17,230 research outputs found
Slowly rotating Kerr black hole as a solution of Einstein-Cartan gravity extended by a Chern-Simons term
We consider the nondynamical Chern-Simons (CS) modification to General
Relativity (GR) in the framework of the Einstein-Cartan formulation, as
providing a way to incorporate a slowly rotating Kerr black hole in the space
of solutions. Our proposal lies on considering the CS term as a source of
torsion and on an iterative procedure to look for vacuum solutions of the
system, by expanding the tetrad, the connection and the embedding parameter, in
powers of a dimensionless small parameter which codifies the CS
coupling. Starting from a torsionless zeroth-order vacuum solution we derive
the second-order differential equation for the corrections
to the metric, for an arbitrary embedding parameter. Furthermore we can show
that the slowly rotating Kerr metric is an solution of the
system either in the canonical or the axial embeddings.Comment: "Presented at the Fifth Meeting on CPT and Lorentz Symmetry,
Bloomington, Indiana, June 28-July 2, 2010
The revival of the Baldwin Effect
The idea that a genetically fixed behavior evolved from the once differential
learning ability of individuals that performed the behavior is known as the
Baldwin effect. A highly influential paper [Hinton G.E., Nowlan S.J., 1987. How
learning can guide evolution. Complex Syst. 1, 495-502] claimed that this
effect can be observed in silico, but here we argue that what was actually
shown is that the learning ability is easily selected for. Then we demonstrate
the Baldwin effect to happen in the in silico scenario by estimating the
probability and waiting times for the learned behavior to become innate.
Depending on parameter values, we find that learning can increase the chance of
fixation of the learned behavior by several orders of magnitude compared with
the non-learning situation
Superlattice nonlinearities for Gigahertz-Terahertz generation in harmonic multipliers
Semiconductor superlattices are strongly nonlinear media offering several
technological challenges associated with the generation of high-frequency
Gigahertz radiation and very effective frequency multiplication up to several
Terahertz. However, charge accumulation, traps and interface defects lead to
pronounced asymmetries in the nonlinear current flow, from which high harmonic
generation stems. This problem requires a full non-perturbative solution of
asymmetric current flow under irradiation, which we deliver in this paper
within the Boltzmann-Bloch approach. We investigate the nonlinear output on
both frequency and time domains and demonstrate a significant enhancement of
even harmonics by tuning the interface quality. Moreover, we find that
increasing arbitrarily the input power is not a solution for high nonlinear
output, in contrast with materials described by conventional susceptibilities.
There is a complex combination of asymmetry and power values leading to maximum
high harmonic generation.Comment: 13 pages, 7 figures, Accepted for Nanophotonics (De Gruyter
Diffuse gamma-ray emission from galactic pulsars
Millisecond Pulsars are second most abundant source population discovered by
the Fermi-LAT. They might contribute non-negligibly to the diffuse emission
measured at high latitudes by Fermi-LAT, the IDGRB. Gamma-ray sources also
contribute to the anisotropy of the IDGRB measured on small scales by
Fermi-LAT. We aim to assess the contribution of the unresolved counterpart of
the detected MSPs population to the IDGRB and the maximal fraction of the
measured anisotropy produced by this source class. We model the MSPs spatial
distribution in the Galaxy and the gamma-ray emission parameters by considering
radio and gamma-ray observational constraints. By simulating a large number of
MSPs populations, we compute the average diffuse emission and the anisotropy
1-sigma upper limit. The emission from unresolved MSPs at 2 GeV, where the peak
of the spectrum is located, is at most 0.9% of the measured IDGRB above 10
degrees in latitude. The 1-sigma upper limit on the angular power for
unresolved MSP sources turns out to be about a factor of 60 smaller than
Fermi-LAT measurements above 30 degrees. Our results indicate that this
galactic source class represents a negligible contributor to the high-latitude
gamma-ray sky and confirm that most of the intensity and geometrical properties
of the measured diffuse emission are imputable to other extragalactic source
classes. Nevertheless, given the MSP distribution, we expect them to contribute
significantly to the gamma-ray diffuse emission at low latitudes. Since, along
the galactic disk, the population of young Pulsars overcomes in number the one
of MSPs, we compute the gamma-ray emission from the whole population of
unresolved Pulsars in two low-latitude regions: the inner Galaxy and the
galactic center.Comment: 19 pages, 26 figures. It matches the published version, minor changes
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Controlling the harmonic conversion efficiency in semiconductor superlattices by interface roughness design
In semiconductor superlattices, when Bragg oscillating electrons interact
with an input electromagnetic field, frequency multiplication is possible. An
ideal superlattice has a purely antisymmetric voltage current response and can
thus produce only odd harmonics. However, real world superlattices can also
have even harmonic response and that increases the range of possible output
frequencies. These effects have been recently explained with a predictive model
that combines an Ansatz solution for the Boltzmann Equation with a
Nonequilibrium Green's Functions approach. This predictive tool, coupled with
recent progress on GHz input sources, support the growing interest in
developing compact room temperature devices that can operate from the GHz to
the THz range. The natural question to ask is what efficiencies can be
expected. This paper addresses this issue by investigating power-conversion
efficiency in irradiated semiconductor superlattices. Interface imperfections
are consistently included in the theory and they strongly influence the power
output of both odd and even harmonics. Good agreement is obtained for predicted
odd harmonic outputs with experimental data for a wide frequency range. The
intrinsic conversion efficiency used is based on the estimated amplitude of the
input field inside the sample and thus independent of geometrical factors that
characterize different setups. The method opens the possibility of designing
even harmonic output power by controlling the interface quality
In the eye of the storm: T cell behavior in the inflammatory microenvironment.
Coordinated unfolding of innate and adaptive immunity is key to the development of protective immune responses. This functional integration occurs within the inflamed tissue, a microenvironment enriched with factors released by innate and subsequently adaptive immune cells and the injured tissue itself. T lymphocytes are key players in the ensuing adaptive immunity and their proper function is instrumental to a successful outcome of immune protection. The site of inflammation is a "harsh" environment in which T cells are exposed to numerous factors that might influence their behavior. Low pH and oxygen concentration, high lactate and organic acid content as well as free fatty acids and reactive oxygen species are found in the inflammatory microenvironment. All these components affect T cells as well as other immune cells during the immune response and impact on the development of chronic inflammation. We here overview the effects of a number of factors present in the inflammatory microenvironment on T cell function and migration and discuss the potential relevance of these components as targets for therapeutic intervention in autoimmune and chronic inflammatory diseases
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