovoltaic devices. We provide evidence ... More
Characterization of TOF-PET Detectors Based on Monolithic Blocks and ASIC-ReadoutJun 22 2018The aim of this work is to show the potential capabilities of monolithic crystals coupled to large SiPM arrays, to be considered as detector blocks for PET scanners enabling Time Of Flight (TOF) capabilities. Monolithic blocks allow one to decode the ... More
An Investigation of Machine Learning Methods Applied to Structure Prediction in Condensed MatterMay 14 2014Materials characterization remains a significant, time-consuming undertaking. Generally speaking, spectroscopic techniques are used in conjunction with empirical and ab-initio calculations in order to elucidate structure. These experimental and computational ... More
Fully retarded van der Waals interaction between dielectric nanoclustersMar 20 2006The fully retarded dispersion interaction between an atom and a cluster or between two clusters is calculated. Results obtained with two different methods are compared. One is to consider a cluster as a collection of many atoms and evaluate the sum of ... More
Heavy Dirac fermions in a graphene/topological insulator hetero-junctionFeb 29 2016The low energy physics of both graphene and surface states of three-dimensional topological insulators is described by gapless Dirac fermions with linear dispersion. In this work, we predict the emergence of a "heavy" Dirac fermion in a graphene/topological ... More
Achromatic Non-Interferometric Single Grating Neutron Phase ImagingMar 23 2019We demonstrate a simple single grating beam modulation technique, which enables the use of a highly intense neutron beam for phase imaging and thus spatially resolved structural correlation measurements in full analogy to quantum interference based methods. ... More
Bilayer graphene under pressure: Electron-hole Symmetry Breaking, Valley Hall Effect, and Landau LevelsFeb 04 2016The electronic structure of bilayer graphene under pressure develops very interesting features with an enhancement of the trigonal warping and a splitting of the parabolic touching bands at the K point of the reciprocal space into four Dirac cones, one ... More
Photoemission study of the skutterudite compounds Co(Sb$_{1-x}$Te$_{x}$)$_3$ and RhSb$_3$Mar 16 2001Mar 19 2001We have studied the electronic structure of the skutterudite compounds Co(Sb$_{1-x}$Te$_{x}$)$_3$ (x= 0, 0.02, 0.04) by photoemission spectroscopy. Valence-band spectra revealed that Sb 5p states are dominant near the Fermi level and are hybridized with ... More
Novel substrates for Helium adsorption: Graphane and Graphene-FluorideApr 13 2012The discovery of fullerenes has stimulated extensive exploration of the resulting behavior of adsorbed films. Our study addresses the planar substrates graphene-fluoride (GF) and graphane (GH) in comparison to graphene. We present initial results concerning ... More
Static Polarizabilities of Dielectric NanoclustersAug 15 2005A cluster consisting of many atoms or molecules may be considered, in some circustances, to be a single large molecule with a well defined polarizability. Once the polarizability of such a cluster is known, one can evaluate certain properties, e.g. the ... More
Some Boas-Bellman Type Inequalities in 2-Inner Product SpacesAug 28 2003Some inequalities in 2-inner product spaces generalizing Bessel's result that are similar to the Boas-Bellman inequality from inner product spaces, are given. Applications for determinantal integral inequalities are also provided.
Development of a novel neutron detection technique by using a boron layer coating a Charge Coupled DeviceAug 14 2014This article describes the design features and the first test measurements obtained during the installation of a novel high resolution 2D neutron detection technique. The technique proposed in this work consists of a boron layer (enriched in ${^{10}}$B) ... More
Electrical Control of the Chemical Bonding of Fluorine on GrapheneJan 24 2011We study the electronic structure of diluted F atoms chemisorbed on graphene using density functional theory calculations. We show that the nature of the chemical bonding of a F atom adsorbed on top of a C atom in graphene strongly depends on carrier ... More
Magnetic Structure of Hydrogen Induced Defects on GrapheneDec 12 2011Using density functional theory (DFT), Hartree-Fock, exact diagonalization, and numerical renormalization group methods we study the electronic structure of diluted hydrogen atoms chemisorbed on graphene. A comparison between DFT and Hartree-Fock calculations ... More
Influence of the Hole Transporting Layer on the Thermal Stability of Inverted Organic Photovoltaics Using Accelerated Heat Lifetime ProtocolsJul 19 2018High power conversion efficiency (PCE) inverted organic photovoltaics (OPVs) usually use thermally evaporated MoO3 as a hole transporting layer (HTL). Despite the high PCE values reported, stability investigations are still limited and the exact degradation ... More
A Style Transfer Approach to Source SeparationMay 01 2019Training neural networks for source separation involves presenting a mixture recording at the input of the network and updating network parameters in order to produce an output that resembles the clean source. Consequently, supervised source separation ... More
Electrically tunable multiple Dirac cones in thin films of (LaO)2(SbSe2)2 family of materialsSep 12 2014Oct 20 2015Two-dimensional Dirac physics has aroused great interests in condensed matter physics ever since the discovery of graphene and topological insulators due to its importance in both fundamental physics and device applications. The ability to control the ... More
Low energy phases of bilayer Bi predicted by structure search in two dimensionsJan 15 2019We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the ... More
Single-electron and single-photon sensitivity with a silicon Skipper CCDMay 31 2017We have developed a non-destructive readout system that uses a floating-gate amplifier on a thick, fully depleted charge coupled device (CCD) to achieve ultra-low readout noise of 0.068 e- rms/pix. This is the first time that discrete sub-electron readout ... More
SENSEI: First Direct-Detection Constraints on sub-GeV Dark Matter from a Surface RunMar 30 2018Sep 11 2018The Sub-Electron-Noise Skipper CCD Experimental Instrument (SENSEI) uses the recently developed Skipper-CCD technology to search for electron recoils from the interaction of sub-GeV dark matter particles with electrons in silicon. We report first results ... More
Reversible Fluorination of Graphene: towards a Two-Dimensional Wide Bandgap SemiconductorMay 01 2010We report the synthesis and evidence of graphene fluoride, a two-dimensional wide bandgap semiconductor derived from graphene. Graphene fluoride exhibits hexagonal crystalline order and strongly insulating behavior with resistance exceeding 10 G$\Omega$ ... More
Photoluminescence from nanocrystalline graphite monofluorideJul 29 2010We synthesize and study the structural and optical properties of nanocrystalline graphene monofluoride and graphite monofluoride, which are carbon-based wide bandgap materials. Using laser excitations 2.41 - 5.08 eV, we identify six emission modes of ... More
Reduction Signals Method Preserving Spatial and Temporal CapabilitiesJun 22 2018In gamma ray imaging, a scintillation crystal is typically used to convert the gamma radiation into visible light. Photosensors are used to transform this light into measurable signals. Several types of photosensors are currently in use depending on the ... More
SENSEI: Direct-Detection Constraints on Sub-GeV Dark Matter from a Shallow Underground Run Using a Prototype Skipper-CCDJan 29 2019We present new direct-detection constraints on eV-to-GeV dark matter interacting with electrons using a prototype detector of the Sub-Electron-Noise Skipper-CCD Experimental Instrument. The results are based on data taken in the MINOS cavern at the Fermi ... More
Graphane: a two-dimensional hydrocarbonJun 27 2006We predict the stability of a new extended two-dimensional hydrocarbon on the basis of first-principles total energy calculations. The compound that we call graphane is a fully saturated hydrocarbon derived from a single graphene sheet with formula CH. ... More
DAMIC at SNOLABJun 30 2014We introduce the fully-depleted charge-coupled device (CCD) as a particle detector. We demonstrate its low energy threshold operation, capable of detecting ionizing energy depositions in a single pixel down to 50 eVee. We present results of energy calibrations ... More
DAMIC: a novel dark matter experimentOct 24 2013DAMIC (Dark Matter in CCDs) is a novel dark matter experiment that has unique sensitivity to dark matter particles with masses below 10 GeV. Due to its low electronic readout noise (R.M.S. ~3 e-) this instrument is able to reach a detection threshold ... More
Results of the engineering run of the Coherent Neutrino Nucleus Interaction Experiment (CONNIE)Apr 05 2016The CONNIE detector prototype is operating at a distance of 30 m from the core of a 3.8 GW$_{\rm th}$ nuclear reactor with the goal of establishing Charge-Coupled Devices (CCD) as a new technology for the detection of coherent elastic neutrino-nucleus ... More
Status of the DAMIC direct dark matter search experimentSep 30 2015Dec 08 2015The DAMIC experiment uses fully depleted, high resistivity CCDs to search for dark matter particles. With an energy threshold $\sim$50 eV$_{ee}$, and excellent energy and spatial resolutions, the DAMIC CCDs are well-suited to identify and suppress radioactive ... More
Exploring low-energy neutrino physics with the Coherent Neutrino Nucleus Interaction Experiment (CONNIE)Jun 05 2019The Coherent Neutrino-Nucleus Interaction Experiment (CONNIE) uses low-noise fully depleted charge-coupled devices (CCDs) with the goal of measuring low-energy recoils from coherent elastic scattering (CE$\nu$NS) of reactor antineutrinos with silicon ... More
The CONNIE experimentAug 04 2016Oct 10 2016The CONNIE experiment uses fully depleted, high resistivity CCDs as particle detectors in an attempt to measure for the first time the Coherent Neutrino-Nucleus Elastic Scattering of antineutrinos from a nuclear reactor with silicon nuclei.This talk, ... More
The DAMIC dark matter experimentOct 07 2015The DAMIC (Dark Matter in CCDs) experiment uses high resistivity, scientific grade CCDs to search for dark matter. The CCD's low electronic noise allows an unprecedently low energy threshold of a few tens of eV that make it possible to detect silicon ... More
Measurement of radioactive contamination in the high-resistivity silicon CCDs of the DAMIC experimentJun 08 2015Jul 09 2015We present measurements of radioactive contamination in the high-resistivity silicon charge-coupled devices (CCDs) used by the DAMIC experiment to search for dark matter particles. Novel analysis methods, which exploit the unique spatial resolution of ... More
Search for low-mass WIMPs in a 0.6 kg day exposure of the DAMIC experiment at SNOLABJul 25 2016Nov 10 2016We present results of a dark matter search performed with a 0.6 kg day exposure of the DAMIC experiment at the SNOLAB underground laboratory. We measure the energy spectrum of ionization events in the bulk silicon of charge-coupled devices down to a signal ... More
Search for low-mass WIMPs in a 0.6 kg day exposure of the DAMIC experiment at SNOLABJul 25 2016We present results of a dark matter search performed with a 0.6 kg day exposure of the DAMIC experiment at the SNOLAB underground laboratory. We measured the energy spectrum of ionization events in the bulk silicon of charge-coupled devices (CCDs) down ... More
First direct detection constraints on eV-scale hidden-photon dark matter with DAMIC at SNOLABNov 09 2016We present direct detection constraints on the absorption of hidden-photon dark matter with particle masses in the range 1.2-30 eV$c^{-2}$ with the DAMIC experiment at SNOLAB. Under the assumption that the local dark matter is entirely constituted of ... More
First Direct-Detection Constraints on eV-Scale Hidden-Photon Dark Matter with DAMIC at SNOLABNov 09 2016Apr 11 2017We present direct detection constraints on the absorption of hidden-photon dark matter with particle masses in the range 1.2-30 eV$c^{-2}$ with the DAMIC experiment at SNOLAB. Under the assumption that the local dark matter is entirely constituted of ... More
The Latin American Giant Observatory: Contributions to the 34th International Cosmic Ray Conference (ICRC 2015)May 07 2016The Latin American Giant Observatory (LAGO) is an extended cosmic ray observatory composed by a network of water-Cherenkov detectors spanning over different sites located at significantly different altitudes (from sea level up to more than $5000$\,m a.s.l.) ... More