Dynamic screening of a localized hole during photoemission from a metal cluster

Recent advances in attosecond spectroscopy techniques have fueled the interest in the theoretical description of electronic processes taking place in the subfemtosecond time scale. Here we study the coupled dynamic screening of a localized hole and a photoelectron emitted from a metal cluster using a semi-classical model. Electron density dynamics in the cluster is calculated with Time-Dependent Density Functional Theory and the motion of the photoemitted electron is described classically. We show that the dynamic screening of the hole by the cluster electrons affects the motion of the photoemitted electron. At the very beginning of its trajectory, the photoemitted electron interacts with the cluster electrons that pile up to screen the hole. Within our model, this gives rise to a significant reduction of the energy lost by the photoelectron. Thus, this is a velocity dependent effect that should be accounted for when calculating the average losses suffered by photoemitted electrons in metals.Comment: 15 pages, 5 figure

Analyse de la rupture et évaluation de la durée de vie basée sur la fiabilité des tubes en polyéthylène pour le transport du gaz

L'objectif de la première partie de cette recherche est de mettre en relief les lois statistiques qui s'adaptent à la rupture par fatigue des canalisations de transport de gaz en polyéthylène. Les résultats indiquent qu'il est recommandé d'utiliser la distribution de Weibull à deux paramètres pour l'évaluation de la fiabilité de ces canalisations pour la phase conception vu leur l'aspect conservateur. La seconde partie est une nouvelle contribution pour l'analyse des résultats de fissuration par fatigue dans ces tubes en termes de paramètres énergétiques basés sur le travail irréversible et la vitesse de propagation. La transition de fragile à ductile est ainsi décrite par des énergies critiques obtenues pour différentes charges de fatigue. Enfin, la fiabilité et l'optimisation de la maintenance des pipes en polyéthylène sont approchées par l'étude de l'effet de la température et de la pression sur la probabilité de défaillance. Il en ressort que l'effet des incertitudes relatives à la température et au modèle de durée de vie est plus important que les fluctuations de la pression du gaz et la géométrie, alors que le remplacement groupé des tronçons usés est bien justifié à partir de la vie de service et des coûts correspondantsCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys

Weyl fermions have recently been observed in several time-reversal-invariant semimetals and photonics materials with broken inversion symmetry. These systems are expected to have exotic transport properties such as the chiral anomaly. However, most discovered Weyl materials possess a substantial number of Weyl nodes close to the Fermi level that give rise to complicated transport properties. Here we predict, for the first time, a new family of Weyl systems defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler materials XCo(2)Z (X = IVB or VB; Z = IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be -1, guaranteeing the existence of an odd number of pairs of Weyl fermions. These materials exhibit, when alloyed, only two Weyl nodes at the Fermi level-the minimum number possible in a condensed matter system. The Weyl nodes are protected by the rotational symmetry along the magnetic axis and separated by a large distance (of order 2 pi) in the Brillouin zone. The corresponding Fermi arcs have been calculated as well. This discovery provides a realistic and promising platform for manipulating and studying the magnetic Weyl physics in experiments

Inelastic X-ray scattering and first-principles study of electron excitations in MgB2

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)An experimental and theoretical study of electronic excitations in MgB2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the Gamma A and Gamma K directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed. (c) 2009 Elsevier Ltd. All rights reserved.14939-4017061711SeCyT (Universidad Nacional de Cordoba, Argentina)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)University of the Basque Country [GIC07IT36607]Departamento de Educacion del Gobierno VascoSpanish Ministerio de Ciencia y Technologia (MCyT) [FIS200766711C0101]IKERBASQUE FoundationLNLS-Brazilian Synchrotron Light Laboratory/MCTFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)University of the Basque Country [GIC07IT36607]Spanish Ministerio de Ciencia y Technologia (MCyT) [FIS200766711C0101

Low-energy acoustic plasmons at metal surfaces RID B-5938-2008 RID C-8602-2009

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers(1) and artificial layered materials(2,3) permit the existence of low-energy collective excitations(4,5), called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons(3), and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors(6). Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons(7). Rather, metallic surfaces should support only conventional surface plasmons(8) - higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy(9,10) and sub-wavelength optics(11-13), but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic ( linear) dispersion, different to the q(parallel to)(1/2) dependence of a 2D plasmon, and was observed on Be( 0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications