9,871 research outputs found
Magnetism of two-dimensional defects in Pd: stacking faults, twin boundaries and surfaces
Careful first-principles density functional calculations reveal the
importance of hexagonal versus cubic stacking of closed packed planes of Pd as
far as local magnetic properties are concerned. We find that, contrary to the
stable face centered cubic phase, which is paramagnetic, the hexagonal
close-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35
/atom. Our results show that two-dimensional defects with local hcp
stacking, like twin boundaries and stacking faults, in the otherwise fcc Pd
structure, increase the magnetic susceptibility. The (111) surface also
increases the magnetic susceptibility and it becomes ferromagnetic in
combination with an individual stacking fault or twin boundary close to it. On
the contrary, we find that the (100) surface decreases the tendency to
ferromagnetism. The results are consistent with the magnetic moment recently
observed in small Pd nanoparticles, with a large surface area and a high
concentration of two-dimensional stacking defects.Comment: 8 pages, 10 figure
Memory in the Photon Statistics of Multilevel Quantum Systems
The statistics of photons emitted by single multilevel systems is
investigated with emphasis on the nonrenewal characteristics of the
photon-arrival times. We consider the correlation between consecutive
interphoton times and present closed form expressions for the corresponding
multiple moment analysis. Based on the moments a memory measure is proposed
which provides an easy way of gaging the non-renewal statistics. Monte-Carlo
simulations demonstrate that the experimental verification of non-renewal
statistics is feasible.Comment: 5 pages, 3 figure
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Senior tourism with sustainable marketing perspective in Mazatlan and Los Cabos, Mexico.
The increasing participation of older people in tourist activities, combined with a demographic phenomenon that implies the aging of the world population, made governments and tourism providers in developed regions identify senior market as a priority.
Tourism is a multidimensional activity that have increased substantially in the last years, challenging the destinations\u27 ecosystems since tourism contributes to greenhouse gas emissions globally. Mass tourism has a significant impact on the environment; hence, sustainable tourism pops out in such circumstances. To the theme of sustainability, Mexico deals with important contrasts.
This research outlines the main elements that tourism marketing (MT) could contribute in terms of sustainability as a competitive advantage of a destination that serves the international senior tourist segment, from the perspective of two case studies of sun-and beach tourist destinations in northwestern Mexico
Structural models for the Si(553)-Au atomic chain reconstruction
Recent photoemission experiments on the Si(553)-Au reconstruction show a
one-dimensional band with a peculiar ~1/4 filling. This band could provide an
opportunity for observing large spin-charge separation if electron-electron
interactions could be increased. To this end, it is necessary to understand in
detail the origin of this surface band. A first step is the determination of
the structure of the reconstruction. We present here a study of several
structural models using first-principles density functional calculations. Our
models are based on a plausible analogy with the similar and better known
Si(557)-Au surface, and compared against the sole structure proposed to date
for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ].
Results for the energetics and the band structures are given. Lines for the
future investigation are also sketched
Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydro- dynamic models of filaments.
To date, highly idealized models for prominences have been used, especially
with respect to the overall magnetic configurations.
Aims. We initiate a more systematic survey of filament wave modes, where we
consider full multi-dimensional models with twisted magnetic fields
representative of the surrounding magnetic flux rope. This requires the ability
to compute accurate 2.5 dimensional magnetohydrodynamic equilibria that balance
Lorentz forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion).
Methods. The governing extended Grad-Shafranov equation is discussed, along
with an analytic prediction for circular flux ropes for the Shafranov shift of
the central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy on an
explicitly known, non-trivial test case.
Results. The code is then used to construct more realistic prominence
equilibria, for all three possible choices of a free flux-function. We quantify
the influence of gravity, and generate cool condensations in hot cavities, as
well as multi- layered prominences.
Conclusions. The internal flux rope equilibria computed here have the
prerequisite numerical accuracy to allow a yet more advanced analysis of the
complete spectrum of linear magnetohydrodynamic perturbations, as will be
demonstrated in the companion paper.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 15 figure
Kinetic instability of drift-Alfven waves in solar corona and stochastic heating
The solar atmosphere is structured and inhomogeneous both horizontally and
vertically. The omnipresence of coronal magnetic loops implies gradients of the
equilibrium plasma quantities like the density, magnetic field and temperature.
These gradients are responsible for the excitation of drift waves that grow
both within the two-component fluid description (in the presence of collisions
and without it) and within the two-component kinetic descriptions (due to
purely kinetic effects). In the present work the effects of the density
gradient in the direction perpendicular to the magnetic field vector are
investigated within the kinetic theory, in both electrostatic and
electromagnetic regimes. The electromagnetic regime implies the coupling of the
gradient-driven drift wave with the Alfven wave. The growth rates for the two
cases are calculated and compared. It is found that, in general, the
electrostatic regime is characterized by stronger growth rates, as compared
with the electromagnetic perturbations. Also discussed is the stochastic
heating associated with the drift wave. The released amount of energy density
due to this heating should be more dependent on the magnitude of the background
magnetic field than on the coupling of the drift and Alfven waves. The
stochastic heating is expected to be much higher in regions with a stronger
magnetic field. On the whole, the energy release rate caused by the stochastic
heating can be several orders of magnitude above the value presently accepted
as necessary for a sustainable coronal heating.Comment: To appear in ApJ (2010
The nature of the low energy band of the Fenna-Matthews-Olson complex: vibronic signatures
Based entirely upon actual experimental observations on electron-phonon
coupling, we develop a theoretical framework to show that the lowest energy
band of the Fenna- Matthews-Olson (FMO) complex exhibits observable features
due to the quantum nature of the vibrational manifolds present in its
chromophores. The study of linear spectra provides us with the basis to
understand the dynamical features arising from the vibronic structure in
non-linear spectra in a progressive fashion, starting from a microscopic model
to finally performing an inhomogenous average. We show that the discreteness of
the vibronic structure can be witnessed by probing the diagonal peaks of the
non-linear spectra by means of a relative phase shift in the waiting time
resolved signal. Moreover, we demonstrate the photon-echo and non-rephasing
paths are sensitive to different harmonics in the vibrational manifold when
static disorder is taken into account. Supported by analytical and numerical
calculations, we show that nondiagonal resonances in the 2D spectra in the
waiting time, further capture the discreteness of vibrations through a
modulation of the amplitude without any effect in the signal intrinsic
frequency. This fact generates a signal that is highly sensitive to
correlations in the static disorder of the excitonic energy albeit protected
against dephasing due to inhomogeneities of the vibrational ensemble.Comment: 14 pages, 6 figure
Transition to turbulence in nonuniform coronal loops driven by torsional Alfven waves
Both observations and numerical simulations suggest that Alfvenic waves may
carry sufficient energy to sustain the hot temperatures of the solar
atmospheric plasma. However, the thermalization of wave energy is inefficient
unless very short spatial scales are considered. Phase mixing is a mechanism
that can take energy down to dissipation lengths, but it operates over too long
a timescale. Here, we study how turbulence, driven by the nonlinear evolution
of phase-mixed torsional Alfven waves in coronal loops, is able to take wave
energy down to the dissipative scales much faster than the theory of linear
phase mixing predicts. We consider a simple model of a transversely nonuniform
cylindrical flux tube with a constant axial magnetic field. The flux tube is
perturbed by the fundamental mode of standing torsional Alfven waves. We solved
the three-dimensional (3D) ideal magnetohydrodynamics equations numerically to
study the temporal evolution. Initially, torsional Alfven waves undergo the
process of phase mixing because of the transverse variation of density. After
only few periods of torsional waves, azimuthal shear flows generated by phase
mixing eventually trigger the Kelvin-Helmholtz instability (KHi), and the flux
tube is subsequently driven to a turbulent state. Turbulence is very
anisotropic and develops transversely only to the background magnetic field.
After the onset of turbulence, the effective Reynolds number decreases in the
flux tube much faster than in the initial linear stage governed by phase mixing
alone. We conclude that the nonlinear evolution of torsional Alfven waves, and
the associated KHi, is a viable mechanism for the onset of turbulence in
coronal loops.Comment: Accepted in A&
Hallazgo de Eastonia rugosa (Chemnitz) en el Pleistoceno de Mallorca
Abstract not availabl
Inelastic fingerprints of hydrogen contamination in atomic gold wire systems
We present series of first-principles calculations for both pure and hydrogen
contaminated gold wire systems in order to investigate how such impurities can
be detected. We show how a single H atom or a single H2 molecule in an atomic
gold wire will affect forces and Au-Au atom distances under elongation. We
further determine the corresponding evolution of the low-bias conductance as
well as the inelastic contributions from vibrations. Our results indicate that
the conductance of gold wires is only slightly reduced from the conductance
quantum G0=2e^2/h by the presence of a single hydrogen impurity, hence making
it difficult to use the conductance itself to distinguish between various
configurations. On the other hand, our calculations of the inelastic signals
predict significant differences between pure and hydrogen contaminated wires,
and, importantly, between atomic and molecular forms of the impurity. A
detailed characterization of gold wires with a hydrogen impurity should
therefore be possible from the strain dependence of the inelastic signals in
the conductance.Comment: 5 pages, 3 figures, Contribution to ICN+T2006, Basel, Switzerland,
July-August 200
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