4,938 research outputs found
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
Inflationary and dark energy regimes in 2+1 dimensions
In this work we investigate the behavior of three-dimensional (3D)
cosmological models. The simulation of inflationary and dark-energy-dominated
eras are among the possible results in these 3D formulations; taking as
starting point the results obtained by Cornish and Frankel.
Motivated by those results, we investigate, first, the inflationary case
where we consider a two-constituent cosmological fluid: the scalar field
represents the hypothetical inflaton which is in gravitational interaction with
a matter/radiation contribution. For the description of an old universe, it is
possible to simulate its evolution starting with a matter dominated universe
that faces a decelerated/accelerated transition due to the presence of the
additional constituent (simulated by the scalar field or ruled by an exotic
equation of state) that plays the role of dark energy. We obtain, through
numerical analysis, the evolution in time of the scale factor, the
acceleration, the energy densities, and the hydrostatic pressure of the
constituents. The alternative scalar cosmology proposed by Cornish and Frankel
is also under investigation in this work. In this case an inflationary model
can be constructed when another non-polytropic equation of state (the van der
Waals equation) is used to simulate the behavior of an early 3D universe.Comment: Latex file, plus 9 figures. To appear in General Relativity and
Gravitatio
Effect of Pressure on Superconducting Ca-intercalated Graphite CaC
The pressure effect on the superconducting transition temperature () of
the newly-discovered Ca-intercalated graphite compound CaC has been
investigated up to 16 kbar. is found to increase under pressure
with a large relative ratio / of +0.4 %/kbar. Using
first-principles calculations, we show that the large and positive effect of
pressure on can be explained in the scope of electron-phonon theory due
to the presence of a soft phonon branch associated to in-plane vibrations of Ca
atoms. Implications of the present findings on the current debate about the
superconducting mechanism in graphite intercalation compounds are discussed.Comment: 6 pages, 5 figs, final PRB versio
3D gravity and non-linear cosmology
By the inclusion of an additional term, non-linear in the scalar curvature
, it is tested if dark energy could rise as a geometrical effect in 3D
gravitational formulations. We investigate a cosmological fluid obeying a
non-polytropic equation of state (the van der Waals equation) that is used to
construct the energy-momentum tensor of the sources, representing the
hypothetical inflaton in gravitational interaction with a matter contribution.
Following the evolution in time of the scale factor, its acceleration, and
the energy densities of constituents it is possible to construct the
description of an inflationary 3D universe, followed by a matter dominated era.
For later times it is verified that, under certain conditions, the non-linear
term in can generate the old 3D universe in accelerated expansion, where
the ordinary matter is represented by the barotropic limit of the van der Waals
constituent.Comment: 7 pages, to appear in Mod. Phys. Let
Cosmological model with non-minimally coupled fermionic field
A model for the Universe is proposed whose constituents are: (a) a dark
energy field modeled by a fermionic field non-minimally coupled with the
gravitational field, (b) a matter field which consists of pressureless baryonic
and dark matter fields and (c) a field which represents the radiation and the
neutrinos. The coupled system of Dirac's equations and Einstein field equations
is solved numerically by considering a spatially flat homogeneous and isotropic
Universe. It is shown that the proposed model can reproduce the expected
red-shift behaviors of the deceleration parameter, of the density parameters of
each constituent and of the luminosity distance. Furthermore, for small values
of the red-shift the constant which couples the fermionic and gravitational
fields has a remarkable influence on the density and deceleration parameters.Comment: Accepted for publication in Europhysics Letter
Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure
We present an experimental lattice location study of Ga atoms in Ge after ion implantation at elevated temperature (250°C). Using extended x-rayabsorption fine structure (EXAFS) experiments and a dedicated sample preparation method, we have studied the lattice location of Ga atoms in Ge with a concentration ranging from 0.5 at. % down to 0.005 at. %. At Ga concentrations ≤0.05 at.%, all Ga dopants are substitutional directly after ion implantation, without the need for post-implantation thermal annealing. At higher Ga concentrations, a reduction in the EXAFS amplitude is observed, indicating that a fraction of the Ga atoms is located in a defective environment. The local strain induced by the Ga atoms in the Ge matrix is independent of the Ga concentration and extends only to the first nearest neighbor Ge shell, where a 1% contraction in bond length has been measured, in agreement with density functional theory calculations.We acknowledge the support from the Research Foundation
Flanders, the epi-team from imec, the KU Leuven
GOA 09/06 project, the IUAP program P6/42 and the Australian
Research Council. S.C. acknowledges support from
OCAS NV by an OCAS-endowed chair at Ghent University
Non-linear terms in 2D cosmology
In this work we investigate the behavior of two-dimensional (2D) cosmological
models, starting with the Jackiw-Teitelboim (JT) theory of gravitation. A
geometrical term, non-linear in the scalar curvature , is added to the JT
dynamics to test if it could play the role of dark energy in a 2D expanding
universe. This formulation makes possible, first, the description of an early
(inflationary) 2D universe, when the van der Waals (vdW) equation of state is
used to construct the energy-momentum tensor of the gravitational sources.
Second, it is found that for later times the non-linear term in can
generate an old 2D universe in accelerated expansion, where an ordinary matter
dominated era evolves into a decelerated/accelerated transition, giving to the
dark energy effects a geometrical origin. The results emerge through numerical
analysis, following the evolution in time of the scale factor, its
acceleration, and the energy densities of constituents.Comment: tex file plus figures in two zipped files. To appear in Europhys.
Let
Self-similar chain conformations in polymer gels
We use molecular dynamics simulations to study the swelling of randomly
end-cross-linked polymer networks in good solvent conditions. We find that the
equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand
lengths N_s exceeding the melt entanglement length N_e. The internal structure
of the network strands in the swollen state is characterized by a new exponent
nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which
predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory
argument for a self-similar structure of mutually interpenetrating network
strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner
theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand
length.Comment: 4 pages, RevTex, 3 Figure
Confinement effects on glass forming liquids probed by DMA
Many molecular glass forming liquids show a shift of the glass transition T-g
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 T-g increasing with decreasing pore size. Second,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts T-g to lower temperatures. Here we present 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 T-g proportional to 1/d in perfect
agreement with recent differential scanning calorimetry (DSC) measurements.
Thermal expansion measurements of empty and salol filled mesoporous samples
revealed that the contribution of negative pressure to the downshift of T-g is
small (<30%) and the main effect is due to the suppression of dynamically
correlated regions of size xi when the pore size xi approaches
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