23,888 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
New Numerical Results Indicate a Half-Filling SU(4) Kondo State in Carbon Nanotubes
Numerical calculations simulate transport experiments in carbon nanotube
quantum dots (P. Jarillo-Herrero et al., Nature 434, 484 (2005)), where a
strongly enhanced Kondo temperature T_K ~ 8K was associated with the SU(4)
symmetry of the Hamiltonian at quarter-filling for an orbitally
double-degenerate single-occupied electronic shell. Our results clearly suggest
that the Kondo conductance measured for an adjacent shell with T_K ~ 16K,
interpreted as a singlet-triplet Kondo effect, can be associated instead to an
SU(4) Kondo effect at half-filling. Besides presenting spin-charge Kondo
screening similar to the quarter-filling SU(4), the half-filling SU(4) has been
recently associated to very rich physical behavior, including a
non-Fermi-liquid state (M. R. Galpin et al., Phys. Rev. Lett. 94, 186406
(2005)).Comment: 7 pages, 7 figure
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
Low redshift constraints on energy-momentum-powered gravity models
There has been recent interest in the cosmological consequences of
energy-momentum-powered gravity models, in which the matter side of Einstein's
equations is modified by the addition of a term proportional to some power,
, of the energy-momentum tensor, in addition to the canonical linear term.
In this work we treat these models as phenomenological extensions of the
standard CDM, containing both matter and a cosmological constant. We
also quantitatively constrain the additional model parameters using low
redshift background cosmology data that are specifically from Type Ia
supernovas and Hubble parameter measurements. We start by studying specific
cases of these models with fixed values of which lead to an analytic
expression for the Friedmann equation; we discuss both their current
constraints and how the models may be further constrained by future
observations of Type Ia supernovas for WFIRST complemented by measurements of
the redshift drift by the ELT. We then consider and constrain a more extended
parameter space, allowing to be a free parameter and considering scenarios
with and without a cosmological constant. These models do not solve the
cosmological constant problem per se. Nonetheless these models can
phenomenologically lead to a recent accelerating universe without a
cosmological constant at the cost of having a preferred matter density of
around instead of the usual . Finally we
also briefly constrain scenarios without a cosmological constant, where the
single component has a constant equation of state which needs not be that of
matter; we provide an illustrative comparison of this model with a more
standard dynamical dark energy model with a constant equation of state.Comment: 13+2 pages, 12+1 figures; A&A (in press
Social Effects in Science: Modelling Agents for a Better Scientific Practice
Science is a fundamental human activity and we trust its results because it
has several error-correcting mechanisms. Its is subject to experimental tests
that are replicated by independent parts. Given the huge amount of information
available, scientists have to rely on the reports of others. This makes it
possible for social effects to influence the scientific community. Here, an
Opinion Dynamics agent model is proposed to describe this situation. The
influence of Nature through experiments is described as an external field that
acts on the experimental agents. We will see that the retirement of old
scientists can be fundamental in the acceptance of a new theory. We will also
investigate the interplay between social influence and observations. This will
allow us to gain insight in the problem of when social effects can have
negligible effects in the conclusions of a scientific community and when we
should worry about them.Comment: 14 pages, 5 figure
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