18,172 research outputs found
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
ARES+MOOG - a practical overview of an EW method to derive stellar parameters
The goal of this document is to describe the important practical aspects in
the use of an Equivalent Width (EW) method for the derivation of spectroscopic
stellar parameters. A general description of the fundamental steps composing
any EW method is given, together with possible differences that may be found in
different methods used in the literature. Then ARES+MOOG is then used as an
example where each step of the method is described in detail. A special focus
is given for the specific steps of this method, namely the use of a
differential analysis to define the atomic data for the adopted line list, the
automatic EW determinations, and the way to find the best parameters at the end
of the procedure. Finally, a practical tutorial is given, where we focus on
simple exercises useful to illustrate and explain the dependence of the
abundances with the assumed stellar parameters. The interdependences are
described and a clear procedure is given to find the "final" stellar
parameters.Comment: 15 pages, 4 figures, accepted for publication as a chapter in
"Determination of Atmospheric Parameters of B, A, F and G Type Stars",
Springer (2014), eds. E. Niemczura, B. Smalley, W. Pyc
Standard map in magnetized relativistic systems: fixed points and regular acceleration
We investigate the concept of a standard map for the interaction of
relativistic particles and electrostatic waves of arbitrary amplitudes, under
the action of external magnetic fields. The map is adequate for physical
settings where waves and particles interact impulsively, and allows for a
series of analytical result to be exactly obtained. Unlike the traditional form
of the standard map, the present map is nonlinear in the wave amplitude and
displays a series of peculiar properties. Among these properties we discuss the
relation involving fixed points of the maps and accelerator regimes.Comment: Work to appear in Phys. Rev. E. 2 figure
Exponential Distributions in a Mechanical Model for Earthquakes
We study statistical distributions in a mechanical model for an earthquake
fault introduced by Burridge and Knopoff [R. Burridge and L. Knopoff, {\sl
Bull. Seismol. Soc. Am.} {\bf 57}, 341 (1967)]. Our investigations on the size
(moment), time duration and number of blocks involved in an event show that
exponential distributions are found in a given range of the paramenter space.
This occurs when the two kinds of springs present in the model have the same,
or approximately the same, value for the elastic constants. Exponential
distributions have also been seen recently in an experimental system to model
earthquake-like dynamics [M. A. Rubio and J. Galeano, {\sl Phys. Rev. E} {\bf
50}, 1000 (1994)].Comment: 11 pages, uuencoded (submitted to Phys. Rev. E
Environmental Epidemiology of Intestinal Schistosomiasis in Uganda: Population Dynamics of Biomphalaria (Gastropoda: Planorbidae) in Lake Albert and Lake Victoria with Observations on Natural Infections with Digenetic Trematodes
This study documented the population dynamics of Biomphalaria and associated natural infections with digenetic trematodes, along the shores of Lake Albert and Lake Victoria, recording local physicochemical factors. Over a two-and-a-half-year study period with monthly sampling, physicochemical factors were measured at 12 survey sites and all freshwater snails were collected. Retained Biomphalaria were subsequently monitored in laboratory aquaria for shedding trematode cercariae, which were classified as either human infective (Schistosoma mansoni) or nonhuman infective. The population dynamics of Biomphalaria differed by location and by lake and had positive relationship with pH (P < 0.001) in both lakes and negative relationship with conductivity (P = 0.04) in Lake Albert. Of the Biomphalaria collected in Lake Albert (N = 6,183), 8.9% were infected with digenetic trematodes of which 15.8% were shedding S. mansoni cercariae and 84.2% with nonhuman infective cercariae. In Lake Victoria, 2.1% of collected Biomphalaria (N = 13,172) were infected with digenetic trematodes with 13.9% shedding S. mansoni cercariae, 85.7% shedding nonhuman infective cercariae, and 0.4% of infected snails shedding both types of cercariae. Upon morphological identification, species of Biomphalaria infected included B. sudanica, B. pfeifferi, and B. stanleyi in Lake Albert and B. sudanica, B. pfeifferi, and B. choanomphala in Lake Victoria. The study found the physicochemical factors that influenced Biomphalaria population and infections. The number and extent of snails shedding S. mansoni cercariae illustrate the high risk of transmission within these lake settings. For better control of this disease, greater effort should be placed on reducing environmental contamination by improvement of local water sanitation and hygiene
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
Alternate islands of multiple isochronous chains in wave-particle interactions
We analyze the dynamics of a relativistic particle moving in a uniform
magnetic field and perturbed by a standing electrostatic wave. We show that a
pulsed wave produces an infinite number of perturbative terms with the same
winding number, which may generate islands in the same region of phase space.
As a consequence, the number of isochronous island chains varies as a function
of the wave parameters. We observe that in all the resonances, the number of
chains is related to the amplitude of the various resonant terms. We determine
analytically the position of the periodic points and the number of island
chains as a function of the wave number and wave period. Such information is
very important when one is concerned with regular particle acceleration, since
it is necessary to adjust the initial conditions of the particle to obtain the
maximum acceleration.Comment: Submitte
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