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Graphene is a very attractive material for nanomechanical devices and membrane applications. Graphene blisters based on silicon oxide micro-cavities are a simple but relevant example of nanoactuators. A drawback of this experimental set up is that gas leakage through the graphene-SiO2 interface contributes significantly to the total leak rate. Here we study the diffusion of air from pressurized graphene drumheads on SiO2 micro-cavities and propose a straightforward method to improve the already strong adhesion between graphene and the underlying SiO2 substrate, resulting in reduced leak rates. This is carried out by applying controlled and localized ultrahigh pressure (> 10 GPa) with an Atomic Force Microscopy diamond tip. With this procedure, we are able to significantly approach the graphene layer to the SiO2 surface around the drumheads, thus enhancing the interaction between them allowing us to better seal the graphene-SiO2 interface, which is reflected in up to ~ 4 times lower leakage rates. Our work opens an easy way to improve the performance of graphene as a gas membrane on a technological relevant substrate such as SiO2., Comment: pages 19, 4 figures + supplementary information

Contrary to most materials, graphene exhibits a negative thermal expansion coefficient (TEC), i.e it contracts when heated. This contraction is due to the thermal excitation of low energy out-of-plane vibration modes. These flexural modes have been reported to govern the electronic transport and the mechanical response of suspended graphene. In this work, we systematically investigate the influence of defects in the TEC of suspended graphene membranes. Controlled introduction of low densities of mono-vacancies reduces the graphene TEC, up to one order of magnitude for a defect density of $5\times10^{12}$~cm$^{-2}$ . Our molecular dynamics simulations reproduce the observed trend and show that TEC reduction is due to the suppression of out-of-plane fluctuations caused by the strain fields created by mono-vacancies in their surrounding areas. These results highlight the key role of defects in the properties of "real-life" graphene, and pave the way for future proposals of electronic and mechanical defect engineering., Comment: 19

The purpose of this research is to study the connection of global properties of eight young stellar clusters projected in the Vista Variables in the Via Lactea (VVV) ESO Large Public Survey disk area and their young stellar object population. The analysis in based on the combination of spectroscopic parallax-based reddening and distance determinations with main sequence and pre-main sequence ishochrone fitting to determine the basic parameters (reddening, age, distance) of the sample clusters. The lower mass limit estimations show that all clusters are low or intermediate mass (between 110 and 1800 Mo), the slope Gamma of the obtained present-day mass functions of the clusters is close to the Kroupa initial mass function. On the other hand, the young stellar objects in the surrounding cluster's fields are classified by low resolution spectra, spectral energy distribution fit with theoretical predictions, and variability, taking advantage of multi-epoch VVV observations. All spectroscopically confirmed young stellar objects (except one) are found to be massive (more than 8 Mo). Using VVV and GLIMPSE color-color cuts we have selected a large number of new young stellar object candidates, which are checked for variability and 57% are found to show at least low-amplitude variations. In few cases it was possible to distinguish between YSO and AGB classification on the basis of the light curves., Comment: 45 pages; 17 figures; published in AJ

We present the first physical characterization of the young open cluster VVV CL041. We spectroscopically observed the cluster main-sequence stellar population and a very-massive star candidate: WR62-2. CMFGEN modeling to our near-infrared spectra indicates that WR62-2 is a very luminous (10$^{6.4\pm0.2} L_{\odot}$) and massive ($\sim80 M_{\odot}$) star., Comment: 1 page, 1 figure, to be published in the "International Workshop on Wolf-Rayet Stars conference proceedings" by Universit\"atsverlag Potsdam (editors: W.-R. Hamann, A. Sander, and H. Todt)

We present an adaptive multi-GPU Exchange Monte Carlo method designed for the simulation of the 3D Random Field Model. The algorithm design is based on a two-level parallelization scheme that allows the method to scale its performance in the presence of faster and GPUs as well as multiple GPUs. The set of temperatures is adapted according to the exchange rate observed from short trial runs, leading to an increased exchange rate at zones where the exchange process is sporadic. Performance results show that parallel tempering is an ideal strategy for being implemented on the GPU, and runs between one to two orders of magnitude with respect to a single-core CPU version, with multi-GPU scaling being approximately $99\%$ efficient. The results obtained extend the possibilities of simulation to sizes of $L = 32, 64$ for a workstation with two GPUs., Comment: 15 pages, 10 figures

Context. The Vista Variables in the V\'ia L\'actea (VVV) ESO Public Survey is a variability survey of the Milky Way bulge and an adjacent section of the disk carried out from 2010 on ESO Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV will eventually deliver a deep near-IR atlas with photometry and positions in five passbands (ZYJHK_S) and a catalogue of 1-10 million variable point sources - mostly unknown - which require classifications. Aims. The main goal of the VVV Templates Project, that we introduce in this work, is to develop and test the machine-learning algorithms for the automated classification of the VVV light-curves. As VVV is the first massive, multi-epoch survey of stellar variability in the near-infrared, the template light-curves that are required for training the classification algorithms are not available. In the first paper of the series we describe the construction of this comprehensive database of infrared stellar variability. Methods. First we performed a systematic search in the literature and public data archives, second, we coordinated a worldwide observational campaign, and third we exploited the VVV variability database itself on (optically) well-known stars to gather high-quality infrared light-curves of several hundreds of variable stars. Results. We have now collected a significant (and still increasing) number of infrared template light-curves. This database will be used as a training-set for the machine-learning algorithms that will automatically classify the light-curves produced by VVV. The results of such an automated classification will be covered in forthcoming papers of the series., Comment: 12 pages, 16 figures, 3 tables, accepted for publication in A&A. Most of the data are now accessible through http://www.vvvtemplates.org/

The Vista Variables in the V\'ia L\'actea (VVV) ESO Public Survey is an ongoing time-series, near-infrared (IR) survey of the Galactic bulge and an adjacent portion of the inner disk, covering 562 square degrees of the sky, using ESO's VISTA telescope. The survey has provided superb multi-color photometry in 5 broadband filters ($Z$, $Y$, $J$, $H$, and $K_s$), leading to the best map of the inner Milky Way ever obtained, particularly in the near-IR. The main variability part of the survey, which is focused on $K_s$-band observations, is currently underway, with bulge fields having been observed between 31 and 70 times, and disk fields between 17 and 36 times. When the survey is complete, bulge (disk) fields will have been observed up to a total of 100 (60) times, providing unprecedented depth and time coverage. Here we provide a first overview of stellar variability in the VVV data, including examples of the light curves that have been collected thus far, scientific applications, and our efforts towards the automated classification of VVV light curves., Comment: Presented at the "40 Years of Variable Stars: A Celebration of Contributions by Horace A. Smith" conference (arXiv:1310.0149). 49 pages, 28 figures

We report and analyze low-temperature measurements of the conductance of partially disordered reduced graphene oxide, finding that the data follow a simple crossover scenario. At room temperature, conductance is dominated by two-dimensional (2D) electric field-assisted, thermally-driven (Pollak-Riess) variable-range hopping (VRH) through highly-disordered regions. However, at lower temperatures T, we find a smooth crossover to follow the exp(-E_0/E)^(1/3) field-driven (Shklovskii) 2D VRH conductance behaviour when the electric field E exceeds a specific crossover value E_C (T)_2D = (E_a E_0^(1/3) /3)^(3/4) determined by the scale factors E_0 and E_a for the high-field and intermediate field regimes respectively. Our crossover scenario also accounts well for experimental data reported by other authors for three-dimensional disordered carbon networks, suggesting wide applicability., Comment: 8 pages, 4 figures; Corrected typo in Table 1, value in the last row of column E_a should be 21.74

We present the results of a search for variable stars in the globular cluster NGC 5286, which has recently been suggested to be associated with the Canis Major dwarf spheroidal galaxy. 57 variable stars were detected, only 19 of which had previously been known. Among our detections one finds 52 RR Lyrae (22 RRc and 30 RRab), 4 LPV's, and 1 type II Cepheid of the BL Herculis type. Periods are derived for all of the RR Lyrae as well as the Cepheid, and BV light curves are provided for all the variables. The mean period of the RRab variables is = 0.656 days, and the number fraction of RRc stars is N(c)/N(RR) = 0.42, both consistent with an Oosterhoff II (OoII) type -- thus making NGC 5286 one of the most metal-rich ([Fe/H] = -1.67; Harris 1996) OoII globulars known to date. The minimum period of the \RRab's, namely Pab,min = 0.513 d, while still consistent with an OoII classification, falls towards the short end of the observed Pab,min distribution for OoII globular clusters. As was recently found in the case of the prototypical OoII globular cluster M15 (NGC 7078), the distribution of stars in the Bailey diagram does not strictly conform to the previously reported locus for OoII stars. We provide Fourier decomposition parameters for all of the RR Lyrae stars detected in our survey, and discuss the physical parameters derived therefrom. The values derived for the RRc's are not consistent with those typically found for OoII clusters, which may be due to the cluster's relatively high metallicity -- the latter being confirmed by our Fourier analysis of the ab-type RR Lyrae light curves. We derive for the cluster a revised distance modulus of (m-M)V = 16.04 mag. (ABRIDGED), Comment: 15 pages, 8 figures (plus 57 light curve sets, in an Appendix). The Astronomical Journal, in press

Carbon nanotubes are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge. Defects are known to modify the electrical resistance of carbon nanotubes. They can be present in as-grown carbon nanotubes, but controlling externally their density opens a path towards the tuning of the nanotube electronic characteristics. In this work consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room temperature resistance versus SWNT-length [R(L)] along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a 400 nm SWNT length., Comment: 16 pages, 4 figures

We present a long-term project aimed at completing the census of (bright) variable stars in Galactic globular clusters. While our main aim is to obtain a reliable assessment of the populations of RR Lyrae and type II Cepheid stars in the Galactic globular cluster system, due attention is also being paid to other types of variables, including SX Phoenicis stars, long-period variables, and eclipsing binaries., Comment: 4 pages, 4 figures. MSAIt, in press