9,772 research outputs found
Acceleration field of a Universe modeled as a mixture of scalar and matter fields
A model of the Universe as a mixture of a scalar (inflaton or rolling tachyon
from the string theory) and a matter field (classical particles) is analyzed.
The particles are created at the expense of the gravitational energy through an
irreversible process whereas the scalar field is supposed to interact only with
itself and to be minimally coupled with the gravitational field. The
irreversible processes of particle creation are related to the non-equilibrium
pressure within the framework of the extended (causal or second-order)
thermodynamic theory. The scalar field (inflaton or tachyon) is described by an
exponential potential density added by a parameter which represents its
asymptotic value and can be interpreted as the vacuum energy. This model can
simulate three phases of the acceleration field of the Universe, namely,(a) an
inflationary epoch with a positive acceleration followed by a decrease of the
acceleration field towards zero, (b) a past decelerated period where the
acceleration field decreases to a maximum negative value followed by an
increase towards zero, and (c) a present accelerated epoch. For the energy
densities there exist also three distinct epochs which begin with a scalar
field dominated period followed by a matter field dominated epoch and coming
back to a scalar field dominated phase.Comment: 9 pages, 2 figures, to be published in General Relativity and
Gravitatio
Specific Heat of the Ca-Intercalated Graphite Superconductor CaC
The superconducting state of Ca-intercalated graphite CaC6 has been
investigated by specific heat measurements. The characteristic anomaly at the
superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of
the superconductivity. The temperature and magnetic field dependence of the
electronic specific heat are consistent with a fully-gapped superconducting
order parameter. The estimated electron-phonon coupling constant is lambda =
0.60 - 0.74 suggesting that the relatively high Tc of CaC6 can be explained
within the weak-coupling BCS approach.Comment: 4 pages, 4 figs, submitted to Phys. Rev. Let
Tests for Establishing Security Properties
Ensuring strong security properties in some cases requires participants to carry out tests during the execution of a protocol. A classical example is electronic voting: participants are required to verify the presence of their ballots on a bulletin board, and to verify the computation of the election outcome. The notion of certificate transparency is another example, in which participants in the protocol are required to perform tests to verify the integrity of a certificate log.
We present a framework for modelling systems with such `testable properties', using the applied pi calculus. We model the tests that are made by participants in order to obtain the security properties. Underlying our work is an attacker model called ``malicious but cautious'', which lies in between the Dolev-Yao model and the ``honest but curious'' model. The malicious-but-cautious model is appropriate for cloud computing providers that are potentially malicious but are assumed to be cautious about launching attacks that might cause user tests to fail
Stress Relaxation of Entangled Polymer Networks
The non-linear stress-strain relation for crosslinked polymer networks is
studied using molecular dynamics simulations. Previously we demonstrated the
importance of trapped entanglements in determining the elastic and relaxational
properties of networks. Here we present new results for the stress versus
strain for both dry and swollen networks. Models which limit the fluctuations
of the network strands like the tube model are shown to describe the stress for
both elongation and compression. For swollen networks, the total modulus is
found to decrease like (V_0/V)^{2/3} and goes to the phantom model result only
for short strand networks.Comment: 9 pages, 3 figures, RevTe
Palatini approach to 1/R gravity and its implications to the late Universe
By applying the Palatini approach to the 1/R-gravity model it is possible to
explain the present accelerated expansion of the Universe. Investigation of the
late Universe limiting case shows that: (i) due to the curvature effects the
energy-momentum tensor of the matter field is not covariantly conserved; (ii)
however, it is possible to reinterpret the curvature corrections as sources of
the gravitational field, by defining a modified energy-momentum tensor; (iii)
with the adoption of this modified energy-momentum tensor the Einstein's field
equations are recovered with two main modifications: the first one is the
weakening of the gravitational effects of matter whereas the second is the
emergence of an effective varying "cosmological constant"; (iv) there is a
transition in the evolution of the cosmic scale factor from a power-law scaling
to an asymptotically exponential scaling ; (v) the energy density of the matter field scales as ; (vi) the present age of the Universe and the
decelerated-accelerated transition redshift are smaller than the corresponding
ones in the CDM model.Comment: 5 pages and 2 figures. Accepted in PR
Evolution of transport properties of BaFe2-xRuxAs2 in a wide range of isovalent Ru substitution
The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are
investigated by measuring resistivity and Hall coefficient on high-quality
single crystals in a wide range of doping (0 < x < 1.4). Ru substitution
weakens the antiferromagnetic (AFM) order, inducing superconductivity for
relatively high doping level of 0.4 < x < 0.9. Near the AFM phase boundary, the
transport properties show non-Fermi-liquid-like behaviors with a
linear-temperature dependence of resistivity and a strong temperature
dependence of Hall coefficient with a sign change. Upon higher doping, however,
both of them recover conventional Fermi-liquid behaviors. Strong doping
dependence of Hall coefficient together with a small magnetoresistance suggest
that the anomalous transport properties can be explained in terms of
anisotropic charge carrier scattering due to interband AFM fluctuations rather
than a conventional multi-band scenario.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Superconductivity in Heavy Alkaline-Earths Intercalated Graphites
We report the discovery of superconductivity below 1.65(6) K in
Sr-intercalated graphite SrC6, by susceptibility and specific heat (Cp)
measurements. In comparison with CaC6, we found that the anisotropy of the
upper critical fields for SrC6 is much reduced. The Cp anomaly at Tc is smaller
than the BCS prediction indicating an anisotropic superconducting gap for SrC6
similar to CaC6. The significantly lower Tc of SrC6 as compared to CaC6 can be
understood in terms of "negative" pressure effects, which decreases the
electron-phonon coupling for both in-plane intercalant and the out-of-plane C
phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
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