9,248 research outputs found
A model for a non-minimally coupled scalar field interacting with dark matter
In this work we investigate the evolution of a Universe consisted of a scalar
field, a dark matter field and non-interacting baryonic matter and radiation.
The scalar field, which plays the role of dark energy, is non-minimally coupled
to space-time curvature, and drives the Universe to a present accelerated
expansion. The non-relativistic dark matter field interacts directly with the
dark energy and has a pressure which follows from a thermodynamic theory. We
show that this model can reproduce the expected behavior of the density
parameters, deceleration parameter and luminosity distance.Comment: 3 pages, 4 figures. To appear in Brazilian Journal of Physic
Decidability of quantified propositional intuitionistic logic and S4 on trees
Quantified propositional intuitionistic logic is obtained from propositional
intuitionistic logic by adding quantifiers \forall p, \exists p over
propositions. In the context of Kripke semantics, a proposition is a subset of
the worlds in a model structure which is upward closed. Kremer (1997) has shown
that the quantified propositional intuitionistic logic H\pi+ based on the class
of all partial orders is recursively isomorphic to full second-order logic. He
raised the question of whether the logic resulting from restriction to trees is
axiomatizable. It is shown that it is, in fact, decidable. The methods used can
also be used to establish the decidability of modal S4 with propositional
quantification on similar types of Kripke structures.Comment: v2, 9 pages, corrections and additions; v1 8 page
Ferroelectricity from spin supercurrents in LiCuVO4
We have studied the magnetic structure of the ferroelectric frustrated
spin-1/2 chain material LiCuVO4 in applied electric and magnetic fields using
polarized neutrons. A symmetry and mean-field analysis of the data rules out
the presence of static Dzyaloshinskii-Moriya interaction, while exchange
striction is shown to be negligible by our specific-heat measurements. The
experimentally observed magnetoelectric coupling is in excellent agreement with
the predictions of a purely electronic mechanism based on spin supercurrents.Comment: 4 pages, 3 figures, final versio
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.
Irreversible Processes in a Universe modelled as a mixture of a Chaplygin gas and radiation
The evolution of a Universe modelled as a mixture of a Chaplygin gas and
radiation is determined by taking into account irreversible processes. This
mixture could interpolate periods of a radiation dominated, a matter dominated
and a cosmological constant dominated Universe. The results of a Universe
modelled by this mixture are compared with the results of a mixture whose
constituents are radiation and quintessence. Among other results it is shown
that: (a) for both models there exists a period of a past deceleration with a
present acceleration; (b) the slope of the acceleration of the Universe
modelled as a mixture of a Chaplygin gas with radiation is more pronounced than
that modelled as a mixture of quintessence and radiation; (c) the energy
density of the Chaplygin gas tends to a constant value at earlier times than
the energy density of quintessence does; (d) the energy density of radiation
for both mixtures coincide and decay more rapidly than the energy densities of
the Chaplygin gas and of quintessence.Comment: 8 pages, 1 figure, to be published in GR
Revealing the pure confinement effect in glass-forming liquids by dynamic mechanical analysis
Many molecular glass forming liquids show a shift of the glass transition Tg
to lower temperatures when the liquid is confined into mesoporous host
matrices. Two contrary explanations for this effect are given in literature:
First, confinement induced acceleration of the dynamics of the molecules leads
to an effective downshift of Tg increasing with decreasing pore size. Secondly,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts Tg to lower temperatures. Here we present novel dynamic mechanical
analysis measurements of the glass forming liquid salol in Vycor and Gelsil
with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic
susceptibility data can be consistently described with the assumption of two
relaxation processes inside the pores: A surface induced slowed down relaxation
due to interaction with rough pore interfaces and a second relaxation within
the core of the pores. This core relaxation time is reduced with decreasing
pore size d, leading to a downshift of Tg in perfect agreement with recent DSC
measurements
Evidence of a bond-nematic phase in LiCuVO4
Polarized and unpolarized neutron scattering experiments on the frustrated
ferromagnetic spin-1/2 chain LiCuVO4 show that the phase transition at HQ of 8
Tesla is driven by quadrupolar fluctuations and that dipolar correlations are
short-range with moments parallel to the applied magnetic field in the
high-field phase. Heat-capacity measurements evidence a phase transition into
this high-field phase, with an anomaly clearly different from that at low
magnetic fields. Our experimental data are consistent with a picture where the
ground state above HQ has a next-nearest neighbour bond-nematic order along the
chains with a fluid-like coherence between weakly coupled chains.Comment: 5 pages, 4 figures. To appear in Phys. Rev. Let
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