arXiv subjects

A. Gomez

Publications and source records attributed to A. Gomez.

At least 109 records · Page 6Linked to original sources

arXiv2017

Different approaches to generate matching effects using arrays in contact with superconducting films

Superconducting films in contact with non-superconducting regular arrays can exhibit commensurability effects between the vortex lattice and the unit cell of the pinning array. These matching effects yield a slowdown of the vortex flow and the corresponding dissipation decrease. The superconducting samples are Nb films grown on Si substrates. We have studied these matching effects with the array on top, embedded or threading the Nb superconducting films and using different materials (Si, Cu, Ni, Py dots and dots fabricated with Co/Pd multilayers). These hybrids allow studying the contribution of different pinning potentials to the matching effects. The main findings are: i) Periodic roughness induced in the superconducting film is enough to generate resistivity minima; ii) A minor effect is achieved by magnetic pinning from periodic magnetic field potentials obtained by dots with out of plane magnetization grown on top of the superconducting film, iii) In the case of array of magnetic dots embedded in the films vortex flow probes the magnetic state; i.e. magnetoresistance measurements detect the magnetic state of very small nanomagnets. In addition, we have studied the role played by the local order in the commensurability effects. This was attained using an array that mimics a smectic crystal. We have found that preserving the local order is crucial. If the local order is not retained the magnetoresistance minima vanish.

cond-mat.supr-con↗

arXiv2016

On the kinematic detection of accreted streams in the Gaia era: a cautionary tale

The $Λ$CDM cosmological scenario predicts that our Galaxy should contain hundreds of stellar streams at the solar vicinity, fossil relics of the merging history of the Milky Way and more generally of the hierarchical growth of galaxies. Because of the mixing time scales in the inner Galaxy, it has been claimed that these streams should be difficult to detect in configuration space but can still be identifiable in kinematic-related spaces like the energy/angular momenta spaces, E-Lz and Lperp-Lz, or spaces of orbital/velocity parameters. By means of high-resolution, dissipationless N-body simulations, containing between 25$\times10^6$ and 35$\times10^6$ particles, we model the accretion of a series of up to four 1:10 mass ratio satellites then up to eight 1:100 satellites and we search systematically for the signature of these accretions in these spaces. In all spaces considered (1) each satellite gives origin to several independent overdensities; (2) overdensities of multiple satellites overlap; (3) satellites of different masses can produce similar substructures; (4) the overlap between the in-situ and the accreted population is considerable everywhere; (5) in-situ stars also form substructures in response to the satellite(s) accretion. These points are valid even if the search is restricted to kinematically-selected halo stars only. As we are now entering the 'Gaia era', our results warn that an extreme caution must be employed before interpreting overdensities in any of those spaces as evidence of relics of accreted satellites. Reconstructing the accretion history of our Galaxy will require a substantial amount of accurate spectroscopic data, that, complemented by the kinematic information, will possibly allow us to (chemically) identify accreted streams and measure their orbital properties. (abridged)

astro-ph.GA↗

arXiv2016

Effective penetration length and interstitial vortex pinning in superconducting films with regular arrays of defects

In order to compare magnetic and non-magnetic pinning we have nanostructured two superconducting films with regular arrays of pinning centers: Cu (non-magnetic) dots in one case, and Py (magnetic) dots in the other. For low applied magnetic fields, when all the vortices are pinned in the artificial inclusions, magnetic dots prove to be better pinning centers, as has been generally accepted. Unexpectedly, when the magnetic field is increased and interstitial vortices appear, the results are very different: we show how the stray field generated by the magnetic dots can produce an effective reduction of the penetration length. This results in strong consequences in the transport properties, which, depending on the dot separation, can lead to an enhancement or worsening of the transport characteristics. Therefore, the election of the magnetic or non-magnetic character of the pinning sites for an effective reduction of dissipation will depend on the range of the applied magnetic field.

cond-mat.supr-con↗

arXiv2016

Superconducting/magnetic three state nanodevice for memory and reading applications

We present a simple nanodevice that can operate in two modes: i) three-state memory and ii) reading device. The nanodevice is fabricated with an array of ordered triangular-shaped nanomagnets embedded in a superconducting thin film. The input signal is ac current and the output signal is dc voltage. Vortex ratchet effect in combination with out of plane magnetic anisotropy of the nanomagnets is the background physics which governs the nanodevice performance.

cond-mat.supr-con↗

arXiv2016

When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy

Quenching, the cessation of star formation, is one of the most significant events in the life cycle of galaxies. We show here the first evidence that the Milky Way experienced a generalised quenching of its star formation at the end of its thick disk formation $\sim$9 Gyr ago. Elemental abundances of stars studied as part of the APOGEE survey reveal indeed that in less than $\sim$2 Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude. Because of the tight correlation between age and alpha abundance, this event reflects in the dearth of stars along the inner disk sequence in the [Fe/H]-[$α$/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the Milky Way was actively forming stars. Afterwards, the star formation resumed at a much lower level to form the thin disk. These events are very well matched by the latest observation of MW-type progenitors at high redshifts. In late type galaxies, quenching is believed to be related to a long and secular exhaustion of gas. In our Galaxy, it occurred on a much shorter time scale, while the chemical continuity before and after the quenching indicates that it was not due to the exhaustion of the gas. While quenching is generally associated with spheroids, our results show that it also occurs in galaxies like the Milky Way, possibly when they are undergoing a morphological transition from thick to thin disks. Given the demographics of late type galaxies in the local universe, in which classical bulges are rare, we suggest further that this may hold true generally in galaxies with mass lower than or approximately $M^*$, where quenching could be directly a consequence of thick disk formation. We emphasize that the quenching phase in the Milky Way could be contemporaneous with, and related to, the formation of the bar. We sketch a scenario on how a strong bar may inhibit star formation.

astro-ph.GA↗

arXiv2016

A new look at the kinematics of the bulge from an N-body model

(Abridged) By using an N-body simulation of a bulge that was formed via a bar instability mechanism, we analyse the imprints of the initial (i.e. before bar formation) location of stars on the bulge kinematics, in particular on the heliocentric radial velocity distribution of bulge stars. Four different latitudes were considered: $b=-4^\circ$, $-6^\circ$, $-8^\circ$, and $-10^\circ$, along the bulge minor axis as well as outside it, at $l=\pm5^\circ$ and $l=\pm10^\circ$. The bulge X-shaped structure comprises stars that formed in the disk at different locations. Stars formed in the outer disk, beyond the end of the bar, which are part of the boxy peanut-bulge structure may show peaks in the velocity distributions at positive and negative heliocentric radial velocities with high absolute values that can be larger than 100 $\rm km$ $\rm s^{-1}$, depending on the observed direction. In some cases the structure of the velocity field is more complex and several peaks are observed. Stars formed in the inner disk, the most numerous, contribute predominantly to the X-shaped structure and present different kinematic characteristics. Our results may enable us to interpret the cold high-velocity peak observed in the APOGEE commissioning data, as well as the excess of high-velocity stars in the near and far arms of the X-shaped structure at $l$=$0^\circ$ and $b$=$-6^\circ$. When compared with real data, the kinematic picture becomes more complex due to the possible presence in the observed samples of classical bulge and/or thick disk stars. Overall, our results point to the existence of complex patterns and structures in the bulge velocity fields, which are generated by the bar. This suggests that caution should be used when interpreting the bulge kinematics: the presence of substructures, peaks and clumps in the velocity fields is not necessarily a sign of past accretion events.

astro-ph.GA↗

arXiv2015

Experimental realization of smectic phase in vortex matter induced by symmetric potentials arranged in two-fold symmetry arrays

Smectic order has been generated in superconducting Nb films with two-fold symmetry arrays of symmetric pinning centers. Magnetic fields applied perpendicularly to the films develop a vortex matter smectic phase that is easily detected when the vortices commensurate with the pinning center array. The smectic phase can be turned on and off with external parameters.

cond-mat.supr-con↗

arXiv2015

Heavy elements Ba, La, Ce, Nd, and Eu in 56 Galactic bulge red giants

Aims. The aim of this work is the study of abundances of the heavy elements Ba, La, Ce, Nd, and Eu in 56 bulge giants (red giant branch and red clump) with metallicities ranging from -1.3 dex to 0.5 dex. Methods. We obtained high-resolution spectra of our giant stars using the FLAMES-UVES spectrograph on the Very Large Telescope. We inspected four bulge fields along the minor axis. Results. We measure the chemical evolution of heavy elements, as a function of metallicity, in the Galactic bulge. Conclusions. The [Ba, La, Ce, Nd/Fe] vs. [Fe/H] ratios decrease with increasing metallicity, in which aspect they differ from disc stars. In our metal-poor bulge stars, La and Ba are enhanced relative to their thick disc counterpart, while in our metal-rich bulge stars La and Ba are underabundant relative to their disc counterpart. Therefore, this contrast between bulge and discs trends indicates that bulge and (solar neighbourhood) thick disc stars could behave differently. An increase in [La, Nd/Eu] with increasing metallicity, for metal-rich stars with [Fe/H] > 0 dex, may indicate that the s-process from AGB stars starts to operate at a metallicity around solar. Finally, [Eu/Fe] follows the [α/Fe] behaviour, as expected, since these elements are produced by SNe type II.

astro-ph.GA↗

arXiv2015

Zinc abundances in Galactic bulge field red giants: implications for DLA systems

Zinc in stars is an important reference element because it is a proxy to Fe in studies of damped Lyman-alpha systems, permitting a comparison of chemical evolution histories of bulge stellar populations and DLAs. In terms of nucleosynthesis, it behaves as an alpha element because it is enhanced in metal-poor stars. The aim of this work is to derive the iron-peak element Zn abundances in 56 bulge giants from high resolution spectra. These results are compared with data from other bulge samples, as well as from disk and halo stars, and damped Lyman-alpha systems, in order to better understand the chemical evolution in these environments. High-resolution spectra were obtained using FLAMES+UVES on the Very Large Telescope. We find [Zn/Fe]=+0.24+-0.02 in the range -1.3 < [Fe/H] < -0.5 and [Zn/Fe]=+0.06+-0.02 in the range -0.5 < [Fe/H] < -0.1, whereas for [Fe/H] > -0.1, it shows a spread of -0.60 < [Zn/Fe] < +0.15, with most of these stars having low [Zn/Fe]<0.0. These low zinc abundances at the high metallicity end of the bulge define a decreasing trend in [Zn/Fe] with increasing metallicities. A comparison with Zn abundances in DLA systems is presented, where a dust-depletion correction was applied for both Zn and Fe. Finally, we present a chemical evolution model of Zn enrichment in massive spheroids, representing a typical classical bulge.

astro-ph.GA↗

arXiv2015

MINERvA neutrino detector response measured with test beam data

The MINERvA collaboration operated a scaled-down replica of the solid scintillator tracking and sampling calorimeter regions of the MINERvA detector in a hadron test beam at the Fermilab Test Beam Facility. This article reports measurements with samples of protons, pions, and electrons from 0.35 to 2.0 GeV/c momentum. The calorimetric response to protons, pions, and electrons are obtained from these data. A measurement of the parameter in Birks' law and an estimate of the tracking efficiency are extracted from the proton sample. Overall the data are well described by a Geant4-based Monte Carlo simulation of the detector and particle interactions with agreements better than 4%, though some features of the data are not precisely modeled. These measurements are used to tune the MINERvA detector simulation and evaluate systematic uncertainties in support of the MINERvA neutrino cross section measurement program.

physics.ins-det↗

arXiv2014

Measurement of Coherent Production of $π^\pm$ in Neutrino and Anti-Neutrino Beams on Carbon from $E_ν$ of $1.5$ to $20$ GeV

Neutrino-induced coherent charged pion production on nuclei, $\stackrel{(-)}ν_μA\toμ^\pmπ^\mp A$ is a rare, inelastic interaction in which a small squared four-momentum $| t|$ is transferred to the recoil nucleus leaving it intact in the reaction. In the scintillator tracker of MINERvA, we remove events with evidence of particles from nuclear breakup and reconstruct $| t|$ from the final state pion and muon. We select low $| t|$ events to isolate a sample rich in coherent candidates. By selecting low $| t|$ events we produce a model-independent measurement of the differential cross section for coherent scattering of neutrinos and anti-neutrinos on carbon. We find poor agreement with the predicted kinematics in neutrino generators used by current oscillation experiments.

hep-ex↗

arXiv2014

Why the Milky Way's bulge is not only a bar formed from a cold thin disk

By analyzing a N-body simulation of a bulge formed simply via a bar instability mechanism operating on a kinematically cold stellar disk, and by comparing the results of this analysis with the structural and kinematic properties of the main stellar populations of the Milky Way bulge, we conclude that the bulge of our Galaxy is not a pure stellar bar formed from a pre-existing thin stellar disk, as some studies have recently suggested. On the basis of several arguments emphasized in this paper, we propose that the bulge population which, in the Milky Way, is observed not to be part of the peanut structure corresponds to the old galactic thick disk, thus implying that the Milky Way is a pure thin+thick disk galaxy, with only a possible limited contribution of a classical bulge.

astro-ph.GA↗

arXiv2014

Vortex pinning vs superconducting wire network: origin of periodic oscillations induced by applied magnetic fields in superconducting films with arrays of nanomagnets

Hybrid magnetic arrays embedded in superconducting films are ideal systems to study the competition between different physical (such as the coherence length) and structural length scales such as available in artificially produced structures. This interplay leads to oscillation in many magnetically dependent superconducting properties such as the critical currents, resistivity and magnetization. These effects are generally analyzed using two distinct models based on vortex pinning or wire network. In this work, we show that for magnetic dot arrays, as opposed to antidot (i.e holes) arrays, vortex pinning is the main mechanism for field induced oscillations in resistance R(H), critical current Ic(H), magnetization M(H) and ac-susceptibility Xac(H) in a broad temperature range. Due to the coherence length divergence at Tc, a crossover to wire network behavior is experimentally found. While pinning occurs in a wide temperature range up to Tc, wire network behavior is only present in a very narrow temperature window close to Tc. In this temperature interval, contributions from both mechanisms are operational but can be experimentally distinguished.

cond-mat.supr-con↗

arXiv2014

Mapping a stellar disk into a boxy bulge: The outside-in part of the Milky Way bulge formation

By means of idealized, dissipationless N-body simulations which follow the formation and subsequent buckling of a stellar bar, we study the characteristics of boxy/peanut-shaped bulges and compare them with the properties of the stellar populations in the Milky Way bulge. The main results of our modeling, valid for the general family of boxy/peanut shaped bulges, are the following: (i) because of the spatial redistribution in the disk initiated at the epoch of bar formation, stars from the innermost regions to the outer Lindblad resonance of the stellar bar are mapped into a boxy bulge; (ii) the contribution of stars to the local bulge density depends on their birth radius: stars born in the innermost disk tend to dominate the innermost regions of the boxy bulge, while stars originating closer to the OLR are preferentially found in the outer regions of the boxy/peanut structure; (iii) stellar birth radii are imprinted in the bulge kinematics, the larger the birth radii of stars ending up in the bulge, the greater their rotational support and the higher their line-of- sight velocity dispersions (but note that this last trend depends on the bar viewing angle); (iv) the higher the classical bulge-over-disk ratio, the larger its fractional contribution of stars at large vertical distance from the galaxy mid-plane. (ABRIDGED) On the basis of their chemical and kinematic characteristics, the results of our modeling suggests that the populations A, B and C, as defined by the ARGOS survey, can be associated, respectively, with the inner thin disk, to the young thick and to the old thick disk, following the nomenclature recently suggested for stars in the solar neighborhood by Haywood et al. (2013).

astro-ph.GA↗

arXiv2014

Metallicity and kinematics of the bar in-situ

Constraints on the Galactic bulge/bar structure and formation history from stellar kinematics and metallicities mainly come from relatively high-latitude fields (|b|>4) where a complex mix of stellar population is seen. We aim here to constrain the formation history of the Galactic bar by studying the radial velocity and metallicity distributions of stars in-situ (|b|<1). We observed red clump stars in four fields along the bar's major axis (l=10,6,-6 and b=0 plus a field at l=0,b=1) with low-resolution spectroscopy from VLT/FLAMES, observing around the CaII triplet. We developed robust methods for extracting radial velocity and metallicity estimates from these low signal-to-noise spectra. We derived distance probability distributions using Bayesian methods rigorously handling the extinction law. We present radial velocities and metallicity distributions, as well as radial velocity trends with distance. We observe an increase in the radial velocity dispersion near the Galactic plane. We detect the streaming motion of the stars induced by the bar in fields at l=+/-6, the highest velocity components of this bar stream being metal-rich ([Fe/H]~0.2 dex). Our data is consistent with a bar inclined at 26+/-3 from the Sun-Galactic centre line. We observe a significant fraction of metal-poor stars, in particular in the field at l=0,b=1. We confirm the flattening of the metallicity gradient along the minor axis when getting closer to the plane, with a hint that it could actually be inverted. Our stellar kinematics corresponds to the expected behaviour of a bar issued from the secular evolution of the Galactic disc. The mix of several populations, seen further away from the plane, is also seen in the bar in-situ since our metallicity distributions highlight a different spatial distribution between metal-poor and metal-rich stars, the more metal-poor stars being more centrally concentrated.

astro-ph.GA↗

arXiv2013

The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk

We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the thick disk set the initial conditions for the formation of the inner thin disk. This provides also an explanation of the apparent coincidence between the step in metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk at about R<10 kpc. We suggest that the outer thin disk developped outside the influence of the thick disk, but also that the high alpha-enrichment of the outer regions may originate from a primordial pollution by the gas expelled from the thick disk. Local metal-poor thin disk stars, whose properties are best explained by an origin in the outer disk, are shown to be as old as the youngest thick disk (9-10 Gyr), implying that the outer thin disk started to form while the thick disk formation was still on-going in the inner Galaxy. We point out that, given the tight age-abundance relations in the thick disk, an inside-out process would give rise to a radial gradient in abundances in this population which is not observed. Finally, we argue that the data discussed here leave little room for radial migration, either to have contaminated the solar vicinity, or, to have redistributed stars in significant proportion across the solar annulus.

astro-ph.GA↗

arXiv2013

Manganese abundances in Galactic bulge red giants

Manganese is mainly produced in type II SNe during explosive silicon burning, in incomplete Si-burning regions, and depends on several nucleosynthesis environment conditions, such as mass cut beween the matter ejected and falling back onto the remnant, electron and neutron excesses, mixing fallback, and explosion energy. Manganese is also produced in type Ia SNe. The aim of this work is the study of abundances of the iron-peak element Mn in 56 bulge giants, among which 13 are red clump stars. Four bulge fields along the minor axis are inspected. The study of abundances of Mn-over-Fe as a function of metallicity in the Galactic bulge may shed light on its production mechanisms. High-resolution spectra were obtained using the FLAMES+UVES spectrograph on the Very Large Telescope. The spectra were obtained within a program to observe 800 stars using the GIRAFFE spectrograph, together with the present UVES spectra. We aim at identifying the chemical evolution of manganese, as a function of metallicity, in the Galactic bulge. We find [Mn/Fe] ~-0.7 at [Fe/H]~-1.3, increasing to a solar value at metallicities close to solar, and showing a spread around -0.7<[Fe/H]<-0.2, in good agreement with other work on Mn in bulge stars. There is also good agreement with chemical evolution models. We find no clear difference in the behaviour of the four bulge fields. Whereas [Mn/Fe] vs. [Fe/H] could be identified with the behaviour of the thick disc stars, [Mn/O] vs. [O/H] has a behaviour running parallel, at higher metallicities, compared to thick disc stars, indicating that the bulge enrichment might have proceeded differently from that of the thick disk.

astro-ph.GA↗

arXiv2013

Juxtaposition of Spin Freezing and Long Range Order in a Series of Geometrically Frustrated Antiferromagnetic Gadolinium Garnets

Specific heat measurements in zero magnetic field are presented on a homologous series of geometrically frustrated, antiferromagnetic, Heisenberg garnet systems. Measurements of Gd3Ga5O12, grown with isotopically pure Gd, agree well with previous results on samples with naturally abundant Gd, showing no ordering features. In contrast, samples of Gd3Te2Li3O12 and Gd3Al5O12 are found to exhibit clear ordering transitions at 243 mK and 175 mK respectively. The effects of low level disorder are studied through dilution of Gd3+ with non-magnetic Y3+ in Gd3Te2Li3O12. A thorough structural characterization, using X-ray diffraction, is performed on all of the samples studied. We discuss possible explanations for such diverse behavior in very similar systems.

cond-mat.dis-nn↗