4,374 research outputs found
Discovery of VHE and HE emission from the blazar 1ES 0414+009 with H.E.S.S and Fermi-LAT
The high energy peaked BL Lac (HBL) object 1ES 0414+009 (z=0.287) is a
distant very high-energy (VHE, E > 100 GeV) blazars with well-determined
redshift. This source was detected with the High Energy Stereoscopic System
(H.E.S.S.) between October 2005 and September 2009. It was also detected with
the Fermi Large Area Telescope (LAT) in 21 months of data. The combined high
energy (HE) and VHE spectra, once corrected for gamma-gamma absorption on the
extragalactic background light (EBL), indicate a Compton peak located above few
TeV, among the highest in the BL Lac class.Comment: proceeding from the 25th Texas Symposium on Relativistic Astrophysics
(Heidelberg, Germany, 2010
Controlling the dynamics of a coupled atom-cavity system by pure dephasing : basics and potential applications in nanophotonics
The influence of pure dephasing on the dynamics of the coupling between a
two-level atom and a cavity mode is systematically addressed. We have derived
an effective atom-cavity coupling rate that is shown to be a key parameter in
the physics of the problem, allowing to generalize the known expression for the
Purcell factor to the case of broad emitters, and to define strategies to
optimize the performances of broad emitters-based single photon sources.
Moreover, pure dephasing is shown to be able to restore lasing in presence of
detuning, a further demonstration that decoherence can be seen as a fundamental
resource in solid-state cavity quantum electrodynamics, offering appealing
perspectives in the context of advanced nano-photonic devices.Comment: 10 pages, 7 figure
Cosmological perturbations of a perfect fluid and noncommutative variables
We describe the linear cosmological perturbations of a perfect fluid at the
level of an action, providing thus an alternative to the standard approach
based only on the equations of motion. This action is suited not only to
perfect fluids with a barotropic equation of state, but also to those for which
the pressure depends on two thermodynamical variables. By quantizing the system
we find that (1) some perturbation fields exhibit a noncommutativity quite
analogous to the one observed for a charged particle moving in a strong
magnetic field, (2) local curvature and pressure perturbations cannot be
measured simultaneously, (3) ghosts appear if the null energy condition is
violated.Comment: 4 pages, uses RevTeX. Title modified, references and comments added
Cosmological perturbation in f(R,G) theories with a perfect fluid
In order to classify modified gravity models according to their physical
properties, we analyze the cosmological linear perturbations for f(R,G)
theories (R being the Ricci scalar and G, the Gauss-Bonnet term) with a
minimally coupled perfect fluid. For the scalar type perturbations, we identify
in general six degrees of freedom. We find that two of these physical modes
obey the same dispersion relation as the one for a non-relativistic de Broglie
wave. This means that spacetime is either highly unstable or its fluctuations
undergo a scale-dependent super-luminal propagation. Two other modes correspond
to the degrees of freedom of the perfect fluid, and propagate with the sound
speed of such a fluid. The remaining two modes correspond to the entropy and
temperature perturbations of the perfect fluid, and completely decouple from
the other modes for a barotropic equation of state. We then provide a concise
condition on f(R,G) theories, that both f(R) and R+f(G) do fulfill, to avoid
the de Broglie type dispersion relation. For the vector type perturbation, we
find that the perturbations decay in time. For the tensor type perturbation,
the perturbations can be either super-luminal or sub-luminal, depending on the
model. No-ghost conditions are also obtained for each type of perturbation.Comment: 12 pages, uses RevTe
Can Mg isotopes be used to trace cyanobacteria-mediated magnesium carbonate precipitation in alkaline lakes?
The fractionation of Mg isotopes was determined during the cyanobacterial mediated precipitation of hydrous magnesium carbonate precipitation in both natural environments and in the laboratory. Natural samples were obtained from Lake Salda (SE Turkey), one of the few modern environments on the Earth's surface where hydrous Mg-carbonates are the dominant precipitating minerals. This precipitation was associated with cyanobacterial stromatolites which were abundant in this aquatic ecosystem. Mg isotope analyses were performed on samples of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments. Laboratory Mg carbonate precipitation experiments were conducted in the presence of purified Synechococcus sp cyanobacteria that were isolated from the lake water and stromatolites. The hydrous magnesium carbonates nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4(OH)25(H2O)) were precipitated in these batch reactor experiments from aqueous solutions containing either synthetic NaHCO3/MgCl2 mixtures or natural Lake Salda water, in the presence and absence of live photosynthesizing Synechococcus sp. Bulk precipitation rates were not to affected by the presence of bacteria when air was bubbled through the system. In the stirred non-bubbled reactors, conditions similar to natural settings, bacterial photosynthesis provoked nesquehonite precipitation, whilst no precipitation occurred in bacteria-free systems in the absence of air bubbling, despite the fluids achieving a similar or higher degree of supersaturation. The extent of Mg isotope fractionation (?26Mgsolid-solution) between the mineral and solution in the abiotic experiments was found to be identical, within uncertainty, to that measured in cyanobacteria-bearing experiments, and ranges from ?1.4 to ?0.7 ‰. This similarity refutes the use of Mg isotopes to validate microbial mediated precipitation of hydrous Mg carbonate
El Sistema Nacional de Investigadores en 2009 : un vector para la internacionalizaciĂłn de las Ă©lites cientĂficas ?
In this paper the authors expect to analyze the following hypothesis from different angles: the National Researchers System (Sistema Nacional de Investigadores, SNI) has created a standardization, progressive although incomplete, of the criteria governing the national scientific elite based on the criteria that prevail amongst the so-called developed countries. We study some of the indicators following which the SNI members who belong to the most aged groups present obvious differences with regard to the less aged ones and are a symptom of this homogenization dynamics, like, for instance, the age at which the doctorate degree is obtained. We also analyze the distortions that characterize this universe, like v.g. the fact that women are under-represented. A systematic analysis of the 2009 data about the SNI researchers, together with a sample of academic trajectories taken from the CVU (Joint Curriculum Vitae, Curriculum vitae Ăşnico) helps us to show at what extent obtaining a degree in a foreign country helps to achieve a rapid career progression within the system, emphasizing the slants by disciplinary area and additional factors that allow to explain how they can get a high scientific prestige capital
Quantum Communication with Quantum Dot Spins
Single electron spins in quantum dots are attractive for quantum
communication because of their expected long coherence times. We propose a
method to create entanglement between two remote spins based on the coincident
detection of two photons emitted by the dots. Local nodes of several qubits can
be realized using the dipole-dipole interaction between trions in neighboring
dots and spectral addressing, allowing the realization of quantum repeater
protocols. We have performed a detailed feasibility study of our proposal based
on tight-binding calculations of quantum dot properties.Comment: 4 pages, 2 figures, new and improved version, explicit performance
estimate
Photoassisted tunneling from free-standing GaAs thin films into metallic surfaces
The tunnel photocurrent between a gold surface and a free-standing
semiconducting thin film excited from the rear by above bandgap light has been
measured as a function of applied bias, tunnel distance and excitation light
power. The results are compared with the predictions of a model which includes
the bias dependence of the tunnel barrier height and the bias-induced decrease
of surface recombination velocity. It is found that i) the tunnel photocurrent
from the conduction band dominates that from surface states. ii) At large
tunnel distance the exponential bias dependence of the current is explained by
that of the tunnel barrier height, while at small distance the change of
surface recombination velocity is dominant
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