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 Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Dias, Wilton S.. Universidade Federal de Itajuba; BrasilFil: Sampedro, Laura M.. Instituto de Astrofísica de Andalucía; 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 $\sim 10^{13}\,\mathrm{cm^{-2}}$, 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 $\lesssim 10^{14}\,\mathrm{cm^{-2}}$ are mainly screened within the first layer, while those induced by larger surface charges $\sim 10^{15}\,\mathrm{cm^{-2}}$ 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 Científicas 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$^{\prime}$ and Q/Q$^{\prime}$ 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$_{2}$ 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$^{\prime}$ 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 Científicas 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