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On the combination of thermal diffuse scattering, inelastic x-ray scattering and ab initio lattice dynamics calculations
No full textLes méthodes classiques dans l'étude de la dynamique de réseau, comme la diffusion inélastique des neutrons et des rayons X, sont et vont rester limitées en flux. En conséquence les mesures sont coûteuse en temps. Pour optimiser le rendement de ces techniques, la stratégie de mesure doit être préparée avant l'expérience. Cette stratégie peut être élaborée et optimisé par des calculs de la dynamique de réseau et de la diffusion diffuse. La mesure de la diffusion diffuse thermique est une expérience simple où les régions étendues dans l'espace réciproque peuvent être explorées rapidement et en détails, ce qui permet d'identifier les caractéristiques dans la dynamique de réseau. Une méthode spectroscopique peut être appliquée ensuite sur les régions d'intérêt donnant accès à l'énergie et à l'intensité des vibrations individuelles. Dans certains cas, la diffusion diffuse thermique devient tellement contraignante pour les calculs de la dynamique de réseau (quasi)harmonique, que l'expérience de diffusion inélastique n'est plus un ingrédient nécessaire dans la reconstruction d'une image cohérente de la dynamique. Dans le cadre de ce travail la combinaison des techniques utilisant la diffusion thermique, la diffusion inélastique des rayons X et les calculs réalisés à partir des premiers principes ab initio est proposée pour l'étude de la dynamique de réseau de monocristaux. Les intensités de diffusion diffuse ainsi que les spectres inélastiques observés sont comparés à ceux calculés ab initio. Ces techniques combinées donnent accès à la description complète de la dynamique de réseau en approximation harmonique, et fournissent des informations supplémentaires précieuses. Le lecteur sera initié au formalisme de la dynamique de réseau et à celui de la diffusion inélastique et thermique. Les méthodes de calculs des propriétés vibrationnelles issues des calculs ab initio vont être introduites suivant un ensemble d'étapes menant à la convergence et donc à la validation de l'ensemble des calculs. Les techniques expérimentales utilisées tout au long de cette étude ainsi que les nouvelles possibilités s'ouvrant désormais grâce aux études combinées, serons présentées. La méthodologie sera illustrée par plusieurs systèmes de référence. Dans le cadre des systèmes à liaisons covalentes, deux polymorphes de silice - coésite et cristobalite - sont à l'étude. Les calculs expérimentaux validés sont utilisés pour l'étude des vecteurs propres, des valeurs propres et de leur contributions a la densité d'états vibrationnels (partiel et total). La comparaison avec le polymorphe de silice le plus abondant - alpha-quartz - et l'oxide de germanium en structure de alpha-quartz - révèle des ressemblances et des différences distinctes dans les propriétés vibrationnelles à basse énergie. Les polymorphes d'étain ont été choisis pour étudier l'impact du sous-système électronique sur les interactions inter-ioniques et la dynamique de réseau de monocristaux. L'étain manifeste des propriétés structurelles intéressantes, et une surface de Fermi relativement complexe. On observe une asymétrie inhabituelle au niveau de la diffusion diffuse, propriété qui peut s'expliquer dans le cadre d'une approximation harmonique de la dynamique de réseau. Enfin, la méthode élaborée est appliquée à de la glace, ce qui démontre non seulement une diffusion thermique caractéristique mais également des contributions statiques provenant du désordre de l'hydrogène. La méthodologie proposée fournit un outil puissant pour l'étude de la dynamique de réseau et sera applicable à une large variété de systèmes. Les études peuvent être étendues à des conditions extrêmes impliquant de très hautes pressions et une large gamme de températures. Cette méthodologie peut également être utilisée pour étudier les propriétés localisées de vibrations atomiques dans les systèmes avec des symétries brisées, par exemple des systèmes avec du désordre ou des effets topologiques.The classical methods in the study of lattice dynamics, such as inelastic neutron and x-ray scattering, are and will remain flux-limited, consequently the measurements are time consuming. To maximise the yield of these techniques, measurement strategies need to be established prior to the experiment. These strategies can be elaborated and optimised by lattice dynamics calculations and thermal diffuse scattering. Measuring thermal diffuse scattering is a simple experiment where extended regions of reciprocal space can be rapidly explored in detail and characteristic features of the lattice dynamics identified. Slower spectroscopy measurements can then be applied on the selected regions of interest to gain access to the energy and intensity of individual vibrations. Moreover, in some cases the input of thermal diffuse scattering may become so constraining for the (quasi)harmonic lattice dynamic calculation, that inelastic scattering experiment will not be a necessary ingredient for the recovery of a self-consistent picture of the dynamics. In the frame of this work, the combination of thermal diffuse scattering, inelastic x-ray scattering and lattice dynamics calculations from first principles ab initio is applied to study the lattice dynamics of single crystals. Both diffuse scattering intensities and inelastic spectra determined by experiment are compared to the ones calculated ab initio. The combination of these three techniques gives access to the full lattice dynamics in the harmonic description and permits valuable new insights into the vibrational properties. The reader will be introduced to the key formalism of lattice dynamics, inelastic and thermal diffuse scattering. Methods for the calculation of vibrational properties from first principles are discussed, followed by a guideline for well converged calculations. The experimental techniques used in this work are presented and new possibilities for combined studies examined. The methodology is illustrated for several benchmark systems. Two silica polymorphs - coesite and alpha-cristobalite – were chosen as examples for covalent systems and investigated in detail. The experimentally validated calculation was used for the analysis of eigenvectors and eigenvalues of different modes, and their contribution to the total and partial density of vibrational states. Comparison with the most abundant silica polymorph - alpha-quartz - and germanium oxide in alpha-quartz structure reveals distinct similarities and differences in the low-energy vibrational properties. Metallic tin polymorphs were chosen to study the influence of the electron subsystem on inter-ionic interactions and the lattice dynamics. Tin exhibits both interesting structural properties and a complex Fermi surface. An unusual asymmetry of thermal diffuse scattering is observed which can be explained within the frame of harmonic lattice dynamics. Finally, the established method is applied to ice which exhibits not only characteristic thermal diffuse scattering but also static contributions from the hydrogen disorder. The methodology proposed in the present work provides a powerful tool in the study of lattice dynamics and will be applicable to a large variety of systems. The studies can be extended to extreme conditions involving very high pressures and a large temperature range. It may be also used to study localised properties of atomic vibrations in systems with broken symmetries, e.g. disorder or surface effects
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Lattice dynamics of VO2 above the metal insulator transition
Full text linkLAUREA MAGISTRALEIn questa tesi è presentato lo studio della dinamica vibrazionale del diossido di vanadio (VO_{2}
), grazie all'utilizzo delle tecniche sperimentali dell'Inelastic X-ray Scattering (IXS) e del Thermal Diffuse Scattering (TDS). Il progetto è stato svolto presso l'European Synchrotron Radiation Facility (ESRF) di Grenoble, in particolare all'interno del gruppo di ID28, beamline specializzata nell'utilizzo della tecnica IXS per lo studio della dinamica vibrazionale. Le misure di scattering diffuso, invece, sono state realizzate nella beamline ID23.
Lo studio della dinamica vibrazionale permette di ottenere informazioni inerenti alle eccitazioni collettive (fononi) di un reticolo periodico. Grazie alla meccanica quantistica, è possibile descrivere un fonone come una quasi-particella, che rappresenta uno stato eccitato dei modi vibrazionali di un cristallo. Le relazioni di dispersione dei fononi costituiscono un importante mezzo per conoscere svariate proprietà di un materiale, ed alcune di queste sono: velocità del suono, costanti elastiche interazioni fonone-fonone, interazioni elettrone-fonone e instabilità del reticolo. Uno dei fenomeni più famosi nel quale i fononi sono coinvolti è la superconduttività: infatti, l'interazione elettrone-fonone è in grado di spiegare il comportamento dei superconduttori che rientrano nella teoria BCS (tradizionali superconduttori a bassa temperatura di transizione).
Il VO_{2}
è stato ampiamente studiato durante gli ultimi decenni, principalmente grazie alla sua interessante caratteristica di mostrare una transizione metallo-isolante (MIT) alla temperatura di circa 340\,
K, dove la fase a più basse temperature è quella isolante, mentre quella ad alte temperature è la fase metallica. Il VO_{2}
non è né l'unico ossido di vanadio, né l'unico ossido di metalli di transizione a mostrare una MIT, ma è l'unico tra questi a possedere una temperatura di transizione così vicina alla temperatura ambiente. Inoltre, il diossido di vanadio sta recentemente attirando ulteriori attenzioni grazie a diverse applicazioni tecnologiche nelle quali può essere impiegato, sia nel campo dell'elettronica che della fotonica.
Ulteriore caratteristica di questo materiale, è la presenza di un cambiamento nella struttura reticolare in concomitanza con la transizione metallo-isolante. Questo fatto porta a pensare che la dinamica vibrazionale possa giocare un ruolo importante nella MIT, grazie a interazioni elettrone-fonone che porterebbero ad una transizione ben descritta nel modello proposto da Peierls. Ciononostante, un secondo meccanismo è stato proposto per spiegare il processo di MIT: questo è il modello di Mott (o Mott-Hubbard), e si basa sull'idea di una transizione guidata da interazioni elettrone-elettrone. Molti risultati sperimentali sono stati presentati a favore di entrambi i modelli proposti, dividendo la comunità scientifica per lungo tempo.
Il principale scopo di questo lavoro è quello di applicare le tecniche sopra citate, di IXS e TDS, in modo tale da fornire una accurata desrizione della dinamica vibrazionale del diossido di vanadio nella fase metallica. La combinazione di queste due tecniche sperimentali, permette di ottenere misure dirette di relazioni di dispersione e mappe di scattering diffuso. Da queste è possibile ricavare importanti informazioni inerenti al ruolo della dinamica vibrazionale nel processo di transizione metallo-isolante.
I risultati sperimentali mostrano in modo innegabile la presenza di instabilità reticolari nella fase metallica del VO_{2}
. Fononi acustici caratterizzati da basse energie sono presenti in più di una direzione di alta simmetria all'interno della zona di Brillouin. Risulta quindi evidente da questi risultati che interazioni elettrone-fonone sono presenti all'interno di questo sistema, prendendo parte al processo di MIT, ma ciò non permette di escludere la presenza di correlazioni elettroniche.In this thesis a study of the lattice dynamics of vanadium dioxide by means of Inelastic X-ray Scattering (IXS) and Thermal Diffuse Scattering (TDS) is presented. The project was conducted at the European Synchrotron Radiation Facility in Grenoble. The main measurements were performed at beam line ID28 using the IXS technique, while the TDS results were obtained at beam line ID23.
The study of lattice dynamics allows to obtain information about the collective excitations (phonons) in a periodic lattice. Thanks to quantum mechanics, a phonon can be seen as a quasiparticle, representing an excited state of the mode of vibrations of a crystal. Knowing the phonon dispersion allows to access various material properties, such as sound velocities, elastic constants, phonon-phonon interactions, electron phonon-interactions and dynamical instabilities. For example, phonons are responsible for the spectacular phenomenon of superconductivity: electron-phonon coupling is the driving mechanism of this process, allowing to explain the behaviour of BCS superconductors.
Vanadium dioxide exhibits a Metal-Insulator Transition (MIT) at about 340\,
K, and this material has been widely studied along the past decades, trying to understand the mechanism driving this transformation. Here the low temperature phase is the insulating phase, while the high temperature one is metallic. The VO_{2}
is neither the only vanadium oxide, nor the only transition metal oxide showing this kind of property, but it is the only one being characterized by a transition temperature so close to room temperature. In addition to being interesting from a fundamental point of view, VO_{2}
has recently drawn a lot of attention since it offers a novel route to novel electronic and photonic applications.
Together with the MIT, a Structural Phase Transition (SPT) takes place, suggesting that the lattice dynamics can play a crucial role across the transition. One possible model which allows to explain the MIT process is known as Peierls transition: this model attributes the radical transformation of this material to electron-lattice interactions. Nevertheless, the MIT process can also originate from electron-electron interactions, and this time the mechanism is referred to as Mott (or Mott-Hubbard) transition. Different theoretical models and a lot of experimental results have been presented to support both the explanations, but an accurate description of lattice dynamics was still missing.
The aim of this work is to provide a thorough description of the lattice dynamics of VO_{2}
in the metallic phase, thus above the transition temperature. Combining the IXS and TDS techniques, it is possible to obtain direct dispersion relation measurements and diffuse scattering maps. These results give a good picture of the phonon energy (and intensity) landscape, highlighting the role that lattice dynamics plays within the metal-insulator transition.
The experimental results show the undeniable presence of lattice instabilities in the high temperature phase of vanadium dioxide. Indeed, low energy acoustic phonons are present in more than one high-symmetry direction of the Brillouin zone. From these results it is evident that electron-lattice interactions play an important role across the MIT, but, of course, the results themselves do not allow to exclude the presence of electron correlations
Lattice dynamics and phonon anomalies in Cerium compounds. The particular case of CeBi and CeSb
Full text linkLAUREA MAGISTRALEQuesta tesi tratta lo studio della dinamica reticolare nel Cerio Bismuto e l'analisi delle anomalie strutturali nel Cerio Antimonio, utilizzando due tecniche sperimentali: IXS e TDS. Lo studio della dinamica reticolare porta ad acquisire le curve di dispersione del composto, dalle quali è possibile determinare diverse proprietà del materiale come velocità del suono e costanti elastiche. L'analisi delle anomalie è invece fatta a bassa temperatura, 14 K, dove si osservano delle super strutture formate da piani atomici paramagnetici e ferromagnetici in successione periodica. Data la particolarità dei dati ottenuti, viene infine presentato un possible modello alternativo per l'interpretazione di queste anomalie.In this thesis work a study on lattice dynamics of Cerium Bismuth and phonon anomalies of Cerium Antimony by means of IXS and TDS is presented. The study of lattice dynamics allow to obtain the dispersion curves of the material, and from these it is possible to have access to a lot of physical properties such as sound velocities and elastic constants. The analysis of the phonon anomalies is performed at low temperature, 14 K, where it is possible to observe super-lattice structures made by a periodic succession of paramagnetic and ferromagnetic planes. At the end, in order to give an explanation to the experimental data, a possible alternative model is presented
Monophosphate tungsten bronzes and antimony : the interplay of framework instability and electron-phonon coupling
No full textLes phonons mous et le couplage électron-phonon sont considérés comme responsables d’un grand nombre de transition de phase. Pour en comprendre complétement les mécanismes, il est nécessaire d’étudier à la fois les modifications structurales, les changements de configuration électronique et les dynamiques de réseau cristallin. De toute évidence, cela représente une charge de travail expérimental et théorique considérable, voire même hors d’atteinte.Néanmoins, il est parfois possible d’introduire certaines simplifications et d’ainsi rendre une telle étude réalisable. C’est le cas pour les deux systèmes au cœur de ce travail de thèse, pour lesquels la transition de phase peut être diviser en deux instabilités : l’une est structurale, intrinsèque aux éléments constitutifs du système et l’autre, superposée, provient de la configuration électronique. L’interaction entre ces instabilités est illustrée à travers l’exemple de deux systèmes à priori hétérogènes, la famille des bronzes monophosphate de tungstène d’une part et l’antimoine d’autre part, qui révèleront finalement posséder des similarités inattendues. La combinaison des techniques de diffusion diffuse et de diffusion inélastique de rayons X permet des observations qualitatives et une meilleure compréhension de la situation pour les deux systèmes.Les bronzes monophosphate de tungstène font partie de la famille des oxydes quasi-2D, (PO2)4(WO3)2m, qui ont la particularité d’être sujet à des instabilités de type onde de densité de charges (ODC). Ces bronzes sont constitués d’une structure de perovskite vide composée par des couches octaédriques (WO3)2m. L’épaisseur de chacune de ces couches est définie par la valeur de m, qui mène ainsi à différents types de phase d’ODC. Le cas du terme m=2 a aussi été étudié car le fait que les chaînes zig-zag y soient isolées conduit à une instabilité quasi-1D. La présence d’une phase d’ODC a été découverte à TC=270K avec q=0.25b*. Cette phase est engendrée par le mouvement à corps rigide, plus exactement, par les basculements corrélés des octaèdres. Pour les autres termes (m=6,7 et 8), l’instabilité structurale a une origine différente et est liée à l’agencement en couches de WO3, plus particulièrement aux déplacements corrélés des chaînes W-O-W-O. Ces derniers sont la cause d’une forte diffusion diffuse sur des plans spécifiques, résultant de la présence de phonons ’relativement’ mous localisés dans la même région. Ensuite, l’emboitement de la surface de Fermi quasi-2D est à l’origine de l’ancrage du vecteur de modulation sur une valeur spécifique de transfert de moment, définit par l’interaction de deux instabilités, structurale et électronique. De façon remarquable, l’amplitude des déplacements des atomes de tungstène dans le terme m=8 est beaucoup plus élevée que dans le m=6.L’antimoine à température ambiante possède une structure rhomboédrique, dérivant d’une légère distorsion de la structure cubique primitive (CP) par transition de Peierls. Sous pression, la distorsion se réduit sans toutefois disparaître complétement, puisque l’antimoine se transforme dans un premier temps en une série de structures complexes, pour finalement adopter celle possédant la plus grande symétrie, la structure cubique centrée (CC). De la même façon que pour les bronzes, les caractéristiques de la diffusion diffuse ainsi que, dans une certaine mesure, les particularités de la dynamique du réseau rhomboédrique, s’expliquent à travers de l’instabilité du réseau cubique primitif. Cette dernière est liée aux déplacements corrélés dans les chaînes avec direction pseudo-cubique . En outre, les détails de la transition de phase peuvent être explicités par l’association de l’analyse des vecteurs critiques de la transformation CC-CP avec les résultats expérimentaux obtenus sur la dépendance en pression de l’énergie des phonons.A large number of phase transitions can be interpreted as being driven by phonon softening and/or electron-phonon coupling. Thus, a full mechanistic description requires the understanding of structural transformation, changes in electronic structure and lattice dynamics. All together this represents an enormous, for many cases unrealisable, experimental and theoretical effort.However, with the introduction of appropriate assumptions the problem may be simplified. Here we concentrate on two systems, where the interpretation of the phase transition may be split into an intrinsic instability of the building blocks combined with a superimposed electronic instability. We illustrate the interplay between the framework and electron-phonon-related instabilities using the seemingly heterogeneous examples of phosphate tungsten bronzes and elementary antimony. Based on the combined results from diffuse and inelastic X-ray scattering, we propose for the two systems a picture that explains the experimental observations. The similarities found between these two systems are deemed to be rather surprising.Monophosphate tungsten bronzes are a family of quasi-2D-oxides, (PO2)4(WO3)2m, that exhibits charge density wave (CDW) instability. They contain empty perovskite WO3 slabs with varying thickness between different members, characterised by the m value. This thickness defines the sequence of charge density wave phases that appear on cooling. The degenerate case of m=2, presenting a quasi-1D instability, was explored since the WO3-octahedra zig-zag chain is isolated. A CDW phase (TC=270K and q=0.25b*) is found to be linked to a rigid-body motion, precisely, to a correlation in the tilting of the octahedra. For the others studied members, as m=6,7 and 8, we found another kind of structural instability. In this case the origin comes from the WO3 slabs framework, realised as correlated displacements of tungsten atoms along the octahedral 4-fold axis direction (W-O-W-O direction). This leads to a strong x-ray diffuse scattering localised in specific planes, linked to relatively soft phonons modes. Specific Fermi surface nesting, close to the 2D case, gives rise to a freezing of the modulations at the specific momentum transfer, defined by the interplay of two instabilities, the structural and electronic one. Remarkably, the displacements of W for m=8 are much superior than in m=6.Elemental antimony at ambient condition has an A7 rhombohedral structure, obtained by small distortion from primitive cubic (PC) lattice through a Peierls transition. Under pressure, the distortion is reduced, but remains finite, as antimony transforms through a series of highly complex structures, before adopting as last the highest-symmetry body-centred cubic (BCC) phase. The main diffuse scattering features and to some extent the peculiarities in the lattice dynamics of the A7 phase – as above - can be explained by the instability of the primitive cubic network with respect to correlated displacements along the chains with pseudo-cubic directions. Analysis of critical vectors for the BCC-PC transformation together with experimentally obtained phonon-energies pressure dependence provides further insights into the details of the phase transformation
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