222 research outputs found
¿Es posible que la competencia entre las grandes potencias en el Atlántico Sur se traslade a la Antártida?
La política internacional se enfrenta al retorno de la competencia entre las grandes potencias. El presidente Joe Biden ha declarado (Delaney, 2021) que los Estados Unidos no permitirán que China se convierta en el líder, más rico y poderoso país en el mundo. En este contexto, un artículo recientemente publicado (Espach, 2021) sugiere que este “nuevo gran juego” ha recalado en el Atlántico Sudoeste y que esto tendrá directas implicancias para la Antártida.
Sin embargo, es importante preguntarse si las tensiones en la política global necesariamente se verán replicadas en la Antártida. Mientras que el Atlántico Sudoeste ha sido propenso a tensiones geopolíticas y disputas territoriales han existido en la región en los últimos tres siglos, el área al sur de los 60º de latitud Sur, bajo la gobernanza del Sistema del Tratado Antártico (STA), goza de un estatus político y jurídico especial el cual ha logrado mantener a esta región libre de conflic-tos armados y tensiones políticas significativas durante las últimas seis décadas.Departamento de Derecho Internacional.Instituto de Relaciones Internacionale
Close encounters of a rotating star with planets in parabolic orbits of varying inclination and the formation of Hot Jupiters
(abbreviated) We extend the theory of close encounters of a planet on a
parabolic orbit with a star to include the effects of tides induced on the
central rotating star. Orbits with arbitrary inclination to the stellar
rotation axis are considered. We obtain results both from an analytic treatment
and numerical one that are in satisfactory agreement. These results are applied
to the initial phase of the tidal circularisation problem. We find that both
tides induced in the star and planet can lead to a significant decrease of the
orbital semi-major axis for orbits having periastron distances smaller than 5-6
stellar radii (corresponding to periods days after the
circularisation has been completed) with tides in the star being much stronger
for retrograde orbits compared to prograde orbits. We use the simple Skumanich
law for the stellar rotation with its rotational period equal to one month at
the age of 5Gyr. The strength of tidal interactions is characterised by
circularisation time scale, defined as a time scale of evolution of
the planet's semi-major axis due to tides considered as a function of orbital
period after the process of tidal circularisation has been completed.
We find that the ratio of the initial circularisation time scales corresponding
to prograde and retrograde orbits is of order 1.5-2 for a planet of one Jupiter
mass and four days. It grows with the mass of the planet, being
of order five for a five Jupiter mass planet with the same . Thus, the
effect of stellar rotation may provide a bias in the formation of planetary
systems having planets on close orbits around their host stars, as a
consequence of planet-planet scattering, favouring systems with retrograde
orbits. The results may also be applied to the problem of tidal capture of
stars in young stellar clusters.Comment: to be published in Celestial Mechanics and Dynamical Astronom
Dynamical Scaling: the Two-Dimensional XY Model Following a Quench
To sensitively test scaling in the 2D XY model quenched from
high-temperatures into the ordered phase, we study the difference between
measured correlations and the (scaling) results of a Gaussian-closure
approximation. We also directly compare various length-scales. All of our
results are consistent with dynamical scaling and an asymptotic growth law , though with a time-scale that depends on the
length-scale in question. We then reconstruct correlations from the
minimal-energy configuration consistent with the vortex positions, and find
them significantly different from the ``natural'' correlations --- though both
scale with . This indicates that both topological (vortex) and
non-topological (``spin-wave'') contributions to correlations are relevant
arbitrarily late after the quench. We also present a consistent definition of
dynamical scaling applicable more generally, and emphasize how to generalize
our approach to other quenched systems where dynamical scaling is in question.
Our approach directly applies to planar liquid-crystal systems.Comment: 10 pages, 10 figure
Evaluation of the BCS Approximation for the Attractive Hubbard Model in One Dimension
The ground state energy and energy gap to the first excited state are
calculated for the attractive Hubbard model in one dimension using both the
Bethe Ansatz equations and the variational BCS wavefunction. Comparisons are
provided as a function of coupling strength and electron density. While the
ground state energies are always in very good agreement, the BCS energy gap is
sometimes incorrect by an order of magnitude, particularly at half-filling.
Finite size effects are also briefly discussed for cases where an exact
solution in the thermodynamic limit is not possible. In general, the BCS result
for the energy gap is poor compared to the exact result.Comment: 25 pages, 5 Postscript figure
Multiplet Effects in the Quasiparticle Band Structure of the Anderson Model
In this paper, we examine the mean field electronic structure of the
Anderson lattice model in a slave boson approximation, which should
be useful in understanding the physics of correlated metals with more than one
f electron per site such as uranium-based heavy fermion superconductors. We
find that the multiplet structure of the ion acts to quench the crystal
field splitting in the quasiparticle electronic structure. This is consistent
with experimental observations in such metals as .Comment: 9 pages, revtex, 3 uuencoded postscript figures attached at en
MeV-mass dark matter and primordial nucleosynthesis
The annihilation of new dark matter candidates with masses in the MeV
range may account for the galactic positrons that are required to explain the
511 keV -ray flux from the galactic bulge. We study the impact of
MeV-mass thermal relic particles on the primordial synthesis of H, He,
and Li. If the new particles are in thermal equilibrium with neutrinos
during the nucleosynthesis epoch they increase the helium mass fraction for
m_X\alt 10 MeV and are thus disfavored. If they couple primarily to the
electromagnetic plasma they can have the opposite effect of lowering both
helium and deuterium. For --10 MeV they can even improve the overall
agreement between the predicted and observed H and He abundances.Comment: 11 pages, 10 figures, references and two appendices added,
conclusions unchanged; accepted for publication in Phys.Rev.
Ground-state energy and entropy of the two-dimensional Edwards-Anderson spin-glass model with different bond distributions
We study the two-dimensional Edwards-Anderson spin-glass model using a
parallel tempering Monte Carlo algorithm. The ground-state energy and entropy
are calculated for different bond distributions. In particular, the entropy is
obtained by using a thermodynamic integration technique and an appropriate
reference state, which is determined with the method of high-temperature
expansion. This strategy provide accurate values of this quantity for
finite-size lattices. By extrapolating to the thermodynamic limit, the
ground-state energy and entropy of the different versions of the spin-glass
model are determined.Comment: 18 pages, 5 figure
The ground state energy of the Edwards-Anderson spin glass model with a parallel tempering Monte Carlo algorithm
We study the efficiency of parallel tempering Monte Carlo technique for
calculating true ground states of the Edwards-Anderson spin glass model.
Bimodal and Gaussian bond distributions were considered in two and
three-dimensional lattices. By a systematic analysis we find a simple formula
to estimate the values of the parameters needed in the algorithm to find the GS
with a fixed average probability. We also study the performance of the
algorithm for single samples, quantifying the difference between samples where
the GS is hard, or easy, to find. The GS energies we obtain are in good
agreement with the values found in the literature. Our results show that the
performance of the parallel tempering technique is comparable to more powerful
heuristics developed to find the ground state of Ising spin glass systems.Comment: 30 pages, 17 figures. A new section added. Accepted for publication
in Physica
Towards coherent optical control of a single hole spin: rabi rotation of a trion conditional on the spin state of the hole
A hole spin is a potential solid-state q-bit, that may be more robust against nuclear spin induced dephasing than an electron spin. Here we propose and demonstrate the sequential preparation, control and detection of a single hole spin trapped on a self-assembled InGaAs/GaAs quantum dot. The dot is embedded in a photodiode structure under an applied electric field. Fast, triggered, initialization of a hole spin is achieved by creating a spin-polarized electron-hole pair with a picosecond laser pulse, and in an applied electric field, waiting for the electron to tunnel leaving a spin-polarized hole. Detection of the hole spin with picoseconds time resolution is achieved using a second picosecond laser pulse to probe the positive trion transition, where a trion is created conditional on the hole spin being detected as a change in photocurrent. Finally, using this setup we observe a Rabi rotation of the hole-trion transition that is conditional on the hole spin, which for a pulse area of 2 pi can be used to impart a phase shift of pi between the hole spin states, a non-general manipulation of the hole spin. (C) 2009 Elsevier Ltd. All rights reserved
Cassini UVIS Observations of the Io Plasma Torus. IV. Modeling Temporal and Azimuthal Variability
In this fourth paper in a series, we present a model of the remarkable
temporal and azimuthal variability of the Io plasma torus observed during the
Cassini encounter with Jupiter. Over a period of three months, the Cassini
Ultraviolet Imaging Spectrograph (UVIS) observed a dramatic variation in the
average torus composition. Superimposed on this long-term variation, is a
10.07-hour periodicity caused by an azimuthal variation in plasma composition
subcorotating relative to System III longitude. Quite surprisingly, the
amplitude of the azimuthal variation appears to be modulated at the beat
frequency between the System III period and the observed 10.07-hour period.
Previously, we have successfully modeled the months-long compositional change
by supposing a factor of three increase in the amount of material supplied to
Io's extended neutral clouds. Here, we extend our torus chemistry model to
include an azimuthal dimension. We postulate the existence of two azimuthal
variations in the number of super-thermal electrons in the torus: a primary
variation that subcorotates with a period of 10.07 hours and a secondary
variation that remains fixed in System III longitude. Using these two hot
electron variations, our model can reproduce the observed temporal and
azimuthal variations observed by Cassini UVIS.Comment: Revised 24 August 2007 Accepted by Icarus, 50 pages, 2 Tables, 8
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