30,765 research outputs found

    Curved Graphene Nanoribbons: Structure and Dynamics of Carbon Nanobelts

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    Carbon nanoribbons (CNRs) are graphene (planar) structures with large aspect ratio. Carbon nanobelts (CNBs) are small graphene nanoribbons rolled up into spiral-like structures, i. e., carbon nanoscrolls (CNSs) with large aspect ratio. In this work we investigated the energetics and dynamical aspects of CNBs formed from rolling up CNRs. We have carried out molecular dynamics simulations using reactive empirical bond-order potentials. Our results show that similarly to CNSs, CNBs formation is dominated by two major energy contribution, the increase in the elastic energy due to the bending of the initial planar configuration (decreasing structural stability) and the energetic gain due to van der Waals interactions of the overlapping surface of the rolled layers (increasing structural stability). Beyond a critical diameter value these scrolled structures can be even more stable (in terms of energy) than their equivalent planar configurations. In contrast to CNSs that require energy assisted processes (sonication, chemical reactions, etc.) to be formed, CNBs can be spontaneously formed from low temperature driven processes. Long CNBs (length of ∼\sim 30.0 nm) tend to exhibit self-folded racket-like conformations with formation dynamics very similar to the one observed for long carbon nanotubes. Shorter CNBs will be more likely to form perfect scrolled structures. Possible synthetic routes to fabricate CNBs from graphene membranes are also addressed

    The first analytical expression to estimate photometric redshifts suggested by a machine

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    We report the first analytical expression purely constructed by a machine to determine photometric redshifts (zphotz_{\rm phot}) of galaxies. A simple and reliable functional form is derived using 41,21441,214 galaxies from the Sloan Digital Sky Survey Data Release 10 (SDSS-DR10) spectroscopic sample. The method automatically dropped the uu and zz bands, relying only on gg, rr and ii for the final solution. Applying this expression to other 1,417,1811,417,181 SDSS-DR10 galaxies, with measured spectroscopic redshifts (zspecz_{\rm spec}), we achieved a mean ⟨(zphot−zspec)/(1+zspec)⟩≲0.0086\langle (z_{\rm phot} - z_{\rm spec})/(1+z_{\rm spec})\rangle\lesssim 0.0086 and a scatter σ(zphot−zspec)/(1+zspec)≲0.045\sigma_{(z_{\rm phot} - z_{\rm spec})/(1+z_{\rm spec})}\lesssim 0.045 when averaged up to z≲1.0z \lesssim 1.0. The method was also applied to the PHAT0 dataset, confirming the competitiveness of our results when faced with other methods from the literature. This is the first use of symbolic regression in cosmology, representing a leap forward in astronomy-data-mining connection.Comment: 6 pages, 4 figures. Accepted for publication in MNRAS Letter

    Gravitational energy of rotating black holes

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    In the teleparallel equivalent of general relativity the energy density of asymptotically flat gravitational fields can be naturaly defined as a scalar density restricted to a three-dimensional spacelike hypersurface Σ\Sigma. Integration over the whole Σ\Sigma yields the standard ADM energy. After establishing the reference space with zero gravitational energy we obtain the expression of the localized energy for a Kerr black hole. The expression of the energy inside a surface of constant radius can be explicitly calculated in the limit of small aa, the specific angular momentum. Such expression turns out to be exactly the same as the one obtained by means of the method preposed recently by Brown and York. We also calculate the energy contained within the outer horizon of the black hole for {\it any} value of aa. The result is practically indistinguishable from E=2MirE=2M_{ir}, where MirM_{ir} is the irreducible mass of the black hole.Comment: 18 pages, LaTex file, one figur

    Measurements of \gamma \gamma \to \mbox{Higgs} and γγ→W+W−\gamma \gamma \to W^{+}W^{-} in e+e−e^{+}e^{-} collisions at the Future Circular Collider

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    The measurements of the two-photon production of the Higgs boson and of W±W^{\pm} boson pairs in e+e−e^{+}e^{-} collisions at the Future Circular Collider (FCC-ee) are investigated. The processes e+e−→γγe+ H e−,e+ W+W− e−e^{+}e^{-}\xrightarrow{\gamma \gamma}e^+\,{\rm H}\,e^-,e^+\,{\rm W^+W^-}\,e^- are computed using the effective photon approximation for electron-positron beams, and studied in their H→bbˉ{\rm H}\to b\bar{b} and W+W−→4j{\rm W^+W^-}\to 4j decay final-states including parton showering and hadronization, jet reconstruction, e±e^\pm forward tagging, and realistic experimental cuts. After selection criteria, up to 75 Higgs bosons and 6600 W±\rm W^{\pm} pairs will be reconstructed on top of controllable continuum backgrounds at s=\sqrt{s} = 240 and 350 GeV for the total expected integrated luminosities, by tagging the scattered e±e^\pm with near-beam detectors. A 5σ\sigma observation of γγ→\gamma \gamma \toH is thereby warranted, as well as high-statistics studies of triple γWW\rm \gamma WW and quartic γγWW\rm \gamma\gamma WW electroweak couplings, improving by at least factors of 2 and 10 the current limits on dimension-6 anomalous quartic gauge couplings.Comment: Presented at EDS Blois 2017 Conference , Prague, Czech Republic, June 26--30, 201

    Evidence for quasi-chemically homogeneous evolution of massive stars up to solar metallicity

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    Long soft gamma ray bursts (LGRBs) are usually associated with the death of the most massive stars. A large amount of core angular momentum in the phases preceding the explosion is required to form LGRBs. A very high initial rotational velocity can provide this angular momentum. Such a velocity strongly influences the way the star evolves: it is chemically homogeneously mixed and evolves directly towards the blue part of the HR diagram from the main sequence. We have shown that chemically homogeneous evolution (CHE) takes place in the SMC, at low metallicity. We want to see if there is a metallicity threshold above which such an evolution does not exist. We perform a spectroscopic analysis of H-rich early-type WN stars in the LMC and the Galaxy. We use the code CMFGEN to determine the fundamental properties and the surface composition of the target stars. We then place the stars in the HR diagram and determine their evolution. We show that both the LMC and Galactic WNh stars we selected cannot be explained by standard stellar evolution. They are located on the left of the main sequence but show surface abundances typical of CN equilibrium. In addition, they still contain a large amount of hydrogen. They are thus core-H burning objects. Their properties are consistent with CHE. We determine the metallicity of the Galactic stars from their position and Galactic metallicity gradients, and conclude that they have 0.6<Z<1.0. A moderate coupling between the core and the envelope is required to explain that stellar winds do not extract to much angular momentum to prevent a blueward evolution. In view of the findings that some long gamma ray bursts appear in solar environments, CHE may be a viable way to form them over a wide range of metallicities.Comment: 10 pages, 10 figures. Accepted in Astronomy and Astrophysic
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