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 $\mu_{B}$/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

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