4,657 research outputs found
Strain rate, temperature, and humidity on strength and moduli of a graphite/epoxy composite
Results of an experimental study of the influence of strain rate, temperature and humidity on the mechanical behavior of a graphite/epoxy fiber composite are presented. Three principal strengths (longitudinal, transverse and shear) and four basic moduli (E1, E2, G12 and U12) of a unidirectional graphite/epoxy composite were followed as a function of strain rate, temperature and humidity. Each test was performed at a constant tensile strain rate in an environmental chamber providing simultaneous temperature and humidity control. Prior to testing, specimens were given a moisture preconditioning treatment at 60 C. Values for the matrix dominated moduli and strength were significantly influenced by both environmental and rate parameters, whereas the fiber dominated moduli were not. However, the longitudinal strength was significantly influenced by temperature and moisture content. A qualitative explanation for these observations is presented
Casimir Force between a Small Dielectric Sphere and a Dielectric Wall
The possibility of repulsive Casimir forces between small metal spheres and a
dielectric half-space is discussed. We treat a model in which the spheres have
a dielectric function given by the Drude model, and the radius of the sphere is
small compared to the corresponding plasma wavelength. The half-space is also
described by the same model, but with a different plasma frequency. We find
that in the retarded limit, the force is quasi-oscillatory. This leads to the
prediction of stable equilibrium points at which the sphere could levitate in
the Earth's gravitational field. This seems to lead to the possibility of an
experimental test of the model. The effects of finite temperature on the force
are also studied, and found to be rather small at room temperature. However,
thermally activated transitions between equilibrium points could be significant
at room temperature.Comment: 16 pages, 5 figure
Kohn-Luttinger superconductivity in graphene
We investigate the development of superconductivity in graphene when the
Fermi level becomes close to one of the Van Hove singularities of the electron
system. The origin of the pairing instability lies in the strong anisotropy of
the e-e scattering at the Van Hove filling, which leads to a channel with
attractive coupling when making the projection of the BCS vertex on the
symmetry modes with nontrivial angular dependence along the Fermi line. We show
that the scale of the superconducting instability may be pushed up to
temperatures larger than 10 K, depending on the ability to tune the system to
the proximity of the Van Hove singularity.Comment: 5 pages, 3 figure
Vector and tensor perturbations in Horava-Lifshitz cosmology
We study cosmological vector and tensor perturbations in Horava-Lifshitz
gravity, adopting the most general Sotiriou-Visser-Weinfurtner generalization
without the detailed balance but with projectability condition. After deriving
the general formulas in a flat FRW background, we find that the vector
perturbations are identical to those given in general relativity. This is true
also in the non-flat cases. For the tensor perturbations, high order
derivatives of the curvatures produce effectively an anisotropic stress, which
could have significant efforts on the high-frequency modes of gravitational
waves, while for the low-frenquency modes, the efforts are negligible. The
power spectrum is scale-invariant in the UV regime, because of the particular
dispersion relations. But, due to lower-order corrections, it will eventually
reduce to that given in GR in the IR limit. Applying the general formulas to
the de Sitter and power-law backgrounds, we calculate the power spectrum and
index, using the uniform approximations, and obtain their analytical
expressions in both cases.Comment: Correct some typos and add new references. Version to be published in
Physical Reviews
Surface-atom force out of thermal equilibrium and its effect on ultra-cold atoms
The surface-atom Casimir-Polder-Lifshitz force out of thermal equilibrium is
investigated in the framework of macroscopic electrodynamics. Particular
attention is devoted to its large distance limit that shows a new, stronger
behaviour with respect to the equilibrium case. The frequency shift produced by
the surface-atom force on the the center-of-mass oscillations of a harmonically
trapped Bose-Einstein condensate and on the Bloch oscillations of an ultra-cold
fermionic gas in an optical lattice are discussed for configurations out of
thermal equilibrium.Comment: Submitted to JPA Special Issue QFEXT'0
Traversable wormhole in the deformed Ho\v{r}ava-Lifshitz gravity
Asymptotically flat wormhole solutions are found in the deformed
Ho\v{r}ava-Lifshitz gravity. It turns out that higher curvature terms can not
play the role of exotic matters which are crucial to form a traversable
wormhole, and external exotic sources are still needed. In particular, the
exotic matter behaves like phantom energy if Kehagias-Sfetsos vacuum is
considered outside the wormhole. Interestingly, the spherically symmetric
setting makes the matter and the higher curvature contribution satisfy
four-dimensional conservation of energy in the covariant form.Comment: 13 pages, 2 figures, version published in Phys. Rev.
Confined coherence in quasi-one-dimensional metals
We present a functional renormalization group calculation of the effect of
strong interactions on the shape of the Fermi surface of weakly coupled
metallic chains. In the regime where the bare interchain hopping is small, we
show that scattering processes involving large momentum transfers perpendicular
to the chains can completely destroy the warping of the true Fermi surface,
leading to a confined state where the renormalized interchain hopping vanishes
and a coherent motion perpendicular to the chains is impossible.Comment: 4 RevTex pages, 5 figures,final version as published by PR
Lifshitz-type formulas for graphene and single-wall carbon nanotubes: van der Waals and Casimir interations
Lifshitz-type formulas are obtained for the van der Waals and Casimir
interaction between graphene and a material plate, graphene and an atom or a
molecule, and between a single-wall carbon nanotube and a plate. The reflection
properties of electromagnetic oscillations on graphene are governed by the
specific boundary conditions imposed on the infinitely thin positively charged
plasma sheet, carrying a continuous fluid with some mass and charge density.
The obtained formulas are applied to graphene interacting with Au and Si
plates, to hydrogen atoms and molecules interacting with graphene, and to
single-wall carbon nanotubes interacting with Au and Si plates. The
generalizations to more complicated carbon nanostructures are discussed.Comment: 11 pages, 5 figures, 2 tables; to appear in Phys. Rev. B; misprints
in Eqs.(33) and (34) are correcte
Gradient expansion, curvature perturbations and magnetized plasmas
The properties of magnetized plasmas are always investigated under the
hypothesis that the relativistic inhomogeneities stemming from the fluid
sources and from the geometry itself are sufficiently small to allow for a
perturbative description prior to photon decoupling. The latter assumption is
hereby relaxed and pre-decoupling plasmas are described within a suitable
expansion where the inhomogeneities are treated to a given order in the spatial
gradients. It is argued that the (general relativistic) gradient expansion
shares the same features of the drift approximation, customarily employed in
the description of cold plasmas, so that the two schemes are physically
complementary in the large-scale limit and for the low-frequency branch of the
spectrum of plasma modes. The two-fluid description, as well as the
magnetohydrodynamical reduction, are derived and studied in the presence of the
spatial gradients of the geometry. Various solutions of the coupled system of
evolution equations in the anti-Newtonian regime and in the quasi-isotropic
approximation are presented. The relation of this analysis to the so-called
separate Universe paradigm is outlined. The evolution of the magnetized
curvature perturbations in the nonlinear regime is addressed for the magnetized
adiabatic mode in the plasma frame.Comment: 40 pages, no figure
Note on two-dimensional gauged Lifshitz models
We fermionize the two-dimensional free Lifshitz scalar field in order to
identify what the gauge covariant couplings are, and then they are bosonized
back to get the gauged Lifshitz scalar field theories. We show that they give
the same physical modes with those of the corresponding Lorentz invariant
gauged scalar theories, although the dispersion relations are different.Comment: 8 pages, to appear in MPL
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