IPG Photonics

About: IPG Photonics is a based out in . It is known for research contribution in the topics: Laser & Fiber laser. The organization has 903 authors who have published 1241 publications receiving 63339 citations.

Ab initio Raman spectrum of the normal and disordered Mg Al 2 O 4 spinel

Abstract: Ab initio calculations, based on density functional theory, of the vibrational Raman spectra of both normal and cationic disordered $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ spinel are reported. The vibrational properties of the disordered system are obtained in two steps. First, the interatomic force constants of small defective supercells are computed in a fully ab initio way. Later, these force constants are used to build the dynamical matrices of larger disordered systems. The calculated Raman spectrum presents the mode near $727\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ which is due to the presence of cationic disorder and which is commonly attributed to a breathing mode (BM) of $\mathrm{Al}{\mathrm{O}}_{4}$ tetrahedra. Contrary to previous interpretations, the $727\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ mode is due to phonons which are silent in the perfect crystal and become Raman active in the disordered structure because of the coupling with the BM of the $\mathrm{Mg}{\mathrm{O}}_{4}$ tetrahedra.

RegSEM: a versatile code based on the spectral element method to compute seismic wave propagation at the regional scale

Abstract: The spectral element method, which provides an accurate solution of the elastodynamic problem in heterogeneous media, is implemented in a code, called RegSEM, to compute seismic wave propagation at the regional scale. By regional scale we here mean distances ranging from about 1 km (local scale) to 90 • (continental scale). The advantage of RegSEM resides in its ability to accurately take into account 3-D discontinuities such as the sediment-rock interface and the Moho. For this purpose, one version of the code handles local unstructured meshes and another version manages continental structured meshes. The wave equation can be solved in any velocity model, including anisotropy and intrinsic attenuation in the continental version. To validate the code, results from RegSEM are compared to analytical and semi-analytical solutions available in simple cases (e.g. explosion in PREM, plane wave in a hemispherical basin). In addition, realistic simulations of an earthquake in different tomographic models of Europe are performed. All these simulations show the great flexibility of the code and point out the large influence of the shallow layers on the propagation of seismic waves at the regional scale. RegSEM is written in Fortran 90 but it also contains a couple of C routines. It is an open-source software which runs on distributed memory architectures. It can give rise to interesting applications, such as testing regional tomographic models, developing tomography using either passive (i.e. noise correlations) or active (i.e. earthquakes) data, or improving our knowledge on effects linked with sedimentary basins.

NMR measurements and numerical simulation of fluid transport in porous solids

Abstract: Pulsed magnetic field gradient stimulated echo NMR measurements are performed for diffusion and flow of an aqueous phase both within a sample of packed spherical beads and within a 25% porosity Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area q is used to derive the displacement probability distributions P Δ (X) for fixed observation times Δ. The shape of P Δ (X) as a function of Δ is simulated for computer-generated porous media, and a good agreement is obtained between the experimental NMR data and the simulations.