Electronic transport in quantum cascade structures

The transport in complex multiple quantum well heterostructures is theoretically described. The model is focused on quantum cascade detectors, which represent an exciting challenge due to the complexity of the structure containing 7 or 8 quantum wells of different widths. Electronic transport can be fully described without any adjustable parameter. Diffusion from one subband to another is calculated with a standard electron-optical phonon hamiltonian, and the electronic transport results from a parallel flow of electrons using all the possible paths through the different subbands. Finally, the resistance of such a complex device is given by a simple expression, with an excellent agreement with experimental results. This relation involves the sum of transitions rates between subbands, from one period of the device to the next one. This relation appears as an Einstein relation adapted to the case of complex multiple quantum structures.Comment: 6 pages, 5 figures, 1 tabl

Evolution des flores et de la végétation Tertiaires en Afrique, au nord de PEquateur

La repartition de la flore Tertiaire en Afrique, três différente de l’actuelle, se comprend si on tient compte de deux facteurs essentiels: la regression des mers épicontinentales vers le nord et la dérive du continent africain. II en résulte qu’á l'Eocêne inférieur, l’équateur se trouvait á environ 1 000 km au nord de sa position actuelle. Ces modifications aboutissent á des transformations importantes de l’environnement climatique et par consequent de la repartition des flores. Cela explique notamment la presence d’une mangrove en Libye avec Sonneratioxylon aubrevillei et en Egypte avec Nipa burtinii. La forêt dense est présente dans le Paléocêne du Tinrhert avec Entandrophragmoxylon normandii (comparable á l’actuel  Entandrophragma angolense). Les formations de foret dense, développées á l’Eocêne, surtout sur le rivage, sont suivies plus au sud, d’une savane qui s’étend davantage á I’Oligocêne et qui peut atteindre la mer. Cependant, on trouve encore en bordure de mer á l’Oligocêne, des espêces de forêt dense comme Entandrophragmoxylon magnieri (comparable á  Entandrophragma candollei) espêce qui avoisine des dépóts deltaiques avec de grands troncs qui indiquent une flore plus sêche (Detarioxylon aegyptiacum, Combretoxylon bussonii et Pterocarpoxylon tibestiense (rapproché du Pterocarpus erinaceus de savane), mais parfois accompagnée de troncs de forêt-galerie (Atherospermoxylon aegyptiacum, Monimiacée). Au Miocene, l’assechement continue et la savane progresse. Les Palmiers apparaissent et des galeries forestieres subsistent, mais même á proximité de la mer, le biotope est sec au Miocene inférieur dans le Nord de l’Afrique. Ce n’est que vers le sud que la végétation hygrophile apparait au niveau du Tibesti avec Myristicoxylon vincentii. Ainsi. prês de la position présente de l’Equateur, la mangrove se retrouve au Sénégal dans le Paléogêne du Cap-Vert avec des pollens de Rhizophoracées. Sonneratiacées, Avicennia et Nipa (Spinizonocolpites) et la forêt dense subsiste dans sa position actuelle entourée d’une savane

Flood Detection in Time Series of Optical and SAR Images

These last decades, Earth Observation brought a number of new perspectives from geosciences to human activity monitoring. As more data became available, Artificial Intelligence (AI) techniques led to very successful results for understanding remote sensing data. Moreover, various acquisition techniques such as Synthetic Aperture Radar (SAR) can also be used for problems that could not be tackled only through optical images. This is the case for weather-related disasters such as floods or hurricanes, which are generally associated with large clouds cover. Yet, machine learning on SAR data is still considered challenging due to the lack of available labeled data. To help the community go forward, we introduce a new dataset composed of co-registered optical and SAR images time series for the detection of flood events and new neural network approaches to leverage these two modalities

Palaeobotanical studies from tropical Africa: relevance to the evolution of forest, woodland and savannah biomes.

Fossil plants provide data on climate, community composition and structure, all of which are relevant to the definition and recognition of biomes. Macrofossils reflect local vegetation, whereas pollen assemblages sample a larger area. The earliest solid evidence for angiosperm tropical rainforest in Africa is based primarily on Late Eocene to Late Oligocene (ca. 39-26 Myr ago) pollen assemblages from Cameroon, which are rich in forest families. Plant macrofossil assemblages from elsewhere in interior Africa for this time interval are rare, but new work at Chilga in the northwestern Ethiopian Highlands documents forest communities at 28 Myr ago. Initial results indicate botanical affinities with lowland West African forest. The earliest known woodland community in tropical Africa is dated at 46 Myr ago in northern Tanzania, as documented by leaves and fruits from lake deposits. The community around the lake was dominated by caesalpinioid legumes, but included Acacia, for which this, to my knowledge, is the earliest record. This community is structurally similar to modern miombo, although it is different at the generic level. The grass-dominated savannah biome began to expand in the Middle Miocene (16 Myr ago), and became widespread in the Late Miocene (ca. 8 Myr ago), as documented by pollen and carbon isotopes from both West and East Africa