Measurements have been made of the X-ray scatter from an MgO single crystal at low temperatures irradiated by an infra-red laser beam of wavelength 10.6μm. The purpose of these measurements was to obtain information about the anharmonic coupling between the lattice vibrations. The modes investigated were the transverse acoustic phonons in the [100] direction, which it has been suggested have anomalously long lifetimes at very low temperatures. The above method was chosen after considering a variety of possible excitation methods including electron-phonon interactions via the piezoelectric effect and the deformation potential. These other processes had to be rejected because they either excited phonons which were unsuitable for X-ray measurement or required pulsed experiments. The dispersion curves for MgO, obtained from a deformable shell model for the lattice were analysed to provide information as to the most likely positions in reciprocal space for phonon population enhancement to occur. The decay routes for the primary phonons excited directly by the infra-red radiation were predicted from the dispersion curves. A major part of the work involved the design and construction of the helium cryostat on which the sample crystal was mounted and kept at as low a temperature as possible. The standard X-ray detection technique of a scintillating crystal-photomultiplier combination was employed and the infra-red beam was produced by a CO2 gas laser with an output power of up to lOW. Because of the nature of the measurements, the results obtained for the change in X-ray scatter by the MgO crystal when the infra-red beam was shone on the crystal required analysis by statistical methods in order to provide the maximum information. It was possible to set an upper limit to the effects found of a few percent, and to compare the orders of magnitude of the coupling for two different processes.

Thesis (Ph.D.)--McMaster University (Canada), 1995. / Source: Dissertation Abstracts International, Volume: 56-12, Section: B, page: 6838. Adviser: J. P. Carbotte.

Thesis (Ph. D.)--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references (p. 140-142).

Thesis (Ph. D. in Physics)--Vanderbilt University, Dec. 2009. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Le schéma générique de percolation 1-liaison→2-TO mis au point sur site pour la compréhension de base des spectres de vibration (Raman et infrarouge - IR) des alliages semiconducteurs aléatoires usuels, i.e., s’inscrivant dans la structure zincblende, qui a récemment supplanté le modèle standard 1-liaison→1-TO dit MREI [modified-random-element-isodisplacement, (Chang, et al., 1971)] mis au point dans les années soixante, est ici confronté au modèle à clusters (Verleur, et al., 1966), consacré par l’usage pour la discus-sion des spectres de vibration de ces alliages zincblende particuliers, présumés non aléatoires, qui montrent évidemment plus d’un mode phonon par liaison dans leur spectres de vibration. En fait, nous montrons que les spectres de réflectivité IR pionniers de l’alliage GaAs1-xPx représenta-tif de cette dernière classe de comportement, i.e. ceux-là mêmes qui ont motivé le développement du modèle à clusters dans les années soixante, trouvent une explication naturelle dans le cadre du modèle de percolation sur la base d’une substitution As ↔ P parfaitement aléatoire, sans aucun paramètre ajustable. L’alliage GaAs1-xPx et ses semblables se trouvent ainsi réhabilités en tant que systèmes aléatoires vis-à-vis de leurs propriétés vibrationnelles. Du même coup, la classification admise des spectres de vibration des alliages semiconducteurs à structure zincblende, basée sur les modèles MREI-Clusters, en quatre sous-types distincts, se trouve totalement unifiée dans le cadre de notre modèle de percolation. Dans un second temps nous explorons dans quelle mesure une extension du schéma de percolation de la structure zincblende vers la structure diamant, peut aider à comprendre la nature mystérieuse du comportement Raman de l’alliage aléatoire représentatif Si1-xGex. Il s’avère qu’une simulation satisfaisante des spectres Raman de Si1-xGex d’ores et déjà existants dans la littérature peut être accomplie, sans autre para-mètre ajustable que l’efficacité Raman de la liaison mixte Si-Ge, dans le cadre d’une version générique 1-liaison→N-mode du schéma de percolation, où N indique une sensibilité à l’environnement local des liaisons Ge-Ge (N=1, insensibilité), Si-Si (N=2, sensibilité à l’environnement premiers-voisins) et Si-Ge (N=3, sen-sibilité à l’environnement seconds-voisins). Des différences notables entre les schémas de percolation de GaAs1-xPx et de Si1-xGex sont attribuées à la différence de nature de la relaxation du réseau dans les cristaux à structure zincblende et diamant (inversion de l’ordre des branches au sein du triplet Si-Ge) et à la dispersion spectaculaire des modes Ge-Ge (antiparallélisme de la branche unique Ge-Ge) et Si-Si (inversion de l’ordre des branches au sein du doublet Si-Si), qui vient s’ajouter à l’effet de contrainte locale, habituellement seul pris en compte pour les alliages zincblende déjà examinés. L’assignation des branches phonons individuelles au sein de chaque schéma de percolation, i.e. GaAs1-xPx ou Si1-xGex, est réalisé par voie ab initio en se focalisant sur les modes de vibration des liaisons en stretching pur le long de motifs d’impuretés prototypes choisis quasi-linéaires de manière à rester dans l’esprit de l’approximation de la chaîne linéaire (ACL) sur laquelle est basé le modèle phénoménologique de percolation / The 1-bond → 2-TO percolation generic scheme proposed for the basic understanding of the vibra-tion spectra (Raman and Infrared –IR) of usual random semiconductor alloys with zincblende structure, which has recently challenged the standard MREI (modified-random-element-isodisplacement) model with the 1-bond→1-TO behavior, (Chang et Mitra, 1971) developped in the sixties, is here confronted to the cluster model (Verleur et Barker, 1966) which has been accepted through use for the study of the vibration spectra of these particular zincblende alloys, which obviously exhibit more than one phonon mode per bond in their vibration spectra. In fact, the IR reflectivity spectra of GaAs1-xPx, the representative alloy of the last class, i.e. those very ones which motivated the developpement of the cluster model in the sixties, find a natural explanation in the scope of the percolation model on the basis of a perfect random substitution As ↔ P, with no adjusta-ble parameter. With this, GaAs1-xPx and its like are rehabilitated as random alloys in principle, and further, the percolation paradigm generalizes to all types and subtypes of the traditional four-type classification of phonon mode behavior of semiconductor alloys, based on the MREI and cluster models. In a second stage, we investigate to which extent an extension of the percolation scheme to the di-amond structure may help to understand the mysterious nature of the Raman behavior of the representative Si1-xGex alloy. We realize that a good simulation of the Si1-xGex Raman spectra already existing in the literature can be obtained with no adjustable parameter but the Raman efficiency of the Si1-xGex bond within a generic 1-bond→N-TO version of the percolation model, where N indicates a sensitivity to the local environment of the Ge-Ge (N=1 non sensitive), Si-Si (N=2, sensitive to the first neighbor environment) and Si-Ge (N=3, sensitive to the second neighbor environment) bonds. Some differences between the GaAs1-xPx and Si1-xGex percolation patterns are attributed to the different natures of the lattice relaxation in the zincblende and di-amond structures (inversion of the order of like branches in the Si-Ge triplet) and to the spectacular disper-sion of the Ge-Ge like modes (anti parallelism of the unique Ge-Ge branch) and Si-Si (inversion of the order of like branches in the Si-Si doublet), which add to the local constraint effect, the only one usually taken into account in the zincblende alloys examined so far. The assignment of the individual phonons branches in each percolation scheme, i.e. GaAs1-xPx or Si1-xGex, is achieved via home-made ab initio phonon calculations with a focusing on bond-stretching along prototype impurity motives designed as quasi linear so as to be remain in with the spirit of the linear chain approximation on which the percolation model is based

RAMOS, Igor Rochaid Oliveira. Estudo de sistemas coloidais fortemente correlacionados. 2014. 113 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-10-31T20:49:57Z No. of bitstreams: 1 2014_tese_iroramos.pdf: 4160505 bytes, checksum: 1c5b95406b1be080fece98a7bc0bf05a (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-10-31T21:06:33Z (GMT) No. of bitstreams: 1 2014_tese_iroramos.pdf: 4160505 bytes, checksum: 1c5b95406b1be080fece98a7bc0bf05a (MD5) / Made available in DSpace on 2014-10-31T21:06:33Z (GMT). No. of bitstreams: 1 2014_tese_iroramos.pdf: 4160505 bytes, checksum: 1c5b95406b1be080fece98a7bc0bf05a (MD5) Previous issue date: 2014 / This thesis presents the study of the structural and dynamical properties, as well as, melting of colloidal systems. Initially, we study the structure and phonon spectrum of a system of charged magnetic dipoles, organized in a bilayer structure and oriented perpendicular to the plane of the layers. This system can be tuned through six different crystalline phases by changing parameters such as the interlayer separation and/or the charge and/or dipole moment of the particles. The presence of the electric charge on the dipole particles is responsible for the nucleation of five staggered phases and a disordered phase which are not found in the magnetic dipole bilayer system previously presented in the literature. These extra phases are a consequence of the competition between the repulsive Coulomb and the attractive dipole interlayer interaction. The minimum energy structures are summarized in a phase diagram associated to the separation between the layers and to the relative importance between the magnetic and electric interactions. We determine the order of the structural phase transitions. The phonon spectrum of the system was calculated within the harmonic approximation. A non-monotonic behavior of the phonon spectrum is found as a function of the effective strength of the inter-particle interaction. The thermodynamic stability of the different phases is determined. Then, we study the bilayer system of charged magnetic dipoles for nonzero temperatures, investigating the melting behavior of the system through the modified Lindemann criterion, as a function of the parameters: (i) the distance between the layers η and (ii) the relative intensity of the magnetic interaction with respect to the electric interaction λ. For large enough λ, one of the phases (the matching hexagonal phase) exhibits a re-entrant melting behavior as a function of η. Since the charges and the magnetic dipole moment of the colloidal particles can be altered, for example, by changing the pH of the solution in which they are immersed or an external magnetic field, respectively, this system can be in principle verified experimentally. Last, a two-dimensional (2D) binary colloidal system consisting of interacting dipoles is investigated. Within the harmonic approximation, we obtained the phonon spectrum of the system as a function of the composition, dipole moment ratio and mass ratio between the small and big particles. Through a systematic analysis of the phonon spectra, we are able to determine the stability region of the different lattice structures of colloidal alloys. The gaps in the phonon frequency spectrum, the optical frequencies in the long-wavelength limit and the sound velocity are discussed as well. Using the modified Lindemann criterion and within the harmonic approximation, we estimated the melting temperature of the sub-lattice generated by the big particles. / Nesta tese, estudamos as propriedades estruturais e dinâmicas, bem como, a fusão de sistemas coloidais. Inicialmente, abordamos o problema de determinar as estruturas de mínima energia e o espectro de fônons de um sistema de dipolos magnéticos carregados, organizados em uma estrutura de bicamadas e orientados perpendicularmente ao plano das camadas. Este sistema pode ser sintonizado através de seis diferentes fases cristalinas, através da variação de parâmetros tais como a separação entre as camadas e/ou a carga e/ou o momento de dipolo das partículas. A presença de carga elétrica nas partículas dipolares é responsável pela nucleação de cinco fases onde as camadas não estão alinhadas verticalmente e uma fase desordenada, que não são encontradas no sistema em bicamadas de dipolos magnéticos previamente apresentado na literatura. Estas fases extras são uma consequência da competição entre a repulsão coulombiana e a interação atrativa entre os dipolos em diferentes camadas. As estruturas de mínima energia são sumarizadas em um diagrama de fases associado à separação entre camadas e a importância relativa entre as interações elétrica e magnética. Determinamos, ainda, a ordem das transições estruturais entre as várias configurações de mínima energia. O espectro de fônons do sistema foi calculado usando a aproximação harmônica. Um comportamento não-monotônico do espectro de fônons é encontrado como função da interação efetiva entre as partículas. A estabilidade termodinâmica das diferentes fases é determinada. Em seguida, estudamos o sistema de bicamadas de dipolos magnéticos carregados para temperaturas diferentes de zero, investigando a fusão do sistema através do critério de Lindemann modificado, em função dos parâmetros: (i) a distância entre as camadas η e (ii) a intensidade relativa da interação magnética com respeito à interação elétrica λ. Para λ suficientemente grande, uma das fases (a fase hexagonal com alinhamento vertical) exibe um comportamento reentrante na temperatura de fusão em função de η. Uma vez que a carga e o momento de dipolo magnético das partículas coloidais pode ser alterado, por exemplo, pela variação do pH da solução na qual estão imersos e por um campo magnético externo, respectivamente, este sistema pode ser em princípio verificado experimentalmente. Por último, um sistema bidimensional (2D) coloidal binário consistindo de dipolos interagentes é investigado. Dentro da aproximação harmônica, calculamos o espectro de fônons do sistema em função da composição, da razão entre os momentos de dipolo e da razão entre as massas das partículas pequenas e grandes. Através de uma análise sistemática dos espectros de fônons, determinamos a região de estabilidade das diferentes estruturas das ligas coloidais. As lacunas no espectro de frequência dos fônons, as frequências óticas no limite de longos comprimentos de onda e a velocidade do som são também discutidos. Usando o critério de Lindemann modificado e dentro da aproximação harmônica, estimamos a temperatura de fusão da sub-rede gerada pelas partículas grandes.

Fast carrier cooling is important for high power graphene based devices. Strongly coupled optical phonons play a major role in the relaxation of photo-excited carriers in graphene. Heterostructures of graphene and hexagonal boron nitride (hBN) have shown exceptional mobility and high saturation current, which makes them ideal for applications, but the effect of the hBN substrate on carrier cooling mechanisms is not understood. We track the cooling of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast pump-probe spectroscopy. We find that the carriers cool down four times faster in the case of graphene on hBN than on a silicon oxide substrate thus overcoming the hot phonon bottleneck that plagues cooling in graphene devices. (C) 2017 Author(s).

Nos travaux sont consacrés à la promotion du modèle de percolation phénoménologique en employant des méthodes indépendantes et libres de tout paramètre. Nous avons utilisé à cet effet des codes ab initio existants (PWscf et SIESTA) et développé une méthode semi-empirique pour étudier les propriétés vibrationnelles des alliages AxB1 xC. Les paramètres techniques du code PWscf ont été testés sur les composés binaires ZnSe, ZnTe et BeSe pour lesquels nous avons confronté nos résultats aux données expérimentales et théoriques disponibles. La validation du modèle de percolation a et e effectué sur l alliage Zn(Se,Te) appartenant à la catégorie des semi-conducteurs 1-liaison ! 2-modes. Nous avons confirmé le caractère général du comportement bimodal prévu par le modèle de percolation en montrant son existence pour chacune des liaisons Zn Se et Zn Te. Les propriétés vibrationnelles et structurales de Zn(Se,Te) ont et e calculées à différentes concentrations avec des cellules optimisées à 64 atomes. Les résultats obtenus sont en bon accord avec l expérience. Nous présentons en détail notre méthode semi-empirique de calcul de spectre de phonons, applicable en principe à tous les alliages sans restriction sur la taille des super-cellules. La mise au point a été réalisé sur les alliages (Zn,Be)Se et (In,Ga)As avec des cellules à64 atomes relaxées d une façon ab initio. L application de notre méthode à(In,Ga)As à 50% simulé par une super-cellule à 8.103 atomes relaxée avec un potentiel harmonique met en évidence que l étude des phonons demande des super-cellules qui reflètent fidèlement le désordre d un alliage / Our work is devoted to promoting the percolation model using phenomenological methods which are independent and free from any need of external parameters. We used for this purpose existing ab initio codes (PWscf and SIESTA) and developed a semiempirical method to study the vibrational properties of AxB1 xC alloys. The technical parameters of the PWscf code were tested on ZnSe, BeSe ZnTe binary compounds, for which we have confronted our results with theoretical and experimental data available. The percolation model validation was done on the alloy Zn(Se, Te) belonging to the category of 1-bond ! 2-modes semi-conductors. We have confirmed the general bimodal behavior provided by the percolation model showing its existence for each bond Zn Se and Zn Te. The vibrational and structural properties of Zn(Se, Te) were calculated for different concentration using optimized cells of 64 atoms. The results are in good agreement with experiments. We present in detail our semi empirical method of calculating the spectrum of phonons, in principle applicable to all alloys without restriction on the size of super-cells. The development was carried out on (Zn, Be)Se and (In, Ga)As alloys with cells of up to 64 atoms relaxed from first principles. The application of our method to (In, Ga)As at 50% simulated by a super-cell of 8.103 atoms with a harmonic relaxation potential underlines the fact that the study of phonons in alloys requires the use of supercells which adequately describe the alloy disorder

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