Le graphène épitaxié : un cas d'école de graphène isolé

International audienceLe graphène épitaxié multicouches produit sur carbure de silicium, avec lequel sont réalisés des circuits électroniques à grande échelle, est une structure nouvelle de carbone. Des expériences menées sur la ligne de photoémission haute résolution Cassiopée à SOLEIL viennent de démontrer que de façon surprenante, les feuillets de graphène dans cet empilement rotationnel ordonné sont effectivement découplés et possèdent la structure électronique d'un feuillet isolé de graphène

Non-trivial Surface-band Dispersion on Bi(111)

We performed angle-resolved photoelectron spectroscopy of the Bi(111) surface to demonstrate that this surface support edge states of non-trivial topology. Along the $\bar{\Gamma}\bar{M}$-direction of the surface Brillouin zone, a surface-state band disperses from the projected bulk valence bands at $\bar{\Gamma}$ to the conduction bands at $\bar{M}$ continuously, indicating the non-trivial topological order of three-dimensional Bi bands. We ascribe this finding to the absence of band inversion at the $L$ point of the bulk Bi Brillouin zone. According to our analysis, a modification of tight-binding parameters can account for the non-trivial band structure of Bi without any other significant change on other physical properties.Comment: 13 pages, 4 figures. This manuscript has been accepted in New Journal of Physic

Catalytic degradation of O‐cresol using H2O2 onto Algerian Clay‐Na

Clay material is used as a catalyst to degrade an organic pollutant,. This study focused on the O‐cresol oxidative degradation in aqueous solution by adding H2O2 and Mont‐Na. The catalytic tests showed a high catalytic activity of Mont‐Na, which made it possible to achieve more than 84.6% conversion after 90 minutes of reaction time at 55 °C in 23.2 mM H2O2. The pH value was found to be negatively correlated with the degradation rate of O‐cresol. UV‐Vis spectrophotometry revealed that the increase of degradation rate at low pH is related to the formation of 2‐Methylbenzoquinone as intermediate product. In addition, the content of iron in Mont‐Na decreased after the catalytic test, bringing further evidence about the O‐cresol catalytic oxidation. The mineralization of O‐cresol is also confirmed by the different methods of characterization of Mont‐Na after the catalytic oxidation test. The effect of the O‐cresol oxidation catalyzed by natural clay is significant.Financial support for this work by the Algerian Directorate General of Scientific Research and Technological Development (DGRSDT) and the Ministry for Higher Education and Scientific Research is gratefully appreciated

Correlation between the tribology of concrete and the rheology of the corresponding mortars

The present work aims to study the possibility of characterizing the pumpability of concrete from the rheology of the derived mortars, i.e. concrete mortar (CM) and concrete equivalent mortar (CEM). As concrete is a material that is heavier than mortar, it is better to carry out tests on mortar. To this end, two test campaigns are presented in this article; the first campaign concerns the formulation of admixed concrete based on two mineral additions, namely limestone fillers and blast furnace slag. The aim of this first campaign was to study the tribological behavior of the formulated concretes. In the second test campaign, the formulations of the mortars were derived from the formulations of the corresponding concretes. Afterwards, a rheological study was carried out on these mortars. Analysis of the results obtained showed that there is a good correlation between the developed concretes and their corresponding CEMs, with fairly high determination coefficients

Correlation between the tribology of concrete and the rheology of the corresponding mortars

The present work aims to study the possibility of characterizing the pumpability of concrete from the rheology of the derived mortars, i.e. concrete mortar (CM) and concrete equivalent mortar (CEM). As concrete is a material that is heavier than mortar, it is better to carry out tests on mortar. To this end, two test campaigns are presented in this article; the first campaign concerns the formulation of admixed concrete based on two mineral additions, namely limestone fillers and blast furnace slag. The aim of this first campaign was to study the tribological behavior of the formulated concretes. In the second test campaign, the formulations of the mortars were derived from the formulations of the corresponding concretes. Afterwards, a rheological study was carried out on these mortars. Analysis of the results obtained showed that there is a good correlation between the developed concretes and their corresponding CEMs, with fairly high determination coefficients

Facteurs environnementaux influençant la composition et la distribution des larves d’éphémères dans les oueds nord-algériens (échelle régionale)

Les relations entre la distribution des Éphéméroptères et les conditions de leurs habitats ont été étudiées dans 87 stations, dans les oueds du Nord de l’Algérie, en confrontant 17 variables environnementales et 27 espèces recensées (présence/absence). L’objectif de ce travail est (1) d’identifier les variables environnementales qui conditionnent la répartition des Éphéméroptères à l’échelle régionale ; (2) de définir leurs préférences écologiques le long de gradients environnementaux, en utilisant des outils statistiques récemment développés comme l’analyse Outlying Mean Index (OMI) et (3) de décrire les assemblages d’espèces qui se produisent dans des habitats similaires. La richesse spécifique est élevée (> 10), au printemps, dans les rivières de montagne (Atlas blidéen, Djurdjura) entre 300 et 600m d’altitude. La minéralisation des eaux et l’altitude sont les deux principaux paramètres qui affectent l’abondance spatio-temporelle des Éphéméroptères. À un degré moindre, les précipitations, la granulométrie des substrats, la pollution organique et l’ombrage expliquent également leur distribution. Cependant l’oxygène dissous, le pH et les facteurs hydrauliques (vitesse du courant, profondeur) semblent être secondaires. Baetis rhodani, Caenis luctuosa, Ecdyonurus rothschildi et Baetis pavidus sont les plus tolérantes vis-à-vis des variables considérées. La niche écologique la plus étroite appartient à Rhithrogena sp. (RTol = 0,39). Comme pour Habrophlebia gr. fusca et Alainites sp., elle est liée aux stations d’altitude. Cheleocloeon dimorphicum, Choroterpes (Ch.) atlas et Procloeon gr. bifidum occupent les eaux plus chaudes et Cloeon gr. simile semble tolérer les eaux à conductivité élevée. Enfin, Caenis pusilla, Acentrella sinaica et Ecdyonurus rothschildi, en dépit de leur faible marginalité, préfèreraient les sites ombragés, à substrat grossier et peu pollués.Relationships between the assemblage structure of mayflies (Insecta: Ephemeroptera) and habitat conditions in wadis were explored in 87 localities of the North Algeria with 17 environmental variables and presence ⁄ absence data on Ephemeroptera species. The objective of this work is to (1) identify the environmental variables driving the composition of mayfly assemblages at the regional scale; (2) define the ecological preferences of mayflies species collected along environmental gradients, using recently developed statistical tools, including Outlying Mean Index Analysis (OMI) and (3) describe the assemblages of species that occur in similar habitats. 27 species of Ephemeroptera were recorded, about half of the entire Algerian mayfly fauna. Species richness is high (SR > 10), in spring, at stations located in mountain rivers (Blidean Atlas, Djurdjura) between altitude 300 to 600m. Water mineralization and altitude are the most important parameters which affected the abundance of Algerian mayfly species (North Algeria). In less extent, precipitation, substrate size, organic pollution and riparian vegetation explain also this distribution. However dissolved oxygen, pH and hydraulic factors (current velocity, depth) seem to be secondary. Baetis rhodani, Caenis luctuosa, Ecdyonurus rothschildi and Baetis pavidus are the most tolerant species against the considered variables. Rhithrogena sp. (RTol = 0.39) is characterized by narrow habitat tolerance. As for Habrophlebia gr. fusca and Alainites sp., they are linked to the headwaters (altitude), Labiobaetis neglectus and Cloeon dipterum to heavily mineralized waters (Cl and Mg). Cheleocloeon dimorphicum, Choroterpes (Ch.) atlas and Procloeon gr. bifidum occur in warm waters and Cloeon gr. simile is able to tolerate salty waters (high conductivity). Finally, Caenis pusilla, Acentrella sinaica and Ecdyonurus rothschildi, despite their narrow marginality, prefer the covered sites with coarse substrate and tolerate light pollution

Topological surface states of strained Mercury-Telluride probed by ARPES

The topological surface states of strained HgTe have been measured using high-resolution ARPES measurements. The dispersion of surface states form a Dirac cone, which origin is close to the top of the \ghh band: the top half of the Dirac cone is inside the stress-gap while the bottom half lies within the heavy hole bands and keeps a linear dispersion all the way to the X-point. The circular dichroism of the photo-emitted electron intensity has also been measured for all the bands.Comment: with supplementary materia

ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides

We study with ARPES the renormalization and quasiparticle lifetimes of the $d_{xy}$ and $d_{xz}$/$d_{yz}$ orbitals in two iron pnictides, LiFeAs and Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ (Co8). We find that both quantities depend on orbital character rather than on the position on the Fermi Surface (for example hole or electron pocket). In LiFeAs, the renormalizations are larger for $d_{xy}$, while they are similar on both types of orbitals in Co8. The most salient feature, which proved robust against all the ARPES caveats we could think of, is that the lifetimes for $d_{xy}$ exhibit a markedly different behavior than those for $d_{xz}$/$d_{yz}$. They have smaller values near $E_F$ and exhibit larger $\omega$ and temperature dependences. While the behavior of $d_{xy}$ is compatible with a Fermi liquid description, it is not the case for $d_{xz}$/$d_{yz}$. This situation should have important consequences for the physics of iron pnictides, which have not been considered up to now. More generally, it raises interesting questions on how a Fermi liquid regime can be established in a multiband system with small effective bandwidths