arXiv subjects

C. Gonzalez

Publications and source records attributed to C. Gonzalez.

26 records · Page 2Linked to original sources

arXiv2014

Light Echoes from Eta Carinae's Great Eruption: Spectrophotometric Evolution and the Rapid Formation of Nitrogen-rich Molecules

We present follow-up optical imaging and spectroscopy of one of the light echoes of $η$ Carinae's 19th-century Great Eruption discovered by Rest et al. (2012). By obtaining images and spectra at the same light echo position between 2011 and 2014, we follow the evolution of the Great Eruption on a three-year timescale. We find remarkable changes in the photometric and spectroscopic evolution of the echo light. The $i$-band light curve shows a decline of $\sim 0.9$ mag in $\sim 1$ year after the peak observed in early 2011 and a flattening at later times. The spectra show a pure-absorption early G-type stellar spectrum at peak, but a few months after peak the lines of the [Ca II] triplet develop strong P-Cygni profiles and we see the appearance of [Ca II] 7291,7324 doublet in emission. These emission features and their evolution in time resemble those observed in the spectra of some Type IIn supernovae and supernova impostors. Most surprisingly, starting $\sim 300$ days after peak brightness, the spectra show strong molecular transitions of CN at $\gtrsim 6800$ Å. The appearance of these CN features can be explained if the ejecta are strongly Nitrogen enhanced, as it is observed in modern spectroscopic studies of the bipolar Homunculus nebula. Given the spectroscopic evolution of the light echo, velocities of the main features, and detection of strong CN, we are likely seeing ejecta that contributes directly to the Homunculus nebula.

astro-ph.SR↗

arXiv2011

Characterization of silicon thin overlayers on rutile \ce{TiO2} (110)-(1x1)

Silicon thin films for coverages ($θ$) between 0.3 and 3 monolayers have been grown on rutile \ce{TiO2}(110)-(1x1) at room temperature and studied by x-ray and ultra-violet photoelectron spectroscopies, Auger electron spectroscopy, and low energy electron diffraction (LEED). A clear evidence of a strong \ce{Si}/\ce{TiO2} interaction consistent with the high affinity of O for Si has been found. The Ti cations on the substrate are reduced, while the Si film is oxidized, yielding \ce{SiO2} and a mixture of silicon suboxides. Neutral Si atoms are observed at a coverage of 3 monolayers. At the interface region we observe the formation of cross-linking Ti-O-Si bonds. The thin Si overlayer strongly attenuates the $(1 \times 1)$ LEED pattern from the substrate. Finally, thermal annealing results in the improvement of the \ce{SiO2} stoichiometry, but the surface order is not recovered. Using ab-initio density functional theory we have obtained optimum geometrical configurations and corresponding density of states for 1/3 \le θ\le 1$ monolayers of Si adsorbed on the $1 \times 1$ two-dimensional unit cell.

cond-mat.mtrl-sci↗

arXiv2007

On the formation and the stability of suspended transition metal monatomic chains

We present a Tight-Binding Molecular Dynamics investigation of the stability, the geometrical and the electronic structure of suspended monatomic transition metal chains. We show that linear and stable monatomic chains are formed at temperature equal or smaller than 500 K for Au, 200 K for Ag and 4 K for Cu. We also evidence that such stability is associated with the persisting sd orbital hybridization along the chains. The study highlight fundamental limitations of conductance measurement experiments to detect these chains in the breaking process of nanowires.

cond-mat.mtrl-sci↗

arXiv2007

Crystal asymmetry induces single-atom chain formation in gold nanowires

We performed density functional theory and tight-binding molecular dynamics calculations to investigate the formation of suspended linear atom chains when stretching gold nanowires along the [110] crystal orientation. We determined that chain formation can occur only when the crystal symmetry is broken in the early stages of the elongation process. Such crystallographic asymmetry can be induced by stretching the wire along a slightly off-axis tensile direction or by introducing thermal fluctuations. Our observation of the off-axis formation of these chains agrees with experimental findings.

cond-mat.mtrl-sci↗

arXiv2006

A Density Functional Study of Magnetism in Bare Gold Nano-clusters

Magnetism in bare uncapped gold nano-clusters is explored from a density functional theory perspective with scalar relativistic effects included via the pseudo-potential. The computed electronic structures of various nano-clusters reveal that permanent size-dependent spin-polarization appears without geometry relaxation for bare clusters even though bulk gold is diamagnetic. The polarized ground states for clusters are favorable due to the hybridization of the s and d orbitals, and bare octahedral clusters are expected to be magnetic for cluster sizes of approximately 38 atoms and larger. Much larger clusters will be diamagnetic when the surface-to-volume ratio is small and the core diamagnetism prevails. Moderate changes in the inter-atomic distances and cluster geometry are shown not to alter this conclusion. Contrary to LDA and EAM predictions, GGA and hybrid geometry optimizations reveal increased inter-atomic bond distances in bare gold clusters relative to the bulk lattice values. This expansion enhances the preexisting spin polarization.

cond-mat.mtrl-sci↗

arXiv2005

Ga-induced atom wire formation and passivation of stepped Si(112)

We present an in-depth analysis of the atomic and electronic structure of the quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford Backscattering Spectrometry (RBS) and Density Functional Theory (DFT) calculations. A new structural model of the Si(112)6 x 1-Ga surface is inferred. It consists of Ga zig-zag chains that are intersected by quasi-periodic vacancy lines or misfit dislocations. The experimentally observed meandering of the vacancy lines is caused by the co-existence of competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully coordinated, and the surface is chemically passivated. STS data reveal a semiconducting surface and show excellent agreement with calculated Local Density of States (LDOS) and STS curves. The energy gain obtained by fully passivating the surface calls the idea of step-edge decoration as a viable growth method toward 1D metallic structures into question.

cond-mat.other↗

arXiv2004

Formation of atom wires on vicinal silicon

The formation of atomic wires via pseudomorphic step-edge decoration on vicinal silicon surfaces has been analyzed for Ga on the Si(112) surface using Scanning Tunneling Microscopy and Density Functional Theory calculations. Based on a chemical potential analysis involving more than thirty candidate structures and considering various fabrication procedures, it is concluded that pseudomorphic growth on stepped Si(112), both under equilibrium and non-equilibrium conditions, must favor formation of Ga zig-zag chains rather than linear atom chains. The surface is non-metallic and presents quasi-one dimensional character in the lowest conduction band.

cond-mat.mtrl-sci↗

arXiv1998

Universality of the 1/3 shot-noise suppression factor in nondegenerate diffusive conductors

Shot-noise suppression is investigated in nondegenerate diffusive conductors by means of an ensemble Monte Carlo simulator. The universal 1/3 suppression value is obtained when transport occurs under elastic collision regime provided the following conditions are satisfied: (i) The applied voltage is much larger than the thermal value; (ii) the length of the device is much longer than both the elastic mean free path and the Debye length. By fully suppressing carrier-number fluctuations, long range Coulomb interaction is essential to obtain the 1/3 value in the low-frequency limit.

cond-mat.mes-hall↗