793 research outputs found
Highly dynamically evolved intermediate-age open clusters
We present a comprehensive UBVRI and Washington CT1T2 photometric analysis of seven catalogued open clusters, namely: Ruprecht 3, 9, 37, 74, 150, ESO 324-15 and 436-2. The multiband photometric data sets in combination with 2MASS photometry and Gaia astrometry for the brighter stars were used to estimate their structural parameters and fundamental astrophysical properties.We found that Ruprecht 3 and ESO 436-2 do not show self-consistent evidence of being physical systems. The remained studied objects are open clusters of intermediate age (9.0 ≤ log(t yr-1) ≤ 9.6), of relatively small size (rcls ~ 0.4-1.3 pc) and placed between 0.6 and 2.9 kpc from the Sun.We analysed the relationships between core, half-mass, tidal and Jacoby radii as well as half-mass relaxation times to conclude that the studied clusters are in an evolved dynamical stage. The total cluster masses obtained by summing those of the observed cluster stars resulted to be ~10-15 per cent of the masses of open clusters of similar age located closer than 2 kpc from the Sun. We found that cluster stars occupy volumes as large as those for tidally filled clusters.Fil: Piatti, Andres Eduardo. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba; ArgentinaFil: Dias, Wilton S.. Universidade Federal de Itajuba; BrasilFil: Sampedro, Laura M.. Instituto de AstrofÃsica de AndalucÃa; España. Universidade de Sao Paulo; Brasi
Anomalous screening of an electrostatic field at the surface of niobium nitride
The interaction between an electric field and the electric charges in a
material is described by electrostatic screening, which in metallic systems is
commonly thought to be confined within a distance of the order of the
Thomas-Fermi length. The validity of this picture, which holds for surface
charges up to , has been recently questioned by
several experimental results when dealing with larger surface charges, such as
those routinely achieved via the ionic gating technique. Whether these results
can be accounted for in a purely electrostatic picture is still debated. In
this work, we tackle this issue by calculating the spatial dependence of the
charge carrier density in thin slabs of niobium nitride via an ab initio
density functional theory approach in the field-effect transistor
configuration. We find that perturbations induced by surface charges are mainly screened within the first layer, while
those induced by larger surface charges can
penetrate over multiple atomic layers, in reasonable agreement with the
available experimental data. Furthermore, we show that a significant
contribution to the screening of large fields is associated not only to the
accumulation layer of the induced charge carriers at the surface, but also to
the polarization of the pre-existing charge density of the undoped system.Comment: 8 pages, 4 figure
Towards the insulator-to-metal transition at the surface of ion-gated nanocrystalline diamond films
Hole doping can control the conductivity of diamond either through boron
substitution, or carrier accumulation in a field-effect transistor. In this
work, we combine the two methods to investigate the insulator-to-metal
transition at the surface of nanocrystalline diamond films. The finite boron
doping strongly increases the maximum hole density which can be induced
electrostatically with respect to intrinsic diamond. The ionic gate pushes the
conductivity of the film surface away from the variable-range hopping regime
and into the quantum critical regime. However, the combination of the strong
intrinsic surface disorder due to a non-negligible surface roughness, and the
introduction of extra scattering centers by the ionic gate, prevents the
surface accumulation layer to reach the metallic regime.Comment: 5 pages, 4 figure
Mass distribution and structural parameters of Small Magellanic Cloud star clusters
In this work we estimate, for the first time, the total masses and mass
function slopes of a sample of 29 young and intermediate-age SMC clusters from
CCD Washington photometry. We also derive age, interstellar reddening and
structural parameters for most of the studied clusters by employing a
statistical method to remove the unavoidable field star contamination. Only
these 29 clusters out of 68 originally analysed cluster candidates present
stellar overdensities and coherent distribution in their colour-magnitude
diagrams compatible with the existence of a genuine star cluster. We employed
simple stellar population models to derive general equations for estimating the
cluster mass based only on its age and integrated light in the B, V, I, C and
T1 filter. These equations were tested against mass values computed from
luminosity functions, showing an excellent agreement. The sample contains
clusters with ages between 60 Myr and 3 Gyr and masses between 300 and 3000 Mo
distributed between ~0.5 deg. and ~2 deg. from the SMC optical centre. We
determined mass function slopes for 24 clusters, of which 19 have slopes
compatible with that of Kroupa IMF (2.3 +/- 0.7), considering the
uncertainties. The remaining clusters - H86-188, H86-190, K47, K63 and NGC242 -
showed flatter MFs. Additionally, only clusters with masses lower than ~1000 Mo
and flatter MF were found within ~0.6 deg. from the SMC rotational centre.Comment: 12 pages, 19 figures. Includes another 29 full-page figures of
supplementary material. Accepted for publication in the MNRA
Accurate radial velocity and metallicity of the Large Magellanic Cloud old globular clusters NGC1928 and NGC1939
We present results obtained from spectroscopic observations of red giants located in the fields of the Large Magellanic Cloud (LMC) globular clusters (GCs) NGC1928 and NGC1939. We used the GMOS and AAOmega+2dF spectrographs to obtain spectra centred on the Ca II triplet, from which we derived individual radial velocities (RVs) and metallicities. From cluster members we derived mean RVs of RVNGC1928 = 249.58±4.65 km s-1 and RVNGC1939 = 258.85±2.08 km s-1, and mean metallicities of [Fe/H]NGC1928 = -1.30±0.15 dex and [Fe/H]NGC1939 = -2.00±0.15 dex. We found that both GCs have RVs and positions consistent with being part of the LMC disc, so that we rule out any possible origin, but in the same galaxy. By computing the best solution of a disc that fully contains each GC, we obtained circular velocities for the 15 known LMC GCs. We found that 11/15 of the GCs share the LMC rotation derived from HST and Gaia DR2 proper motions. This outcome reveals that the LMC disc existed since the very early epoch of the galaxy formation and experienced the steep relatively fast chemical enrichment shown by its GC metallicities. The four remaining GCs turned out to have circular velocities not compatible with an in situ cluster formation, but rather with being stripped from the SMC.Fil: Piatti, Andres Eduardo. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Hwang, N.. Korea Astronomy And Space Science Institute; Corea del SurFil: Cole, A. A.. University of Tasmania; AustraliaFil: Angelo, M. S.. Laboratorio Nacional de Astrofisica; BrasilFil: Emptage, B.. University of Tasmania; Australi
Gemini/GMOS photometry of intermediate-age star clusters in the Large Magellanic Cloud
We present Gemini South GMOS g,i photometry of 14 intermediate-age Large
Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203,
2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a
continuing project to investigate the extended Main Sequence Turnoff (EMSTO)
phenomenon. Extensive artificial star tests were made over the observed field
of view. These tests reveal the observed behaviour of photometric errors with
magnitude and crowding. The cluster stellar density radial profiles were traced
from star counts over the extent of the observed field. We adopt clus- ter
radii and build colour-magnitude diagrams (CMDs) with cluster features clearly
identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching
of theoretical isochrones. The studied LMC clusters are confirmed to be
intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162
and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the
basis of having dual red clumps.Comment: MNRAS, accepte
Carrier mobility and scattering lifetime in electric double-layer gated few-layer graphene
We fabricate electric double-layer field-effect transistor (EDL-FET) devices
on mechanically exfoliated few-layer graphene. We exploit the large capacitance
of a polymeric electrolyte to study the transport properties of three, four and
five-layer samples under a large induced surface charge density both above and
below the glass transition temperature of the polymer. We find that the carrier
mobility shows a strong asymmetry between the hole and electron doping regime.
We then employ ab-initio density functional theory (DFT) calculations to
determine the average scattering lifetime from the experimental data. We
explain its peculiar dependence on the carrier density in terms of the specific
properties of the electrolyte we used in our experiments.Comment: 6 pages, 3 figure
Mapping multi-valley Lifshitz transitions induced by field-effect doping in strained MoS2 nanolayers
Gate-induced superconductivity at the surface of nanolayers of semiconducting
transition metal dichalcogenides (TMDs) has attracted a lot of attention in
recent years, thanks to the sizeable transition temperature, robustness against
in-plane magnetic fields beyond the Pauli limit, and hints to a
non-conventional nature of the pairing. A key information necessary to unveil
its microscopic origin is the geometry of the Fermi surface hosting the Cooper
pairs as a function of field-effect doping, which is dictated by the filling of
the inequivalent valleys at the K/K and Q/Q points of the
Brillouin Zone. Here, we achieve this by combining Density Functional Theory
calculations of the bandstructure with transport measurements on ion-gated
2H-MoS nanolayers. We show that, when the number of layers and the amount
of strain are set to their experimental values, the Fermi level crosses the
bottom of the high-energy valleys at Q/Q at doping levels where
characteristic kinks in the transconductance are experimentally detected. We
also develop a simple 2D model which is able to quantitatively describe the
broadening of the kinks observed upon increasing temperature. We demonstrate
that this combined approach can be employed to map the dependence of the Fermi
surface of TMD nanolayers on field-effect doping, detect Lifshitz transitions,
and provide a method to determine the amount of strain and spin-orbit splitting
between sub-bands from electric transport measurements in real devices.Comment: 8 pages, 4 figure
Nearly coeval intermediate-age Milky Way star clusters at very different dynamics evolutionary stages
We report astrophysical properties of 12 Milky Way open clusters located beyond a 2 kpc circle around the Sun by using deep optical photometry. We estimated their age and metallicities on the basis of a maximum likelihood approach using astrometric members determined from Gaia DR2 data. The studied clusters turned out to be of intermediate-age (0.8-4.0 Gyr), with metallicities spanning the range [Fe/H] ∼ −0.5-+0.1 dex, and distributed within the general observed trend of the Milky Way disc radial and perpendicular metallicity gradients. As far as we are aware, these are the first metal abundance estimates derived for these clusters so far. From the constructed stellar density radial profiles and cluster mass functions we obtained a variety of structural and internal dynamics evolution parameters. They show that while the innermost cluster regions would seem to be mainly shaped according to the respective internal dynamics evolutionary stages, the outermost ones would seem to be slightly more sensitive to the Milky Way tidal field. The nearly coeval studied clusters are experiencing different levels of two-body relaxation following star evaporation; those at more advanced stages being more compact objects. Likewise, we found that the more important the Milky way tides, the larger the Jacobi volume occupied by the clusters, irrespective of their actual sizes and internal dynamics evolutionary stages.Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Angelo, Mateus S.. Centro Federal de Educacao Tecnologica de Minas Gerais; BrasilFil: Dias, Wilton S.. Universidade Federal de Itajuba; Brasi
Proximity Eliashberg theory of electrostatic field-effect-doping in superconducting films
We calculate the effect of a static electric field on the critical
temperature of a s-wave one band superconductor in the framework of proximity
effect Eliashberg theory. In the weak electrostatic field limit the theory has
no free parameters while, in general, the only free parameter is the thickness
of the surface layer where the electric field acts. We conclude that the best
situation for increasing the critical temperature is to have a very thin film
of a superconducting material with a strong increase of electron-phonon (boson)
constant upon charging.Comment: 9 pages, 5 figure
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