22,523 research outputs found
Energy in an Expanding Universe in the Teleparallel Geometry
The main purpose of this paper is to explicitly verify the consistency of the
energy-momentum and angular momentum tensor of the gravitational field
established in the Hamiltonian structure of the Teleparallel Equivalent of
General Relativity (TEGR). In order to reach these objectives, we obtained the
total energy and angular momentum (matter plus gravitational field) of the
closed universe of the Friedmann-Lemaitre-Robertson-Walker (FLRW). The result
is compared with those obtained from the pseudotensors of Einstein and
Landau-Lifshitz. We also applied the field equations (TEGR) in an expanding
FLRW universe. Considering the stress energy-momentum tensor for a perfect
fluid, we found a teleparallel equivalent of Friedmann equations of General
Relativity (GR).Comment: 19 pages, no figures. Revised in view of Referee's comments. Version
to appear in the Brazilian Journal of Physic
Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory
We propose a model for magnetic noise based on spin-flips (not
electron-trapping) of paramagnetic dangling-bonds at the
amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times
is derived from the single-phonon cross-relaxation mechanism for a
dangling-bond interacting with the tunneling two-level systems of the amorphous
interface. The temperature and frequency dependence is sensitive to three
energy scales: The dangling-bond spin Zeeman energy delta, as well as the
minimum (E_min) and maximum (E_max) values for the energy splittings of the
tunneling two-level systems. We compare and fit our model parameters to a
recent experiment probing spin coherence of antimony donors implanted in
nuclear-spin-free silicon [T. Schenkel {\it et al.}, Appl. Phys. Lett. 88,
112101 (2006)], and conclude that a dangling-bond area density of the order of
10^{14}cm^{-2} is consistent with the data. This enables the prediction of
single spin qubit coherence times as a function of the distance from the
interface and the dangling-bond area density in a real device structure. We
apply our theory to calculations of magnetic flux noise affecting SQUID devices
due to their Si/SiO_2 substrate. Our explicit estimates of flux noise in SQUIDs
lead to a noise spectral density of the order of 10^{-12}Phi_{0}^{2} {Hz}^{-1}
at f=1Hz. This value might explain the origin of flux noise in some SQUID
devices. Finally, we consider the suppression of these effects using surface
passivation with hydrogen, and the residual nuclear-spin noise resulting from a
perfect silicon-hydride surface.Comment: Final published versio
Space-time Torsion and Neutrino Oscillations in Vacuum
The objective of this study is to verify the consistency of the prescription
of alternative minimum coupling (connection) proposed by the Teleparallel
Equivalent to General Relativity (TEGR) for the Dirac equation. With this aim,
we studied the problem of neutrino oscillations in Weitzenbock space-time in
the Schwarzschild metric. In particular, we calculate the phase dynamics of
neutrinos. The relation of spin of the neutrino with the space-time torsion is
clarified through the determination of the phase differences between spin
eigenstates of the neutrinos.Comment: 07 pages, no figure
Angular Momentum of the BTZ Black Hole in the Teleparallel Geometry
We carry out the Hamiltonian formulation of the three- dimensional
gravitational teleparallelism without imposing the time gauge condition, by
rigorously performing the Legendre transform. Definition of the gravitational
angular momentum arises by suitably interpreting the integral form of the
constraint equation Gama^ik=0 as an angular momentum equation. The
gravitational angular momentum is evaluated for the gravitational field of a
rotating BTZ black hole.Comment: 17 pages, no figures, v2: some misprints corrected, Ref.s added, Eq.s
revised, submitted to General Relativity and Gravitatio
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Enhancing Fault / Intrusion Tolerance through Design and Configuration Diversity
Fault/intrusion tolerance is usually the only viable way of improving the system dependability and security in the presence of continuously evolving threats. Many of the solutions in the literature concern a specific snapshot in the production or deployment of a fault-tolerant system and no immediate considerations are made about how the system should evolve to deal with novel threats. In this paper we outline and evaluate a set of operating systems’ and applications’ reconfiguration rules which can be used to modify the state of a system replica prior to deployment or in between recoveries, and hence increase the replicas chance of a longer intrusion-free operation
Scale insects (Hemiptera: Coccoidea) of ornamental plants from Sao Carlos, Sao Paulo, Brazil
A list of 35 scale insects collected from 72 ornamental plant species in Sao Carlos, Sao Paulo, Brazil is provided. Regarding host specificity, 30 scale insects were polyphagous, 4 oligophagous, and 1 monophagous. A total of 102 coccoid/plant associations are recorded, 29 of which are new host records for the species; 60 are new host records for the species in Brazil. Pulvinaria urbicola Cockerell, 1893 (Coccidae), Phenacoccus similis Granara de Willink, 1983 (Pseudococcidae), and Orthezia molinarii (Morrison, 1952) (Ortheziidae) are recorded for the first time in Brazil. In addition, we describe the injury caused by scale insects on ornamental plants
Is the Industrial Product-Service System Really Sustainable
As the product-service system has shifted from its original concept to the Industrial PSS, its scope has expanded to include industrial products. Furthermore, the overall goal of reducing environmental impacts has been left behind. Despite the PSS's potential as a business model for a more sustainable production and consumption system, the mere addition of services to conventional products does not necessarily lead to a reduction of environmental impacts. This paper aims to discuss the concepts related to PSS, the need for considering environmental impact reduction as a critical issue for sustainability, and the role of ecodesign practices in the development of PSS
Statistical Dynamics of Religions and Adherents
Religiosity is one of the most important sociological aspects of populations.
All religions may evolve in their beliefs and adapt to the society
developments. A religion is a social variable, like a language or wealth, to be
studied like any other organizational parameter.
Several questions can be raised, as considered in this study: e.g. (i) from a
``macroscopic'' point of view : How many religions exist at a given time? (ii)
from a ``microscopic'' view point: How many adherents belong to one religion?
Does the number of adherents increase or not, and how? No need to say that if
quantitative answers and mathematical laws are found, agent based models can be
imagined to describe such non-equilibrium processes.
It is found that empirical laws can be deduced and related to preferential
attachment processes, like on evolving network; we propose two different
algorithmic models reproducing as well the data. Moreover, a population
growth-death equation is shown to be a plausible modeling of evolution dynamics
in a continuous time framework. Differences with language dynamic competition
is emphasized.Comment: submitted to EP
Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions
The relative contributions of in-plane (damping-like) and out-of-plane
(field-like) spin-transfer-torques in the magnetization switching of
out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically
analyzed using the transformed Landau-Lifshitz (LL) equation with the STT
terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics,
the out-of-plane torque influences the precession frequency but it does not
contribute significantly to the STT switching process (in particular to the
switching time and switching current density), which is mostly determined by
the in-plane STT contribution. This conclusion is confirmed by finite
temperature and finite writing pulse macrospin simulations of the current-field
switching diagrams. It contrasts with the case of STT-switching in in-plane
magnetized MTJ in which the field-like term also influences the switching
critical current. This theoretical analysis was successfully applied to the
interpretation of voltage-field STT switching diagrams experimentally measured
on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like
behavior. The physical nonequivalence of Landau and Gilbert dissipation terms
in presence of STT-induced dynamics is also discussed
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