30,765 research outputs found
Curved Graphene Nanoribbons: Structure and Dynamics of Carbon Nanobelts
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 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
We report the first analytical expression purely constructed by a machine to
determine photometric redshifts () of galaxies. A simple and
reliable functional form is derived using galaxies from the Sloan
Digital Sky Survey Data Release 10 (SDSS-DR10) spectroscopic sample. The method
automatically dropped the and bands, relying only on , and
for the final solution. Applying this expression to other SDSS-DR10
galaxies, with measured spectroscopic redshifts (), we achieved a
mean and a scatter when averaged up to . 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
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 .
Integration over the whole 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 , 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 . The result is
practically indistinguishable from , where is the
irreducible mass of the black hole.Comment: 18 pages, LaTex file, one figur
Measurements of \gamma \gamma \to \mbox{Higgs} and in collisions at the Future Circular Collider
The measurements of the two-photon production of the Higgs boson and of
boson pairs in collisions at the Future Circular
Collider (FCC-ee) are investigated. The processes
are computed using the effective photon approximation for electron-positron
beams, and studied in their and
decay final-states including parton showering and hadronization, jet
reconstruction, forward tagging, and realistic experimental cuts. After
selection criteria, up to 75 Higgs bosons and 6600 pairs will be
reconstructed on top of controllable continuum backgrounds at 240
and 350 GeV for the total expected integrated luminosities, by tagging the
scattered with near-beam detectors. A 5 observation of H is thereby warranted, as well as high-statistics studies of triple
and quartic 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
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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