The x-ray standing wave method is used to investigate some crystallographic features of the first stages of growth of ultrathin pseudomorphic MnTe(001) strained layers buried in CdTe on CdTe(001) substrates. Experiments with 004 and 113 reflecting planes show evidence of the presence of both MnTe clusters and diluted CdMnTe alloy.
À travers une approche historique, épistémologique et patrimoniale des minéraux de collection, les auteurs analysent le changement d’image des minéraux et de la façon de les collectionner – notamment leur passage progressif d’objet précieux participant à une représentation idéale de la Nature à spécimens classés et inventoriés, destinés à l’enseignement et à la recherche – et s’interrogent sur l’évolution du statut des collections et les enjeux actuels de leur patrimonialisation.
By taking advantage of the channeling phenomenon in solids, stepped structures are particularly well suited for studying surface atomic displacements. In particular 90\ifmmode^\circ\else\textdegree\fi{} double-alignment experiments, in which the blocking of the backscattered particles by axes of the terrace plane is studied, provide exclusive information on the first atomic plane, without any bulk contribution. Surface thermal vibrations and disorder can be studied in this way. Moreover, in these experiments, the specific relaxation of the step-edge atoms can be measured. Channeling experiments in a more standard geometry can also be performed as on flat surfaces in order to extract information on the relaxation of the whole terrace. The (16,1,1) stepped, kinkless copper structure has been studied. The main results obtained for a clean surface are the following: (1) The coherent relaxation of the surface plane is very small (at the limit of our experimental precision); however, the best fit of our results corresponds to a contraction of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ \AA{}. (2) A coherent relaxation, perpendicular to the terraces, specific to the ledge atoms, has not been detected and is hence certainly smaller than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ \AA{}. (3) There is evidence for small, quasi-isotropic, static displacements of the surface atoms, all over the surface plane of concern. These displacements, with a mean value around 0.1 \AA{} at 300 K, are somewhat correlated and are seen in channeling in the same way as thermal vibrations, but they have a stronger temperature dependence. This type of defect is perhaps related to the stepped structure. However, since it can only be detected unambiguously in 90\ifmmode^\circ\else\textdegree\fi{} double-alignment backscattering experiments, which require a stepped surface, it may well be present but unobserved in simpler structures. We have also studied the effect of oxygen coverage. The saturation coverage ${\ensuremath{\bigominus}}_{s}$ has been determined quantitatively by nuclear microanalysis; it corresponds to ${\ensuremath{\bigominus}}_{s}=0.5$ of a monolayer. In this case low-energy electron-diffraction patterns show the existence of surface reconstruction and faceting. The channeling results are consistent with a change from the $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ \AA{} contraction observed on the clean surface to a $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ \AA{} dilatation. However, they mainly point to an increase of surface disorder. The very high sensitivity of the channeling technique in the 90\ifmmode^\circ\else\textdegree\fi{}-backscattering double-alignment geometry for studying surface defects and surface thermal vibrations is also discussed.
Abstract The SuperCam instrument suite onboard the Mars 2020 Perseverance rover uses the laser-induced breakdown spectroscopy (LIBS) technique to determine the elemental composition of rocks and soils of the Mars surface. It is associated with a microphone to retrieve the physical properties of the ablated targets when listening to the laser-induced acoustic signal. In this study, we report the monitoring of laser-induced mineral phase transitions in acoustic data. Sound data recorded during the laser ablation of hematite, goethite and diamond showed a sharp increase of the acoustic signal amplitude over the first laser shots. Analyses of the laser-induced craters with Raman spectroscopy and scanning electron microscopy indicate that both hematite and goethite have been transformed into magnetite and that diamond has been transformed into amorphous-like carbon over the first laser shots. It is shown that these transitions are the root cause of the increase in acoustic signal, likely due to a change in target’s physical properties as the material is transformed. These results give insights into the influence of the target’s optical and thermal properties over the acoustic signal. But most importantly, in the context of the Mars surface exploration with SuperCam, as this behavior occurs only for specific phases, it demonstrates that the microphone data may help discriminating mineral phases whereas LIBS data only have limited capabilities.
