Pages scientifiques 2014

Quantitative ion beam analysis of M–C–O systems: application to an oxidized uranium carbide sample
G. Martin, G. Raveu, P. Garcia, G. Carlot, H. Khodja, I. Vickridge, M.F. Barthe & T. Sauvage, Philosophical Magazine Volume 94, Issue 11, 2014.

Les carbures mixtes uranium-plutonium sont explorés comme combustibles potentiels pour les réacteurs de génération IV, et ce en raison de leur forte densité d’actinides, leur haute température de fusion et leur excellente conductivité thermique. La synthèse de tels matériaux est néanmoins délicate du fait de leur forte réactivité à l’air, qui se traduit par une prise d’oxygène.

Le cycle géologique des éléments volatils est régi par des transferts entre les réservoirs de la surface (ex. atmosphère, océans) et les réservoirs profonds (ex. manteau terrestre). Ces transferts s’opèrent via le volcanisme et via le recyclage de la croûte terrestre dans les zones de subduction.

Ainsi il est important de déterminer la capacité d’insertion d’un élément tel le fluor dans des minéraux hôtes présents dans le manteau. Dans le cadre d’une collaboration entre l’UPMC, le CEA, l’université d’Edinburgh et l’université of Western Ontario, l’étude de la co-incorporation du fluor et de l’eau dans la forstérite, minéral représentatif du manteau supérieur (jusqu’à 400 km de profondeur environ) a été entreprise. Ces minéraux ont été synthétisés dans des presses piston cylindre afin de reproduire les conditions de pression et de température du manteau supérieur.

En combinant deux techniques analytiques disponibles à la microsonde nucléaire du LEEL, il a été possible de doser l’eau et le fluor, éléments difficiles à mesurer dans ce type d’échantillons. Le fluor est quantifié par la réaction nucléaire ¹⁹F(p,p’γ)¹⁹F, l’eau (via l’hydrogène) par la technique ERDA ¹H(α,p)⁴He.

Les résultats ont montré qu’il existe bien un lien entre l’incorporation du fluor et celle de l’eau. Le mécanisme d’incorporation du fluor, qui fait intervenir des défauts mixtes hydroxyle-fluorure, a par ailleurs pu être élucidé en combinant modélisation moléculaire ab initio et observations par spectroscopie infrarouge. Ces résultats suggèrent que les minéraux majeurs du manteau terrestre peuvent accommoder la totalité du fluor transitant par ce réservoir et que les cycles de l’eau et du fluor sont liés.

Clumped fluoride-hydroxyl defects in forsterite: Implications for the upper-mantle

C. Crépisson, M. Blanchard, H. Bureau, C. Sanloup, A. C. Withers, H. Khodja, S. Surblé, C. Raepsaet, K. Béneut, C. Leroy, P. Giura, E. Balan, Earth and Planetary Science Letters 390 (2014) 287.

Understanding the formation of nanoparticles in water at room temperature is a major challenge, both to tackle the mechanisms of mineral growth in biological systems, and to synthesise efficiently nanomaterials. However, the growth mechanisms usually invoked sometimes fail dramatically to describe reality, especially the final size, the crystalline quality and rate of production of the nanocrystals. Progress in this area are slow because of the great difficulty in observing the intermediate steps between the solution before reaction, and the construction of the first nanocrystal. A group of researchers from the IRAMIS/NIMBE, the Laboratory of Condensed Matter Physics at the École Polytechnique and the Université Pierre-et-Marie-Curie has managed this feat by using in particular the SOLEIL synchrotron radiation. Their observations allowed to propose a formation scenario which is unusual, but consistent with reality, and probably very generic. These results have been recently published in ACS Nano.

Since the beginning of the years 2000, more and more examples of functional amyloid fibers have been discovered in bacteria and fungi as well as in the mammals’ realm[1-2]. Very recently, a study on about forty peptide hormones has demonstrated that those were stored inside pituitary secretory granules under the form of amyloid fibers[3].

Beyond the structural biology question, we are especially interested in these molecules because we think they might be appropriate models to tackle broader issues like biomimetic self-assembly and amyloid formation – amyloid understood as “beta-sheet rich assembly”.

Indeed, peptides are generally small and easily synthesizable molecules that, if our hypothesis is right, will form reversible amyloid-like assemblies whose structure and assembly mechanism will be carefully studied.

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Electric field switching of Rashba splitting using ferroelectric polarization

The ambition of our work is to control spin-orbit coupling in an emerging class of spintronics materials, such as topological insulators and their parent compounds, deposited on thin films ferroelectric (FE) substrate (Figs. 1, 2 and 3). One of the most interesting and useful features of FE materials is the strong coupling of the polarization with elastic strain that can be switched by an external electric field. The remnant polarization in a FE should provide a robust and reproducible control of the Rashba-type electron band spin splitting in a 2D system and therefore potentially of spin transport properties.

Experimental and theoretical studies within the project will be closely matched to obtain valuable information on spin polarisation and contribute to providing a framework for new type of applications in spintronics.

Topological Insulators

Topological insulators (TI) are new states of quantum matter in which surface states residing in the bulk insulating gap of such systems are protected by time-reversal symmetry ([HAS10], [Qi11]). The electronic band structure of these materials includes characteristic conical features at high-symmetry points within the Brillouin zone. Electrons in the vicinity of a cone exhibit linear dispersion and are expected to behave like massless Dirac fermions confined within a two-dimensional plane (Fig. 4).

The transport properties of a topological insulator are closely related to the two-dimensional (2D) integer quantum Hall effect, which also has unique edge states. These electronic states are insensitive to disorder because there are no states available for backscattering. They were originally predicted to exist in graphene [Kan05, Ber06]. Graphene is made out of carbon, which is a light element with a weak spin-orbit interaction. Clearly, a better place to look for this physics would be in materials with strong spin-orbit interactions, made from heavy elements near the bottom of the Periodic Table.

Figure 4: Spin-momentum locked helical surface Dirac fermions are hallmark signatures of topological insulators. a) ARPES data for Bi₂Se₃ reveal surface electronic states with a single spin-polarized Dirac cone. b) The surface Fermi surface exhibits a chiral left-handed spin texture. c) Surface electronic structure of Bi₂Se₃ computed in the local-density approximation. The shaded regions describe bulk states, and the lines are surface states. d) Schematic of the spin-polarized surface-state dispersion in Bi₂X₃ (1;000) topological insulators.
From: [Has10]

Topological Insulators: Thin films

Whereas the surfaces of bulk topological insulators provide a promising basis for spintronics application, thin films are more relevant for actual device architecture. Effort has been made to reduce the thickness of known bulk topological insulators. The choice of available materials is thus substantially broadened. However, the usable film thickness has a natural lower limit, below which the two film boundaries may be coupled by quantum tunnelling, so that a small, thickness-dependent gap opens up at the Dirac point, cutting off the spin conduction.

Thin film parent compound of topological insulators

The requirement of fine-tuning the thickness makes the experimental fabrication of 2D topological insulators rather difficult. Therefore, it is desirable to search for new materials or new schemes to obtain a 2D topological insulator. The Bi(111) film on a suitable substrate actually represents a special interesting class of films having an intermediate inter-bilayer (BL) coupling strength that may have a significant influence on the topological property [Liu11]. It should be noted that the substrate may have additional effects on the electronic structure and hence the topological properties of Bi films, which are of central interest for these studies.

Figure 5: ARPES spectra of: a) (111) face of a Bi monocrystal (12 ML) prepared on a InAs(111)-A face; b) (111) face of a Sb monocrystal prepared on a Bi(111) buffer layer shown in a).

We have extensive experience with thin layer growth. Bi growth is epitaxial and a Bi monocrystal of very high quality is obtained after depositing several monolayers (Fig. 5a). The band dispersion fits well with literature data on Bi single crystals [Hof06, Hir06]. On top of Bi(111) we were able to prepare high quality Sb(111) monocrystals (Fig. 5b). The Sb(111) and the Bi(111) surfaces have a hexagonal structure; their lattice cons­tants are 4.30 and 4.54 Å, respectively; so the lattice mismatch is ~5% for Sb grown on a Bi buffer layer.

Theoretical description

This part of the work will be done in close collaboration with our colleagues from Ludwigs-Maximilians-Universität, Munich, Germany. The interpretation of the spin dependent band structure, as observed in ARPES, will be supported by ab-initio Korringa-Kohn-Rostoker (KKR) electronic structure calculations which account for all aspects of the investigated TIs: spin-orbit coupling without approximations by a fully relativistic approach, interdiffusion and disorder by means of the coherent potential approximation (CPA) and correlations by the dynamical mean field theory (DMFT). For this the layer dependent spin texture, as well as the spin and orbital resolved Bloch-spectral function will be determined. The measured ARPES spectra will be complemented by calculations on the basis of the one-step model of photoemission that accounts for all relevant aspects like matrix elements and final state effects, in a state-of-the art way. [Min11b, Min11, Min13].