Enhancing the superconducting transition temperature of BaSi2 by structural tuning

We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi2. Compared with the layered AlB2 structure of graphite or diboride-like superconductors, in the hexagonal structure of binary silicides the sp3 arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, obtaining the enhancement of the superconducting transition temperature from 4 K to 8.7 K when the silicon planes flatten out. By performing ab-initio calculations based on density functional theory we explain how the electronic and phononic properties of the system are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters.Comment: Submitte

Giant tuning of electronic and thermoelectric properties by epitaxial strain in p-type Sr-doped LaCrO3 transparent thin films

The impact of epitaxial strain on the structural, electronic, and thermoelectric properties of p-type transparent Sr-doped LaCrO3 thin films has been investigated. For this purpose, high-quality fully-strained La0.75Sr0.25CrO3 (LSCO) epitaxial thin films were grown by molecular beam epitaxy on three different (pseudo)cubic (001)-oriented perovskite-oxide substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and DyScO3. The lattice mismatch between the LSCO films and the substrates induces in-plane strain ranging from -2.06% (compressive) to +1.75% (tensile). The electric conductivity can be controlled over two orders of magnitude, σ ranging from ~0.5 S cm-1 (tensile strain) to 35 S cm-1 (compressive strain). Consistently, the Seebeck coefficient S can be finely tuned by a factor of almost two from ~127 μV K-1 (compressive strain) to 208 μV K-1 (tensile strain). Interestingly, we show that the thermoelectric power factor (PF = S2 σ) can consequently be tuned by almost two orders of magnitude. The compressive strain yields a remarkable enhancement by a factor of three for 2% compressive strain with respect to almost relaxed films. These results demonstrate that epitaxial strain is a powerful lever to control the electric properties of LSCO and enhance its thermoelectric properties, which is of high interest for various devices and key applications such as thermal energy harvesters, coolers, transparent conductors, photo-catalyzers and spintronic memories.Financial support from the European Commission through the project TIPS (H2020-ICT-02-2014-1-644453), the French national research agency (ANR) through the projects MITO (ANR-17-CE05-0018), LILIT (ANR-16-CE24-0022), DIAMWAFEL (ANR-15-CE08-0034-02), the CNRS through the MITI interdisciplinary programs (project NOTE), IDEX Lyon-St-Etienne through the project IPPON, the Spanish Ministerio de Ciencia e Innovación, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV2015-0496) and the MAT2017-85232-R (AEI/FEDER, EU), PID2019-107727RB-I00 (AEI/FEDER, EU), and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. The China Scholarship Council (CSC) is acknowledged for the grant of Dong Han. Ignasi Fina acknowledges Ramón y Cajal contract RYC-2017-22531. Seebeck measurements at ILM were made within the ILMTech transport platform. The authors are also grateful to Jean-Baptiste Goure, Philippe Regreny, Aziz Benamrouche, and Bernat Bozzo for their technical support and the reviewers for their valuable and constructive comments that have improved the quality of the manuscript.Peer reviewe

Développement d'une cellule haute pression haute température dans la presse Paris-Edimbourg pour la mesure de propriétés élastiques et de densité : Application sur les oxydes de fer.

The aim of the present work was to develop a new experimental apparatus which allows the measurement of elastic properties by ultrasonic wave propagation under extreme conditions in the Paris-Edinburgh press. A numerical modelling of the sample environment has been realised by finite element method and leads to an appropriate conception with minimum thermal gradients. The measurements of the ultrasonic velocity on magnetite with the new dedicated apparatus lead us to the determination of the three elastic moduli and of the density under high pressure (6 GPa) and temperature (100°C). Similar measurement on wüstite for the elastic modulus C11 has been realised (6 GPa; 200°C). The abnormal behaviour under pressure (12 GPa) of the shear moduli of wüstite C44 and C' has been measured by neutron scattering and interpreted as pre-transitional phenomena of the para-antiferromagnetical transitionL'objectif de ce travail de thèse a été de mettre en place un dispositif expérimental permettant la mesure des propriétés élastiques sous conditions extrêmes par propagation d'onde ultrasonore dans la presse Paris Edimbourg. La modélisation numérique du volume expérimental par la méthode des éléments finis a conduit à une conception appropriée assurant une température homogène. Les mesures de vitesse ultrasonore effectuées sur la magnétite avec la cellule optimisée ont permis la détermination des modules élastiques et de la densité sous haute pression (6 GPa) et température (100°C). Pour la wüstite sont présentées des mesures ultrasonores sous pression (6 GPa) et température (200°C) du module de compression C11. Le comportement anormal en pression (12 GPa) des constantes élastiques C44 et C' de la wüstite mesuré par diffusion inélastique de neutrons est interprété comme un phénomène pré-transitionnel à la transition para-antiferromagnétique

Développement d'une cellule haute pression haute température dans la presse Paris-Edimbourg pour la mesure de propriétés élastiques et de densité (application sur les oxydes de fer)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

HYPERBAR, Applicabilité de la technologie Haute Pression en Biotechnologie

International audienceLa technologie des hautes pressions isostatiques est bien connue en agroalimentaire, car elle permet de réduire la charge bactérienne et virale de produits alimentaires. Pourtant en 2006, elle était encore complètement inconnue du monde pharmaceutique où cette technologie constituerait un facteur clé de compétitivité pour la production de vaccins. C’est pourquoi le groupe Mérial a fait appel à l’expertise technologique et scientifique du réseau de technologie des hautes pressions du CNRS. Labellisé par Lyonbiopôle fin 2006, le projet FUI « HYPERBAR », porté par Mérial, le Laboratoire Unité de Biologie Structurale des Interactions entre Virus et cellule Hôte de l’Université de Grenoble, le réseau des Hautes Pressions du CNRS et Top Industrie, le projet HYPERBAR se proposait d’évaluer la faisabilité d’inactivation de bactéries et de virus d’intérêt vaccinal et d’évaluer également la solubilisation et le repliement de protéines recombinantes à destinée pharmaceutique. Dans le même temps, la faisabilité de réalisation d’un équipement haute pression de qualité pharmaceutique était étudiée

HYPERBAR, Applicabilité de la technologie Haute Pression en Biotechnologie

International audienceLa technologie des hautes pressions isostatiques est bien connue en agroalimentaire, car elle permet de réduire la charge bactérienne et virale de produits alimentaires. Pourtant en 2006, elle était encore complètement inconnue du monde pharmaceutique où cette technologie constituerait un facteur clé de compétitivité pour la production de vaccins. C’est pourquoi le groupe Mérial a fait appel à l’expertise technologique et scientifique du réseau de technologie des hautes pressions du CNRS. Labellisé par Lyonbiopôle fin 2006, le projet FUI « HYPERBAR », porté par Mérial, le Laboratoire Unité de Biologie Structurale des Interactions entre Virus et cellule Hôte de l’Université de Grenoble, le réseau des Hautes Pressions du CNRS et Top Industrie, le projet HYPERBAR se proposait d’évaluer la faisabilité d’inactivation de bactéries et de virus d’intérêt vaccinal et d’évaluer également la solubilisation et le repliement de protéines recombinantes à destinée pharmaceutique. Dans le même temps, la faisabilité de réalisation d’un équipement haute pression de qualité pharmaceutique était étudiée

Anisotropic low-energy vibrational modes as an effect of cage geometry in the binary barium silicon clathrate Ba24Si100

International audienceThe low lattice thermal conductivity in inorganic clathrates has been shown recently to be related to the low-energy range of optical phonons dominated by motions of guest atoms trapped in a network of host covalent cages. A promising route to further reduce the heat conduction, and increase the material efficiency for thermoelectric heat waste conversion, is then to lower the energy of these guest-weighted optical phonons. In the present work, the effect of the host cage geometry is explored. The lattice dynamics of the binary type-IX clathrate, Ba24Si100, has been investigated experimentally by means of inelastic neutron scattering as a function of temperature between 5 K and 280 K, and computed by ab-initio density functional techniques. It is compared with the lattice dynamics of Ba8Si46, the simplest representative of the well-known type-I clathrate structure. The binary Ba8Si46 and Ba24Si100 materials have both a cubic unit cell made of different Si cages. The energies, the degree of anharmonicity as well as the anisotropy of the optical phonon modes weighted by Ba motions are found to depend strongly on 2 the size and shape of the cages. The lowest optical phonon energies in Ba24Si100 are found around 2.5-4 meV, while those in Ba8Si46 have higher energies around 7-9 meV. The low-lying optical phonons in Ba24Si100 are mainly weighted by the motion of Ba in the opened Si20 cage, which doesn't exist in Ba8Si46. Moreover, the Ba vibrations within the opened Si20 cages are found intrinsically anisotropic, strongly dispersionless in some directions and exhibit a significant anharmonicity, which is not observed for any optical phonon modes in Ba8Si46

Shear wave anisotropy in textured phase D and constraints on deep water recycling in subduction zones

International audienceRegions of low seismic velocity and high shear anisotropies in cold subducted slabs have often been related to anisotropic fabrics in hydrous phases mainly induced by slab deformation. The interpretation of these seismic anomalies in terms of hydration thus relies on a better knowledge of the elasticity and plastic deformation mechanisms of candidate hydrous phases. Here we investigate the development of lattice preferred orientations (LPO) in phase D [MgSi2H2O6, 10-18 wt% H2O], the ultimate water carrier in hydrous subducted peridotite. The samples were deformed non-hydrostatically up to 48 GPa in a diamond anvil cell and the texture and strength were obtained from analysis of the X-ray diffraction patterns collected in radial diffraction geometry. We find that at low strains the layered structure of phase D displays strong 0001 texture, where the stacking fault axis (c-axis) preferentially align parallel to the compression axis. A subsidiary 1010 texture develops at higher strains. Plasticity simulations in polycrystalline aggregates using a viscoplastic self-consistent model suggest that these LPO patterns are consistent with shape preferred orientation mechanism during the first compaction steps and, with dominant easy glide on basal planes and harder first order pyramidal slip, respectively, upon further compression. We find that phase D displays the lowest strength and the highest anisotropy among phases in hydrous peridotite in the uppermost lower mantle and might thus control the shear wave anisotropy generated in subducted slabs below the transition zone. We further evaluate the effect of textured phase D on the seismic velocity structure and shear wave anisotropy of deformed hydrous peridotite and compare the results to seismic observations in Tonga subduction. We show that 16 vol% of phase D in hydrous subducted peridotite is required to explain the negative velocity anomalies of 3%, the extent of shear wave splitting (0.9 +/- 0.3%) and the shear wave ray polarization geometry (V-SH > V-SV) observed in a detached fragment of the Tonga slab laying sub-horizontally below the transition zone. Seismic observations may thus place constraints on the degree of hydration of the Tonga slab beyond the transition zone, leading to the requirement that a minimum of 1.2 wt% H2O is retained in the slab by hydrous peridotite below 670 km depth

Mechanical alloying as a new synthesis route for metastable silicon clathrates

International audienc