arXiv subjects

J. Gonzalez

Publications and source records attributed to J. Gonzalez.

At least 73 records · Page 4Linked to original sources

arXiv2009

Water Cherenkov Detectors response to a Gamma Ray Burst in the Large Aperture GRB Observatory

In order to characterise the behaviour of Water Cherenkov Detectors (WCD) under a sudden increase of 1 GeV - 1 TeV background photons from a Gamma Ray Burst (GRB), simulations were conducted and compared to data acquired by the WCD of the Large Aperture GRB Observatory (LAGO). The LAGO operates arrays of WCD at high altitude to detect GRBs using the single particle technique. The LAGO sensitivity to GRBs is derived from the reported simulations of the gamma initiated particle showers in the atmosphere and the WCD response to secondaries.

astro-ph.HE↗

arXiv2009

A template bank for gravitational waveforms from coalescing binary black holes: non-spinning binaries

Gravitational waveforms from the inspiral and ring-down stages of the binary black hole coalescences can be modelled accurately by approximation/perturbation techniques in general relativity. Recent progress in numerical relativity has enabled us to model also the non-perturbative merger phase of the binary black-hole coalescence problem. This enables us to \emph{coherently} search for all three stages of the coalescence of non-spinning binary black holes using a single template bank. Taking our motivation from these results, we propose a family of template waveforms which can model the inspiral, merger, and ring-down stages of the coalescence of non-spinning binary black holes that follow quasi-circular inspiral. This two-dimensional template family is explicitly parametrized by the physical parameters of the binary. We show that the template family is not only \emph{effectual} in detecting the signals from black hole coalescences, but also \emph{faithful} in estimating the parameters of the binary. We compare the sensitivity of a search (in the context of different ground-based interferometers) using all three stages of the black hole coalescence with other template-based searches which look for individual stages separately. We find that the proposed search is significantly more sensitive than other template-based searches for a substantial mass-range, potentially bringing about remarkable improvement in the event-rate of ground-based interferometers. As part of this work, we also prescribe a general procedure to construct interpolated template banks using non-spinning black hole waveforms produced by numerical relativity.

gr-qc↗

arXiv2009

Propagating, evanescent, and localized states in carbon nanotube-graphene junctions

We study the electronic structure of the junctions between a single graphene layer and carbon nanotubes, using a tight-binding model and the continuum theory based on Dirac fermion fields. The latter provides a unified description of different lattice structures with curvature, which is always localized at six heptagonal carbon rings around each junction. When these are evenly spaced, we find that it is possible to curve the planar lattice into armchair (6n,6n) as well as zig-zag (6n,0) nanotubes. We show that the junctions fall into two different classes, regarding the low-energy electronic behavior. One of them, constituted by the junctions made of the armchair nanotubes and the zig-zag (6n,0) geometries when n is a multiple of 3, is characterized by the presence of two quasi-bound states at the Fermi level, which are absent for the rest of the zig-zag nanotubes. These states, localized at the junction, are shown to arise from the effective gauge flux induced by the heptagonal carbon rings, which has a direct reflection in the local density of states around the junction. Furthermore, we also analyze the band structure of the arrays of junctions, finding out that they can also be classified into two different groups according to the low-energy behavior. In this regard, the arrays made of armchair and (6n,0) nanotubes with n equal to a multiple of 3 are characterized by the presence of a series of flat bands, whose number grows with the length of the nanotubes. We show that such flat bands have their origin in the formation of states confined to the nanotubes in the array. This is explained in the continuum theory from the possibility of forming standing waves in the mentioned nanotube geometries, as a superposition of modes with opposite momenta and the same quantum numbers under the C_6v symmetry of the junction.

cond-mat.mes-hall↗

arXiv2008

Kohn-Luttinger superconductivity in graphene

We investigate the development of superconductivity in graphene when the Fermi level becomes close to one of the Van Hove singularities of the electron system. The origin of the pairing instability lies in the strong anisotropy of the e-e scattering at the Van Hove filling, which leads to a channel with attractive coupling when making the projection of the BCS vertex on the symmetry modes with nontrivial angular dependence along the Fermi line. We show that the scale of the superconducting instability may be pushed up to temperatures larger than 10 K, depending on the ability to tune the system to the proximity of the Van Hove singularity.

cond-mat.mes-hall↗

arXiv2008

Unconventional quasiparticle lifetime in undoped graphene

We address the question of how small can the quasiparticle decay rate be at low energies in undoped graphene, where kinematical constraints are known to prevent the decay into particle-hole excitations. For this purpose, we study the renormalization of the phonon dispersion by many-body effects, which turns out to be very strong in the case of the out-of-plane phonons at the K point of the spectrum. We show that these evolve into a branch of very soft modes that provide the relevant channel for quasiparticle decay, at energies below the scale of the optical phonon modes. In this regime, we find that the decay rate is proportional to the cube of the quasiparticle energy. This implies that a crossover should be observed in transport properties from the linear dependence characteristic of the high-energy regime to the much slower decay rate due to the soft phonon modes.

cond-mat.mes-hall↗

arXiv2008

Critical currents in graphene Josephson junctions

We study the superconducting correlations induced in graphene when it is placed between two superconductors, focusing in particular on the supercurrents supported by the 2D system. For this purpose we make use of a formalism placing the emphasis on the many-body aspects of the problem, with the aim of investigating the dependence of the critical currents on relevant variables like the distance L between the superconducting contacts, the temperature, and the doping level. Thus we show that, despite the vanishing density of states at the Fermi level in undoped graphene, supercurrents may exist at zero temperature with a natural 1/L^3 dependence at large L. When temperature effects are taken into account, the supercurrents are further suppressed beyond the thermal length L_T (~ v_F / k_B T, in terms of the Fermi velocity v_F of graphene), entering a regime where the decay is given by a 1/L^5 dependence. On the other hand, the supercurrents can be enhanced upon doping, as the Fermi level is shifted by a chemical potential μfrom the charge neutrality point. This introduces a new crossover length L* ~ v_F / μ, at which the effects of the finite charge density start being felt, marking the transition from the short-distance 1/L^3 behavior to a softer 1/L^2 decay of the supercurrents at large L. It turns out that the decay of the critical currents is given in general by a power-law behavior, which can be seen as a consequence of the perfect scaling of the Dirac theory applied to the low-energy description of graphene.

cond-mat.mes-hall↗

arXiv2007

From the Magnetization Profile to the Stray Field of Bistable Wires

We present new analytical calculations of the spatial dependence of the stray field of a bistable magnetic wire from the magnetization profile of the wire. Contributions from the outer shell and from the wire ends are neglected. The results qualitatively agree with experimental data, taken from literature.

cond-mat.mtrl-sci↗

arXiv2007

Quantum Hall effect in carbon nanotubes and curved graphene strips

We develop a long wavelength approximation in order to describe the low-energy states of carbon nanotubes in a transverse magnetic field. We show that, in the limit where the square of the magnetic length $l = \sqrt{\hbar c /e B}$ is much larger than the $C$-$C$ distance times the nanotube radius $R$, the low-energy theory is given by the linear coupling of a two-component Dirac spinor to the corresponding vector potential. We investigate in this regime the evolution of the band structure of zig-zag nanotubes for values of $R/l > 1$, showing that for radius $R \approx 20$ nm a clear pattern of Landau levels start to develop for magnetic field strength $B \gtrsim 10$ T. The levels tend to be four-fold degenerate, and we clarify the transition to the typical two-fold degeneracy of graphene as the nanotube is unrolled to form a curved strip. We show that the dynamics of the Dirac fermions leads to states which are localized at the flanks of the nanotube and that carry chiral currents in the longitudinal direction. We discuss the possibility to observe the quantization of the Hall conductivity in thick carbon nanotubes, which should display steps at even multiples of $2 e^2/h$, with values doubled with respect to those in the odd-integer quantization of graphene.

cond-mat.mes-hall↗

arXiv2007

The Offline Software Framework of the Pierre Auger Observatory

The Pierre Auger Observatory is designed to unveil the nature and the origins of the highest energy cosmic rays. The large and geographically dispersed collaboration of physicists and the wide-ranging collection of simulation and reconstruction tasks pose some special challenges for the offline analysis software. We have designed and implemented a general purpose framework which allows collaborators to contribute algorithms and sequencing instructions to build up the variety of applications they require. The framework includes machinery to manage these user codes, to organize the abundance of user-contributed configuration files, to facilitate multi-format file handling, and to provide access to event and time-dependent detector information which can reside in various data sources. A number of utilities are also provided, including a novel geometry package which allows manipulation of abstract geometrical objects independent of coordinate system choice. The framework is implemented in C++, and takes advantage of object oriented design and common open source tools, while keeping the user side simple enough for C++ novices to learn in a reasonable time. The distribution system incorporates unit and acceptance testing in order to support rapid development of both the core framework and contributed user code.

astro-ph↗

arXiv2007

Phenomenological template family for black-hole coalescence waveforms

Recent progress in numerical relativity has enabled us to model the non-perturbative merger phase of the binary black-hole coalescence problem. Based on these results, we propose a phenomenological family of waveforms which can model the inspiral, merger, and ring-down stages of black hole coalescence. We also construct a template bank using this family of waveforms and discuss its implementation in the search for signatures of gravitational waves produced by black-hole coalescences in the data of ground-based interferometers. This template bank might enable us to extend the present inspiral searches to higher-mass binary black-hole systems, i.e., systems with total mass greater than about 80 solar masses, thereby increasing the reach of the current generation of ground-based detectors.

gr-qc↗

arXiv2007

Electronic instabilities of a Hubbard model approached as a large array of coupled chains: competition between d-wave superconductivity and pseudogap phase

We study the electronic instabilities in a 2D Hubbard model where one of the dimensions has a finite width, so that it can be considered as a large array of coupled chains. The finite transverse size of the system gives rise to a discrete string of Fermi points, with respective electron fields that, due to their mutual interaction, acquire anomalous scaling dimensions depending on the point of the string. Using bosonization methods, we show that the anomalous scaling dimensions vanish when the number of coupled chains goes to infinity, implying the Fermi liquid behavior of a 2D system in that limit. However, when the Fermi level is at the Van Hove singularity arising from the saddle points of the 2D dispersion, backscattering and Cooper-pair scattering lead to the breakdown of the metallic behavior at low energies. These interactions are taken into account through their renormalization group scaling, studying in turn their influence on the nonperturbative bosonization of the model. We show that, at a certain low-energy scale, the anomalous electron dimension diverges at the Fermi points closer to the saddle points of the 2D dispersion. The d-wave superconducting correlations become also large at low energies, but their growth is cut off as the suppression of fermion excitations takes place first, extending progressively along the Fermi points towards the diagonals of the 2D Brillouin zone. We stress that this effect arises from the vanishing of the charge stiffness at the Fermi points, characterizing a critical behavior that is well captured within our nonperturbative approach.

cond-mat.str-el↗

arXiv2007

Cooper-pair propagation and superconducting correlations in graphene

We investigate the Cooper-pair propagation and the proximity effect in graphene under conditions in which the distance L between superconducting electrodes is much larger than the width W of the contacts. In the case of undoped graphene, supercurrents may exist with a spatial decay proportional to W^2/L^3. This changes upon doping into a 1/L^2 behavior, opening the possibility to observe a supercurrent over length scales above 1 micron at suitable doping levels. We also show that there is in general a crossover temperature T ~ v_F/k_B L that marks the onset of the strong decay of the supercurrent, and that corresponds to the scale below which the Cooper pairs are not disrupted by thermal effects during their propagation.

cond-mat.mes-hall↗

arXiv2007

OPserver: interactive online-computations of opacities and radiative accelerations

Codes to compute mean opacities and radiative accelerations for arbitrary chemical mixtures using the Opacity Project recently revised data have been restructured in a client--server architecture and transcribed as a subroutine library. This implementation increases efficiency in stellar modelling where element stratification due to diffusion processes is depth dependent, and thus requires repeated fast opacity reestimates. Three user modes are provided to fit different computing environments, namely a web browser, a local workstation and a distributed grid.

astro-ph↗

arXiv2006

A theory of superconductivity in multi-walled carbon nanotubes

We devise an approach to describe the electronic instabilities of doped multi-walled nanotubes, where each shell has in general a manifold of Fermi points. Our analysis relies on the scale dependence of the different scattering processes, showing that a pairing instability arises for a large enough number of Fermi points as a consequence of their particular geometric arrangement. The instability is enhanced by the tunneling of Cooper pairs between nearest shells, giving rise to a transition from the Luttinger liquid to a superconducting state in a wide region of the phase diagram.

cond-mat.supr-con↗

arXiv2006

Stellar populations of early-type galaxies in different environments I. Line-strength indices

Aims: This paper commences a series devoted to the study of the stellar content of early-type galaxies. The goal of the series is to set constraints on the evolutionary status of these objects. Methods: In this paper we describe the details of the galaxy sample, the observations, and the data reduction. Line-strength indices and velocity dispersions sigma are measured in 98 early-type galaxies drawn from different environments, and the relation of the indices with the velocity dispersion analysed in detail. Results: The present sample indicates that some of the index-sigma relations depend on galaxy environment. In particular, the slope of the relation between Balmer lines and sigma is steeper for galaxies in the Virgo cluster, small groups, and in the field than for galaxies in the Coma cluster. In several indices there is also a significant offset in the zero point between the relations defined by the different subsamples. The slopes of the index-sigma relation for the Virgo and low-density environment galaxies are explained by a variation of both age and metallicity with velocity dispersion, as previously noted in other studies. For the galaxies in the Coma cluster, however, the relation of the indices with sigma only requires a variation of the abundance along the sigma sequence. In agreement with other studies we find that the models that better reproduce the slopes are those in which the alpha elements vary more than the Fe-peak elements along the sigma sequence, while, at a given sigma, older galaxies show an higher alpha/Fe ratio. Conclusions: The results can be explained assuming that galaxies in the Coma cluster have experienced a truncated star formation and chemical enrichment history compared to a more continuous time-extended history for their counterparts in lower density environments.

astro-ph↗

arXiv2006

Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves

We demonstrate composite media with ferromagnetic wires that exhibit a frequency region at the microwave regime with scattering spectra strongly dependent on an external magnetic field or stress. These tunable composite materials have recently been proposed theoretically; however, no direct experimental verification has been reported. We used composite materials with predominantly oriented CoFeCrSiB glass-coated amorphous wires having large magnetoimpedance at GHz frequencies. The free space measurements of reflection and transmission coefficients were conducted in the frequency range 1-8 GHz in the presence of an external static magnetic field or stress applied to the whole sample. In general, the transmission spectra show greater changes in the range of 10dB for a relatively small magnetic field of few Oe or stress of 0.1 MPa. The obtained results are quantitatively consistent with the analytical expressions predicted by the effective medium arguments. The incident electromagnetic wave induces an electrical dipole moment in each wire, the aggregate of which forms the effective dipole response of the whole composite structure in the radiative near or far field region. The field and stress dependences of the effective response arise from a field or tensile stress sensitivity of the ac surface impedance of a ferromagnetic wire. In the vicinity of the antenna resonance the variations in the magneto-impedance of the wire inclusions result in large changes of the total effective response. A number of applications of proposed materials is discussed including the field tunable microwave surfaces and the self-sensing media for the remote non-destructive evaluation of structural materials.

cond-mat.mtrl-sci↗

arXiv2006

Quantum Hall effect in carbon nanotubes

We investigate the effects of a transverse magnetic field on the transport properties of carbon nanotubes, making use of a long-wavelength description in terms of Dirac fermion fields. For values of the magnetic length smaller than the nanotube radius, we observe that the electronic states organize into incipient Landau subbands, with a highly degenerate level at zero energy. We show that only the states in dispersive branches, localized at the flanks of the nanotube, are able to transport current in the longitudinal direction. This is at the origin of the quantization of the Hall conductivity, that turns out to be given by even multiples of 2 e^2 /h. We also analyze the effects of the electron-electron interaction, showing that the magnetic field induces a suppression of the electronic correlations, reflected in particular in the enhancement of the tunneling density of states near the Fermi level.

cond-mat.mes-hall↗

arXiv2006

The Offline Software Framework of the Pierre Auger Observatory

astro-ph↗