14,229 research outputs found
Two-band superconductivity in doped SrTiO films and interfaces
We investigate the possibility of multi-band superconductivity in SrTiO
films and interfaces using a two-dimensional two-band model. In the undoped
compound, one of the bands is occupied whereas the other is empty. As the
chemical potential shifts due to doping by negative charge carriers or
application of an electric field, the second band becomes occupied, giving rise
to a strong enhancement of the transition temperature and a sharp feature in
the gap functions, which is manifested in the local density of states spectrum.
By comparing our results with tunneling experiments in Nb-doped SrTiO, we
find that intra-band pairing dominates over inter-band pairing, unlike other
known multi-band superconductors. Given the similarities with the value of the
transition temperature and with the band structure of LaAlO/SrTiO
heterostructures, we speculate that the superconductivity observed in
SrTiO interfaces may be similar in nature to that of bulk SrTiO,
involving multiple bands with distinct electronic occupations.Comment: revised expanded versio
Stellar population gradients in Seyfert 2 galaxies. Northern sample
We use high signal-to-noise ratio long-slit spectra in the 3600-4700A range
of the twenty brightest northern Seyfert 2 galaxies to study the variation of
the stellar population properties as a function of distance from the nucleus.
In order to characterize the stellar population and other continuum sources
(e.g. featureless continuum FC) we have measured equivalent widths Ws of six
absorption features, four continuum colours and their radial variations, and
performed spectral population synthesis as a function of distance from the
nucleus. About half the sample has CaIIK and G-band W values smaller at the
nucleus than at 1 kpc from it, due to a younger population and/or FC. The
stellar population synthesis shows that, while at the nucleus, 75% of the
galaxies present contribution > 20% of ages younger or equal than 100Myr and/or
of a FC, this proportion decreases to 45% at 3 kpc. In particular, 55% of the
galaxies have contribution > 10% of the 3 Myr/FC component (a degenerate
component in which one cannot separate what is due to a FC or to a 3 Myr
stellar population) at the nucleus, but only 25% of them have this contribution
at 3 kpc. As reference, the stellar population of 10 non-Seyfert galaxies,
spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A
comparison between the stellar population of the Seyferts and that of the
non-Seyferts shows systematic differences: the contribution of ages younger
than 1 Gyr is in most cases larger in the Seyfert galaxies than in
non-Seyferts, not only at the nucleus but up to 1 kpc from it.Comment: 23 pages, 18 figures, MNRAS in pres
Knowledge-based entrepreneurship and employment growth in Portugal
In this paper we try to assess the direct impact of knowledge-based entrepreneurship (understood as the creation of new firms that intensively rely on more educated workers) on net changes in employment at both the economy and the industry levels. Based on a descriptive analysis, we show that new knowledge-based firms (KBF) differentiate themselves from other firms (new or not new) by having a positive direct contribution on net employment growth (whereas most groups of incumbents contribute negatively to changes in employment in subsequent years). Notwithstanding, new KBFs’ direct impact on net employment growth is rather small, for two reasons: first, although KBFs tend to add jobs to the existing ones in net terms, the number of additional jobs for each employee in the base year tends to be modest; secondly, the share of new KBFs in overall employment is marginal. These results show some variability across industries, as a result of both the knowledge-intensity and the rate of firm turbulence of each industry. In any case, only in a few industries can new KBF be said to make a difference in net employment growth (among which stand out some knowledge-intensive services).Project AEGIS,co-funded by the European Commission under Theme 8 “Socio-Economic Sciences and Humanities” of the 7th Framework Programme for Research and Technological. Development (Project Number: 225134)
The AGN properties of the starburst galaxy NGC 7582
NGC 7582 was identified as a Starburst galaxy in the optical \cite[(Veron et
al. 1981)]{Veron et al.(1981)} but its X-Ray emission is typical of a Seyfert 1
galaxy \cite[(Ward et al. 1978)]{Ward et al.(1978)}. We analyzed a datacube of
this object obtained with the GMOS-IFU on the Gemini-South telescope. After a
subtraction of the stellar component using the {\sc starlight} code \cite[(Cid
Fernandes et al. 2005)]{Cid Fernandes et al. (2005)}, we looked for optical
signatures of the AGN. We detected a broad component (figure
\ref{fig1}) in the source where \cite[Bianchi et al.(2007)]{Bianchi et
al.(2007)} identified the AGN in an HST optical image. We also found a broad
feature (figure \ref{fig2}), but its emission reveals a extended
source. We suggest that it is the light of the AGN scattered in the ionization
cone. We propose that NGC 7582 is a Seyfert 1 galaxy. A number of other
"hot-spots" and Wolf-Rayet features were also identified.Comment: 1 page, 2 figures, to be published in the Proceedings of the IAU
Symposium no. 26
Application of Neural Networks to the study of stellar model solutions
Artificial neural networks (ANN) have different applications in Astronomy,
including data reduction and data mining. In this work we propose the use ANNs
in the identification of stellar model solutions. We illustrate this method, by
applying an ANN to the 0.8M star CG Cyg B. Our ANN was trained using
60,000 different 0.8M stellar models. With this approach we identify
the models which reproduce CG Cyg B's position in the HR diagram. We observe a
correlation between the model's initial metal and helium abundance which, in
most cases, does not agree with a helium to metal enrichment ratio
Y/Z=2. Moreover, we identify a correlation between the model's
initial helium/metal abundance and both its age and mixing-length parameter.
Additionally, every model found has a mixing-length parameter below 1.3. This
means that CG Cyg B's mixing-length parameter is clearly smaller than the solar
one. From this study we conclude that ANNs are well suited to deal with the
degeneracy of model solutions of solar type stars.Comment: Accepted for publication in New Astronom
- …