Papers by stéphane pailhès
The work presented in this thesis aims at achieving an increased understanding of the electronic ... more The work presented in this thesis aims at achieving an increased understanding of the electronic structure of materials whose properties are to a large extent governed by transition metals. All studies are based on data from angle-resolved photoelectron spectroscopy (ARPES) employed on the valence band of the studied systems. In some cases spectroscopic data have also been combined with numerical electronic structure studies performed using a density functional theory computer code. Five of the studies make extensive use of the benefits provided by performing angle-resolved photoelectron spectroscopy at high photon energies. When probed with soft x-ray photons, the near Fermi edge distribution of spectral weight in Nd2-xCexCuO4 and La2-x-yNdySrxCuO4 display significant differences as compared to similar studies performed in the 20-100 eV range of photon energies. This effect can be attributed to the approximate two-fold increase in probing depth as compared to the low photon energy ...
SSRN Electronic Journal
Abstract Competition between point defect (vacancy and interstitial) microscopic configurations i... more Abstract Competition between point defect (vacancy and interstitial) microscopic configurations is inherent to crystalline phases of increased structural complexity. Phase transitions that preserve symmetry between them belong to a specific class of isostructural transitions. Type-I silicon clathrates are complex crystalline phases whose unit cell containing fifty atoms consists of a 3D covalent silicon framework of face-sharing polyhedral cages encapsulating guest cations. At high pressure, an intriguing structural transition is associated with an abrupt reduction of volume with no indication for any breakage of symmetry. Using isothermal high-pressure X-ray diffraction performed on a single crystal of the simplest representative type-I silicon clathrates, binary Ba8Si46, we confirm the isostructural character of the transition and identify the associated mechanism. A detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow us to pinpoint a microscopic mechanism driven by a rearrangement of point defects initially present in the structure. An analysis based on the Landau theory gives a coherent description of the experimental observations. A discussion on the analogy between this transformation and liquid-liquid transitions is proposed.
Ceramics
Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route... more Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size.
Physical Chemistry Chemical Physics
Thermoelectric properties of bulk nanostructured Nb-doped rutile TiO2.
Physical Review B
The low lattice thermal conductivity in inorganic clathrates has been shown recently to be relate... more The 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, Ba 24 Si 100 , has been investigated experimentally by means of inelastic neutron scattering as a function of temperature between 5 and 280 K, and computed by ab initio density functional techniques. It is compared with the lattice dynamics of Ba 8 Si 46 , the simplest representative of the well-known type-I clathrate structure. The binary Ba 8 Si 46 and Ba 24 Si 100 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 the size and shape of the cages. The lowest optical phonon energies in Ba 24 Si 100 are found around 2.5-4 meV, while those in Ba 8 Si 46 have higher energies around 7-9 meV. The low-lying optical phonons in Ba 24 Si 100 are mainly weighted by the motion of Ba in the opened Si 20 cage, which does not exist in Ba 8 Si 46. Moreover, the Ba vibrations within the opened Si 20 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 Ba 8 Si 46 .
Physical Review B
Despite their crystalline nature, thermoelectric clathrates exhibit a strongly reduced lattice th... more Despite their crystalline nature, thermoelectric clathrates exhibit a strongly reduced lattice thermal conductivity. While the reason for this unexpected behavior is known to lie in the peculiarities of the complex crystal structure and the interplay of the underlying guest-host framework, their respective roles are still not fully disentangled and understood. Our ab initio study of the most simple type-I clathrate phase, the binary compound Ba 8 Si 46 and its derivatives Ba 8−x Si 46 seeks to identify these mechanisms and provides insight into their origin. Indeed, the strongly decreased lattice thermal conductivity in thermoelectric clathrates is a consequence of a reduction of the acoustic phonon bandwidth, a lowering of the acoustic phonon group velocities, and the amplification of three-phonon-scattering processes. While the complexity of the crystal structure is demonstrated not to be the leading factor, the reasons are manifold. A modified Si-Si interaction causes a first decrease of the sound velocity, whereas the presence of flat Ba modes results in an additional lowering. These modes correspond to confined Bloch states that are localized on the Ba atoms and significantly increase the scattering phase space and, together with an increased anharmonicity of the interatomic interactions, strongly affect the phonon lifetimes.
Science and Technology of Advanced Materials
High-quality thermoelectric La x Sr 1-x TiO 3 (LSTO) layers (here with x = 0.2), with thicknesses... more High-quality thermoelectric La x Sr 1-x TiO 3 (LSTO) layers (here with x = 0.2), with thicknesses ranging from 20 nm to 0.7 m, have been epitaxially grown on SrTiO 3 (001) substrates by enhanced solid-source oxide molecularbeam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 x 10-4 cm at room temperature), one order of magnitude lower than commercial Nb-doped SrTiO 3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of around-60 V/K have been found for all films, accordingly. Finally, a correlation is given between the mosaicity and the (thermo)electric properties. These functional LSTO films can be integrated on Si in opto-microelectronic devices as transparent conductor, thermoelectric elements or in non-volatile memory structures.
Nature communications, Sep 8, 2017
Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration... more Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities. Here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate Ba7.81Ge40.67Au5.33, renowned for its puzzling 'glass-like' thermal conductivity. Surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 Å(-1). Considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one. Our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimenta...
Materials Letters, 2016
Abstract Stability studies by ab-initio calculations for the best-known binary Ba-Si clathrates r... more Abstract Stability studies by ab-initio calculations for the best-known binary Ba-Si clathrates reveal that Ba 24 Si 100 is the most stable composition and that the latter can be stabilized in a relatively small energy range, of the order of a few kJ/mol.at. This suggested alternative synthesis routes, easier and cheaper to the commonly used high pressure-high temperature processes. Here, for the first time, we confirmed the theoretical prediction and were successful in the synthesis of the phase Ba 24 Si 100 by mechanical alloying. The influence of the milling parameters is discussed in order to get a crystalline phase with 90% purity.
Chemistry of Materials, 2016
Angle-resolved photoemission measurements on the electron-doped cuprate Sm1.85Ce0.15CuO4 evidence... more Angle-resolved photoemission measurements on the electron-doped cuprate Sm1.85Ce0.15CuO4 evidence anisotropic dressing of charge-carriers due to many-body interactions. Most significantly, the scattering rate along the zone boundary saturates for binding energies larger than ∼ 200 meV, while along the diagonal direction it increases nearly linearly with the binding energy in the energy range ∼ 150 − 500 meV. These results indicate that many-body interactions along the diagonal direction are strong down to the bottom of the band, while along the zone-bounday they become very weak at energies above ∼ 200 meV.
Applied Physics Letters, 2015
An alternative approach for high-speed temperature measurement with micrometer-scale spatial reso... more An alternative approach for high-speed temperature measurement with micrometer-scale spatial resolution based on the luminescence of Ni-doped diamond micro-particles is described. Under picosecond pulsed laser excitation, these particles exhibit intense green luminescence; the lifetime of this luminescence decreases by several orders of magnitude upon heating from 120 K up to 900 K. The intensity of the luminescence remains constant over this temperature range. Real-time measurements were performed with repetition rates of up to 100 Hz with a temperature resolution of better than 1 • C.
JDN 16 – Diffusion Inélastique des Neutrons pour l'Etude des Excitations dans la Matiére Condensée, 2010
La supraconductivité non conventionnelle est certainement l'un des phénomènes les plus remarquabl... more La supraconductivité non conventionnelle est certainement l'un des phénomènes les plus remarquables associés à la physique des systèmes dits à fortes corrélations électroniques. Dans ces systèmes, les degrés de liberté du spin et de la charge de l'électron sont très fortement couplés, de sorte que le déplacement des charges est contraint par l'environnement magnétique formé par leurs spins. La diffusion inélastique des neutrons est l'unique sonde expérimentale permettant de photographier l'état magnétique de ces systèmes dans tout l'espace réciproque et de suivre son évolution sur une échelle d'énergie parfaitement adaptée à celle de la supraconductivité. A travers l'exemple des cuprates supraconducteurs à haute température critique, nous montrons comment la diffusion inélastique des neutrons apporte des informations uniques permettant de comprendre l'organisation électronique de ces systémes à l'échelle microscopique et les mécanismes mise en jeu dans l'apparition de la supraconductivté. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.
Phys. Chem. Chem. Phys., 2015
Inelastic neutron scattering performed on powders and single crystals indicate that the glass-lik... more Inelastic neutron scattering performed on powders and single crystals indicate that the glass-like thermal conductivity of tetrahedrite minerals originates from a strongly anharmonic low-energy vibrational mode related to the peculiar chemical environment of the Cu2 atoms.
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Papers by stéphane pailhès