Spin wave resonances in La_{0.7}Sr_{0.3}MnO_{3} films: measurement of spin wave stiffness and anisotropy field

We studied magnetic field dependent microwave absorption in epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films using an X-band Bruker ESR spectrometer. By analyzing angular and temperature dependence of the ferromagnetic and spin-wave resonances we determine spin-wave stiffness and anisotropy field. The spin-wave stiffness as found from the spectrum of the standing spin-wave resonances in thin films is in fair agreement with the results of inelastic neutron scattering studies on a single crystal of the same composition [Vasiliu-Doloc et al., J. Appl. Phys. \textbf{83}, 7343 (1998)].Comment: 15 pages, 7 figures (now figure captions are included

Analysis and Control of mortar Quality with ultrasonic wave attenuation

Wave propagation and attenuation on mortar material are discussed in this paper with a Non Destructive Testing (NDT) which can be used to characterize samples of mortars and effect of microstructure of sand in their hardening. Samples were manufactured using same water/cement ration (w/c) 0.65, and cement/sand ration (c/s) 0.5 in order to simulate the attenuation. The characterization of attenuation in mortar material has been performed by ultrasonic reflection technique using a transducer with central frequency 0.5MHz. It is shown that sand particle size exercises significant influence on the evolution of attenuation, the attenuation parameters give information about average state of the hardening of the mortar itself

A Study of Carbon Substitutions in MgB_2 within the two-band Eliashberg theory

We study the effects of C substitutions in MgB_2 within the two-band model in the Eliashberg formulation. We use as input the B-B stretching-mode frequency and the partial densities of states N_{sigma}(EF) and N_{pi}(EF), recently calculated for Mg(B_{1-x}C_{x})_2 at various x values from first-principles density functional methods. We then take the prefactor in the Coulomb pseudopotential matrix, mu, and the interband scattering parameter, Gamma^{sigma pi}, as the only adjustable parameters. The dependence on the C content of Tc and of the gaps (Delta_{sigma} and Delta_{pi}) recently measured in Mg(B_{1-x}C_{x})_2 single crystals indicate an almost linear decrease of mu on increasing x, with an increase in interband scattering that makes the gaps merge at x=0.132. In polycrystals, instead, where the gap merging is not observed, no interband scattering is required to fit the experimental data.Comment: 7 pages, 8 figures, RevTex4. Detailed discussion of the results adde

Comparison of MPPT techniques: “P&O” and “InCd” for PV systems

The energy utilization efficiency of the photovoltaic can be significantly improved by employing the MPPT (Maximum power point trackers) method. The MPPT is one of the key technologies in photovoltaic generation system;it allows extraction of maximum available power from the photovoltaic (PV) array.The maximum available power is tracked using specialized algorithms such as Perturb and Observe (P&O), and Incremental Conductance (InCd), which are widely applied in the MPPTcontroller due to their simplicity and easy implementation. This paper presents a detailed comparison of their theories and their simulations under the Matlab / Simulink environment. The obtained results show that the use of MPPT control improves in a considerable way the performance of PV systems

Vector Control of a Doubly-Fed Induction Generator by Using a Classical PI and a fuzzy PI Controllers

This work deals with the conversion of wind systems in order to improve the quality of the provided energy. To this end, we are interested in the modeling and the simulation of a Doubly-Fed Induction Generator (DFIG) with a wound rotor used in the electromechanical conversion of wind systems. In this paper, we carried out the modeling and the direct and indirect vector control of the (DFIG) by using a classical PI controller and then a fuzzy logic PI controller. The aim of these control systems is to minimize the interaction between active and reactive power and to ensure an efficient decoupling by the use of two algorithms: fuzzy logic control and classical control.nbsp The algorithms are developed and tested under Matlab/Simulink

Modeling and Simulation of an Isolated Site Conversion Chain Driven by a Permanent Magnet Generator

The objective of this work is to study the conversion of wind energy in its entirety in order to optimize the output power and improve the quality of the energy supplied. For this, we are interested in the modeling and simulation of a turbine associated with a speed multiplier, we study the modeling and control of a permanent magnet synchronous generator feeding a three-phase load which corresponds to a chain of Conversion of small-scale wind power into an isolated site. The technique adopted is developed in Matlab / Simulink / SimPowerSystems. The results of the simulation are presented and analyzed at the end of this work

First evidence for a gravitational lensing-induced echo in gamma rays with Fermi LAT

Aims. This article shows the first evidence for gravitational lensing phenomena in high energy gamma-rays. This evidence comes from the observation of a gravitational lens induced echo in the light curve of the distant blazar PKS 1830-211. Methods. Traditional methods for the estimation of time delays in gravitational lensing systems rely on the cross-correlation of the light curves of the individual images. In this paper, we use 300 MeV-30 GeV photons detected by the Fermi-LAT instrument. The Fermi-LAT instrument cannot separate the images of known lenses. The observed light curve is thus the superposition of individual image light curves. The Fermi-LAT instrument has the advantage of providing long, evenly spaced, time series. In addition, the photon noise level is very low. This allows to use directly Fourier transform methods. Results. A time delay between the two compact images of PKS 1830-211 has been searched for both by the autocorrelation method and the "double power spectrum" method. The double power spectrum shows a 3 {\sigma} evidence for a time delay of 27.5 $\pm$ 1.3 days, consistent with the result from Lovell et al. (1998). The relative uncertainty on the time delay estimation is reduced from 20% to 5%.Comment: submitted to A&

Structure and Spin Dynamics of La0.85_{0.85}Sr0.15_{0.15}MnO3_3

Neutron scattering has been used to study the structure and spin dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities and new superlattice peaks consistent with the onset of a spin-canted phase below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0, 0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a concomitant lattice distortion. The long-wavelength magnetic excitations are found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$ dependence at low T, and the damping at small q follows $q^4T^{2}$. An anomalously strong quasielastic component, however, develops at small wave vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At larger q, on the other hand, the magnetic excitations become heavily damped at low temperatures, indicating that spin waves in this regime are not eigenstates of the system, while raising the temperature dramatically increases the damping. The strength of the spin-wave damping also depends strongly on the symmetry direction in the crystal. These anomalous damping effects are likely due to the itinerant character of the $e_g$ electrons.Comment: 8 pages (RevTex), 9 figures (encapsulated postscript

Spatio-temporal analysis of North African forest cover dynamics using time series of vegetation indices – case of the Maamora forest (Morocco)

North African forest areas play several roles and functions and represent a heritage of great economic and ecological importance. As a result of global changes, that act independently or synergistically, these areas are currently undergoing a pronounced degradation and their productivity is decreasing due to several factors. This work aims to characterize spatio-temporal dynamics of vegetation within the Maamora forest. This forest is considered as the most extensive cork oak woodland in the world and is divided, from west to east, into five cantons A, B, C, D and E. The data, extracted between 2000–2021 from MODIS NDVI/EVI images of 250 m, were analyzed using statistical parameters with the Pettitt homogeneity and the Mann-Kendall trend tests, with their seasonal and spatial components, in order to better consider the vegetation distribution of this forest. Results show a clear temporal and spatial (inter-canton) variability of vegetation intensity, unrelated to the continental gradient. In fact, recorded mean values in cantons C and E are significantly higher than those of cantons B and D respectively. This is confirmed by both regressive and progressive trends, which were identified respectively from the months of March 2012 and October 2008, in the data series of cantons B and E successively. Spatially, the regressive dynamic remains generalized and affects more than 26.7% of the Maamora’s total area with extreme rates (46.1% and 14.0%) recorded respectively by the two aforementioned cantons. Similarly, all the stand types in canton B show the highest regressive rates, especially the cork oak regeneration strata (75.4%) and the bare lands (86.1%), which may explain the positive tendencies identified by the related series during the fall season. However, the cantons C and E record the lowest rates, respectively, for natural stands of cork oak and artificial plantations. These results highlight also the absence of a causal relationship between the contrasting vegetation dynamics of the Maamora and the climatic conditions, expressed here by the continental gradient. However, they do highlight the effects of other factors, particularly those of a technical nature

Macroscopic Quantum Coherence of the Spin Triplet in the Spin-Ladder Compound Sr14Cu24O41

International audienceWe report the direct observation by inelastic neutron scattering experiments of a spin triplet of magnetic excitations in the response associated with the ladders in the composite cuprate Sr14Cu24O41. This appears as a peak at q_{Q1D}=\pi and energy \Delta_1=32.5 meV, and we conjecture that all the triplets making up this conspicuous peak have the same phase and therefore interpret it as the signature of the occurrence of quantum coherence along the ladder direction between entangled spin pairs. From the comparison with previous neutron and x-ray data, we conclude that the temperature evolution of this mode is driven by the crystallization of holes into a charge density wave in the ladder sublattic