Compte rendu: La forme de I’espace: polygones, polyèdres et polytopes

Dans ce livre, on entreprend une aventure graphique dans des mondes fantastiques à deux, trois ou quatre dimensions, en utilisant des polygones (ou 2-polytopes), des polyèdres (ou 3-polytopes) et des polytopes (ou 4-polytopes). Les sujets s’étendent sur plusieurs époques et plusieurs pays, mais une emphase particuliére est mise sur le passé du Japon. Les étoiles en sont Platon et les polygones, Kepler et les polyèdres, et Fuller et les polytopes.A graphic adventure in two-, three- and four-dimensional fantastic worlds is undertaken in this book, using polygons (or 2-polytopes), polyhedra (or 3-polytopes), and polytopes (or Cpolytopes). The topics extend over many epochs and countries, with particular emphasis on the past in Japan. The stars are Plato and polygons, Kepler and polyhedra, and Fuller and polytopes.Peer Reviewe

L’hexagone régulier quadridimensionnel

On comprend facilement que les analogues quadridimensionnels du triangle régulier, du carré et du pentagone régulier (dans cet article, ils sont tous composes uniquement d’arêtes et ne comportent aucun élément à deux dimensions) sont respectivement le tétraèdre régulier, le cube et le dodécaèdre régulier (dans cet article, ils sont tous composés uniquement de faces et ne comportent aucun élément à trois dimensions). Alors, quel polyèdre est l’analogue quadridimensionnel de I’hexagone régulier, c’est-à-dire un hexagone régulier quadridimensionnel? Si nous résolvons cette énigme, nous pourrons représenter un flocon de neige, un nid d’abeille, un crayon, etc. quadridimensionnels.It is easily understood that the Cdimensional analogues of the regular triangle, square, and regular pentagon (in this paper, all are composed of only edges and have no portion of 2-space) are the regular tetrahedron, cube, and regular dodecahedron (in this paper, all are composed of only faces and have no portion of 3-space) respectively. Then, which polyhedron is the Cdimensional analogue of the regular hexagon, i.e. Cdimensional regular hexagon? If this riddle is solved, we can see a 4-dimensional snowflake, honeycomb, pencil, etc.Peer Reviewe

High-resolution cryo-EM structure of photosystem II reveals damage from high-dose electron beams

Photosystem II (PSII) plays a key role in water-splitting and oxygen evolution. X-ray crystallography has revealed its atomic structure and some intermediate structures. However, these structures are in the crystalline state and its final state structure has not been solved. Here we analyzed the structure of PSII in solution at 1.95 Å resolution by single-particle cryo-electron microscopy (cryo-EM). The structure obtained is similar to the crystal structure, but a PsbY subunit was visible in the cryo-EM structure, indicating that it represents its physiological state more closely. Electron beam damage was observed at a high-dose in the regions that were easily affected by redox states, and reducing the beam dosage by reducing frames from 50 to 2 yielded a similar resolution but reduced the damage remarkably. This study will serve as a good indicator for determining damage-free cryo-EM structures of not only PSII but also all biological samples, especially redox-active metalloproteins

Thermoelectric properties of n-type nanocrystalline bismuth-telluride-based thin films deposited by flash evaporation

The thermal conductivity of n-type nanocrystalline bismuth-telluride-based thin films (Bi2.0Te2.7Se0.3) is investigated by a differential 3 method at room temperature. The nanocrystalline thin films are grown on a glass substrate by a flash evaporation method, followed by hydrogen annealing at 250 °C. The structure of the thin films is studied by means of atomic force microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy. The thin films exhibit an average grain size of 60 nm and a cross-plane thermal conductivity of 0.8 W/m K. The in-plane electrical conductivity and in-plane Seebeck coefficient are also investigated. Assuming that the in-plane thermal conductivity of the thin films is identical to that of the cross-plane direction, the in-plane figure of merit of the thin films is estimated to be ZT=0.7. As compared with a sintered bulk sample with average grain size of 30 µm and nearly the same composition as the thin films, the nanocrystalline thin films show approximately a 50% reduction in the thermal conductivity, but the electrical conductivity also falls 40%. The reduced thermal and electrical conductivities are attributed to increased carrier trapping and scattering in the nanocrystalline film

Structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin film

The structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin films are investigated for potential application in microthermoelectric devices. Nanocrystalline Si0.8Ge0.2 thin films are grown by low-pressure chemical vapor deposition on a sandwich of Si3N4/SiO2/Si3N4 films deposited on a Si (100) substrate. The Si0.8Ge0.2 film is doped with boron by ion implantation. The structure of the thin film is studied by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the film has column-shaped crystal grains ~100 nm in diameter oriented along the thickness of the film. The electrical conductivity and Seebeck coefficient are measured in the temperature range between 80–300 and 130–300 K, respectively. The thermal conductivity is measured at room temperature by a 3 method. As compared with bulk silicon-germanium and microcrystalline film alloys of nearly the same Si/Ge ratio and doping concentrations, the Si0.8Ge0.2 nanocrystalline film exhibits a twofold reduction in the thermal conductivitity, an enhancement in the Seebeck coefficient, and a reduction in the electrical conductivity. Enhanced heat carrier scattering due to the nanocrystalline structure of the films and a combined effect of boron segregation and carrier trapping at grain boundaries are believed to be responsible for the measured reductions in the thermal and electrical conductivities, respectively

Ab initio calculation for electronic structure and optical property of tungsten carbide in a TiCN-based cermet for solar thermal applications

We present an ab initio calculation to understand electronic structures and optical properties of a tungsten carbide WC being a major component of a TiCN-based cermet. We found that the WC has a fairly low-energy plasma excitation $\sim$0.6 eV (2 $\mu$m) and therefore can be a good constituent of a solar selective absorber. The evaluated figure of merit for photothermal conversion is prominently high compared to those of the other materials included in the TiCN-based cermet. The imaginary part of the dielectric function is considerably small around the zero point of the real part of the dielectric function, corresponding to the plasma excitation energy. Therefore, a clear plasma edge appeared, ensuring the high performance of the WC as the solar absorber.Comment: 13pages, 8 figures, 2table

Stepwise development of Hematopoietic stem Cells from Embryonic Stem Cells

The cellular ontogeny of hematopoietic stem cells (HSCs) remains poorly understood because their isolation from and their identification in early developing small embryos are difficult. We attempted to dissect early developmental stages of HSCs using an in vitro mouse embryonic stem cell (ESC) differentiation system combined with inducible HOXB4 expression. Here we report the identification of pre-HSCs and an embryonic type of HSCs (embryonic HSCs) as intermediate cells between ESCs and HSCs. Both pre-HSCs and embryonic HSCs were isolated by their c-Kit(+)CD41(+)CD45(−) phenotype. Pre-HSCs did not engraft in irradiated adult mice. After co-culture with OP9 stromal cells and conditional expression of HOXB4, pre-HSCs gave rise to embryonic HSCs capable of engraftment and long-term reconstitution in irradiated adult mice. Blast colony assays revealed that most hemangioblast activity was detected apart from the pre-HSC population, implying the early divergence of pre-HSCs from hemangioblasts. Gene expression profiling suggests that a particular set of transcripts closely associated with adult HSCs is involved in the transition of pre-HSC to embryonic HSCs. We propose an HSC developmental model in which pre-HSCs and embryonic HSCs sequentially give rise to adult types of HSCs in a stepwise manner