3,342 research outputs found
Strain balanced quantum posts
Quantum posts are assembled by epitaxial growth of closely spaced quantum dot
layers, modulating the composition of a semiconductor alloy, typically InGaAs.
In contrast with most self-assembled nanostructures, the height of quantum
posts can be controlled with nanometer precision, up to a maximum value limited
by the accumulated stress due to the lattice mismatch. Here we present a strain
compensation technique based on the controlled incorporation of phosphorous,
which substantially increases the maximum attainable quantum post height. The
luminescence from the resulting nanostructures presents giant linear
polarization anisotropy.Comment: Submitted to Applied Physics Letters (7th March 2011). 4 pages, 4
figure
Geodesics in a quasispherical spacetime: A case of gravitational repulsion
Geodesics are studied in one of the Weyl metrics, referred to as the M--Q
solution. First, arguments are provided, supporting our belief that this
space--time is the more suitable (among the known solutions of the Weyl family)
for discussing the properties of strong quasi--spherical gravitational fields.
Then, the behaviour of geodesics is compared with the spherically symmetric
situation, bringing out the sensitivity of the trajectories to deviations from
spherical symmetry. Particular attention deserves the change of sign in proper
radial acceleration of test particles moving radially along symmetry axis,
close to the surface, and related to the quadrupole moment of the
source.Comment: 30 pages late
Nonlocal Equation of State in Anisotropic Static Fluid Spheres in General Relativity
We show that it is possible to obtain credible static anisotropic spherically
symmetric matter configurations starting from known density profiles and
satisfying a nonlocal equation of state. These particular types of equation of
state describe, at a given point, the components of the corresponding
energy-momentum tensor not only as a function at that point, but as a
functional throughout the enclosed configuration. To establish the physical
plausibility of the proposed family of solutions satisfying nonlocal equation
of state, we study the constraints imposed by the junction and energy
conditions on these bounded matter distributions.
We also show that it is possible to obtain physically plausible static
anisotropic spherically symmetric matter configurations, having nonlocal
equations of state\textit{,}concerning the particular cases where the radial
pressure vanishes and, other where the tangential pressures vanishes. The later
very particular type of relativistic sphere with vanishing tangential stresses
is inspired by some of the models proposed to describe extremely magnetized
neutron stars (magnetars) during the transverse quantum collapse.Comment: 21 pages, 1 figure, minor changes in the text, references added, two
new solutions studie
Thermal Conduction in Systems out of Hydrostatic Equilibrium
We analyse the effects of thermal conduction in a relativistic fluid, just
after its departure from hydrostatic equilibrium, on a time scale of the order
of thermal relaxation time. It is obtained that the resulting evolution will
critically depend on a parameter defined in terms of thermodynamic variables,
which is constrained by causality requirements.Comment: 16 pages, emTex (LaTex 2.09). To appear in Classical and Quantum
Gravit
âMale effectâ and âtemporary weaningâ in synchronization of post-partum ovarian activity in Pelibuey ewes
To evaluate the response of the âmale effectâ and âtemporary weaningâ on the synchronization of post-partum ovarian activity in Pelibuey ewes, an experiment was carried out using 78 Pelibuey ewes with suckling lambs. The ewes were separated from their offspring for 48 hours and randomly assigned to one of four treatments derived from the arrangement of two factors, âmale effectâ and âtemporary weaningâ, each at two levels. Treatments were: T1 (n = 20) control ewes, without âmale effectâ and without âtemporary weaningâ); T2 (n = 19) ewes without âmale effectâ and with âtemporary weaningâ; T3 (n = 20) ewes with âmale effectâ and without âtemporary weaningâ; and T4 (n = 19), ewes with âmale effectâ and with âtemporary weaningâ. The response to oestrus, return to oestrus, gestation rate and lambing rate were analysed using logistic regression. The onset of oestrus was analysed using survival curves. No significant differences were found for lambing rate and prolificacy among treatments. âTemporary weaningâ (T2) and âmale effectâ (T3) did not influence the response to oestrus, rate and duration of return to oestrus, or gestation rate and fertility, and was similar to the control group (T1). The interaction of âmale effectâ with âtemporary weaningâ (T4) increased the response to oestrus, reduced the rate and duration of return to oestrus, and the gestation rate, but increased fertility. Synchronizing post-partum ovarian activity with âmale effectâ and âtemporary weaningâ reduces the onset of oestrus and the rate of return to oestrus, but increases the response to oestrus and fecundity in Pelibuey ewes.Keywords: Biostimulation, progestogens, prolificacy, prostaglandins, sucklin
Spherically symmetric dissipative anisotropic fluids: A general study
The full set of equations governing the evolution of self--gravitating
spherically symmetric dissipative fluids with anisotropic stresses is deployed
and used to carry out a general study on the behaviour of such systems, in the
context of general relativity. Emphasis is given to the link between the Weyl
tensor, the shear tensor, the anisotropy of the pressure and the density
inhomogeneity. In particular we provide the general, necessary and sufficient,
condition for the vanishing of the spatial gradients of energy density, which
in turn suggests a possible definition of a gravitational arrow of time. Some
solutions are also exhibited to illustrate the discussion.Comment: 28 pages Latex. To appear in Phys.Rev.
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