Carrier induced ferromagnetism in the insulating Mn doped III-V semiconductor InP

Although InP and GaAs have very similar band-structure their magnetic properties appear to drastically differ. Critical temperatures in (In,Mn)P are much smaller than that of (Ga,Mn)As and scale linearly with Mn concentration. This is in contrast to the square root behaviour found in (Ga,Mn)As. Moreover the magnetization curve exhibits an unconventional shape in (In,Mn)P contrasting with the conventional one of well annealed (Ga,Mn)As. By combining several theoretical approaches, the nature of ferromagnetism in Mn doped InP is investigated. It appears that the magnetic properties are essentially controlled by the position of the Mn acceptor level. Our calculations are in excellent agreement with recent measurements for both critical temperatures and magnetizations. The results are only consistent with a Fermi level lying in an impurity band, ruling out the possibility to understand the physical properties of Mn doped InP within the valence band scenario. The quantitative success found here reveals a predictive tool of choice that should open interesting pathways to address magnetic properties in other compoundsComment: 5 pages and 5 figures, accepted for publication in Phys. Rev.

High-pressure x-ray scattering and computer simulation studies of density-induced polyamorphism in silicon

A low- to high-density pressure-driven phase transition in amorphous silicon is investigated by synchrotron x-ray diffraction in the diamond anvil cell. Complementary atomistic molecular dynamics computer simulations provide insight into the underlying structural transformations and allow us to interpret the structure factors obtained from experiment. During compression the form of the scattering function S (Q) changes abruptly at 13.5 GPa, indicating significant structural rearrangement in the amorphous solid. In particular, the first peak in S (Q) shifts to larger Q values. The changes are correlated with the occurrence of a low- to high-density (LDA-HDA) polyamorphic transition observed previously using Raman scattering and electrical conductivity measurements. The data are analyzed to provide real space (pair distribution function) information. The experimental data are compared with results from molecular dynamics (MD) simulations using a modified Stillinger-Weber many-body potential energy function in order to extract structural information on the densified amorphous material. We deduce that the polyamorphic transition involves an abrupt increase in the proportion of 5- and 6-coordinate Si atoms. The overall structure of the HDA polyamorph can be related to that of the LDA form by creation of highly-coordinated "defects" within the tetrahedrally-bonded LDA network. However classical and quantum MD simulations indicate that an even higher density amorphous state might exist, based on structures that resemble the densely-packed metallic polymorphs of crystalline Si. © 2007 The American Physical Society

TEM observation and in situ compression tests of transition alumina prepared by high pressure compaction at room temperature

The behavior of ceramics at the nanometer scale strongly differs from the one of the corresponding bulk material. For instance, strong plastic deformation has recently been reported in isolated nanometer-sized alumina nanoparticles or MgO nanocubes, when tested in situ in a transmission electron microscope (TEM). This plastic behavior may also occur in a powder during the compaction process, even at room temperature. Controlling plastic deformation of nanoparticles during the ceramics processing might be a way to enhance their properties or to improve the processing route (compaction and sintering steps, for instance). We present here a comprehensive study of the mechanical behavior of transition alumina in the compacted powder. Please click Additional Files below to see the full abstract

The crystal structure of cold compressed graphite

Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp3 bonds and is the only carbon allotrope which provides an excellent match to unexplained features in experimental X-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band gap semiconductor with a hardness comparable to diamond.Comment: 4 pages, 5 figure

From mesoscale to nanoscale mechanics in single-wall carbon nanotubes

The experimental work was carried out in collaboration with W. Wenseleers and S. Cambré at the University of Antwerp, Belgium. The computational results presented have been achieved in part using the Vienna Scientific Cluster (VSC). DJD is grateful for support from the Region Rhône-Alpes through the programme “Accueil-PRO 2014” and from the iMUST Labex programme “Mobility in 2015”. ACTD, TFTC, WC, MALM, SB, DM and ASM acknowledge support from the French Agence Nationale de la Recherche through contract ANR-11-NANO-025 “TRI-CO”. ACTD acknowledges postdoctoral grant from Brazilian Ministry of Education (CAPES)

Wafer-scale detachable monocrystalline Germanium nanomembranes for the growth of III-V materials and substrate reuse

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronic, photovoltaic, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, limitation of waste, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable monocrystalline Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate monocrystalline Ge NMs with surface roughness below 1 nm on top of nanoengineered void layer enabling layer detachment. Furthermore, these Ge NMs exhibit compatibility with the growth of III-V materials. High-resolution transmission electron microscopy (HRTEM) characterization shows Ge NMs crystallinity and high-resolution X-ray diffraction (HRXRD) reciprocal space mapping endorses high-quality GaAs layers. Finally, we demonstrate the chemical reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge during the fabrication process which paves the way for a new generation of low-cost flexible optoelectronics devices.Comment: 17 pages and 6 figures along with 3 figures in supporting informatio

Design of sensors for monitoring the environment and ecosystems: from the definition of specifications to production

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Etudes expérimentales et théoriques des mécanismes d'amorphisation sous pression

La transition cristal-amorphe induite par l'application de haute pression concerne de nombreux matériaux dont des composés majeurs tels que le quartz ou la glace. Le présent travail a consisté à étudier trois matériaux répertoriés dans la littérature comme pouvant présenter une amorphisation sous pression : LiKSO4, Eu2(MoO4)3 et Cs2HgBr4. L'amorphisation du composé observée vers 13 GPa dans la littérature n'a pas été confirmée ici, même dans des conditions non-hydrostatiques. Trois transitions de phases cristal-cristal ont été observées sous pression pour LiKSO4 entre 0 et 32 GPa. Les transformations structurales sous pression du molybdate d'europium Eu2(MoO4)3 ont été étudiées par diffraction de rayons X, spectroscopie Raman et fluorescence. L'amorphisation se déroule en deux étapes : la mise en désordre du réseau des oxygènes est achevée vers 8-9 GPa, celle du réseau des cations métalliques a lieu vers 18 GPa. L'observation d'une nouvelle phase cristalline, issue de la compression de la phase amorphe, vers 28 GPa indique que l'état amorphe correspond à un intermédiaire cinétique entre deux phases cristallines. L'amorphisation de Cs2HgBr4 entre 10 et 15 GPa n'est observée que dans des conditions de compression non-hydrostatiques. L'évolution des diffractogrammes sous pression uniaxiale a pu être simulée en supposant des déformations aléatoires et non-homogènes. L'ensemble de nos observations sur Cs2HgBr4 nous a amené à développer un nouveau mécanisme d'amorphisation : la notion de verre ferroélastique, analogue mécanique des verres ferroelectriques. Ce modèle permet une interprétation satisfaisante des faits expérimentaux concernant le quartz et la glace.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Polarized Raman spectra of Gd₂(MoO₄)₃ in its orthorhombic structure

International audiencePolarized Raman spectra of a single crystal of gadolinium molybdate, Gd2(MoO4)3 were obtained between 400 and 80 K. Tentative assignments of high-wavenumber bands of symmetric and antisymmetric MoO4 stretching modes to the three types of tetrahedra present in the ferroelectric phase described by the C2v8 space group are made on the oriented gas model. A careful analysis of the low-wavenumber spectra allowed identification of new bands compared to those in the literature. The evolution of wavenumbers in the investigated temperature range indicates that a small structural change takes place at about 200 K. Some modes soften when approaching the ferroelectric-paraelectric transition at 159 °C