Problèmes aux limites issus de la supraconductivité (estimations semi-classiques et comportement asymptotique des solutions)

Cette thèse est consacrèe à l'étude de plusieurs modèles de l'effet de proximité' dans le cadre de la thèorie de Ginzburg-Landau de la supraconductivité. Ces modèles proviennent des interfaces entre des supraconducteurs et des métaux normaux.Dans une première partie de cette thèse, nous estimons dans le régime semiclassique le bas du spectre d'un opérateur de Schrôdinger magnétique associé à une condition au bord du type Fourier (Robin), appelée dans ce contexte la condition au bord de de Gennes, et nous étudions également la localisation des états fondamentaux. Nous exhibons des cas où la condition au bord de de Gennes a des effets forts sur cette localisation.Dans une autre partie, nous construisons un problème spectral lié à l'apparition de la supraconductivité pour une fonctionnelle de Ginzburg-Landau généralisée ayant des coefficients discontinues, où le paramètre d'ordre' et le potentiel magnétique' sont définis dans l'espace entier. Pourle régime où le paramétre de Ginzburg-Landau est grand, nous estimons le champ surcritique corespondant au champ pour lequel les états normaux perdent leur stabilité. Dans d'autres régimes asymptotiques, nous retrouvons un développement limité déjà obtenu pour un autre modèle standard'.Dans la dernière partie, nous étudions de nouveau une fonctionnelle de Ginzburg-Landau généralisée mais dans le cas sans champ magnétique appliqué. Nous obtenons dans ce cas le comportement asymptotique du paramètre d'ordre pour le régime où le paramètre de Ginzburg-Landau est grand. Ceci montre en particulier que la supraconductivité persiste dans une bande mince dans le métal normal, près du bord du supraconducteur.This thesis is devoted to the study of various models for the proximity effect' in the frame work of the Ginzburg-Landau theory of superconductivity. These models arise in the situation when a superconductor is adjacent to a normal metal.In a first part of this thesis, we estimate in the semi-classical limit the ground state energy of a magnetic Schrôdinger operator associated to a Fourier (Robin) type boundary condition, called in this context the de Gennes boundary condition, and we study the localization of the ground states. We exhibit cases when the de Gennes boundary condition has strong effects on this localization.In another part, we formulate a spectral problem related to the onset of superconductivity for a generalized Ginzburg-Landau functional having discontinuous coefficients, where the order parameter and the magnetic potential are defined in the whole space In the regime when the Ginzburg-Landau parameter (of the superconducting material) is large, we estimate the critical applied magnetic field for which the normal state will lose its stability. In some asymptotic situations, we recover results related to the standard' Ginzburg-Landau model.In the final part, we study again a generalized Ginzburg-Landau functional in the case without an applied magnetic field. We determine in this case the asymptotic behavior of the order parameter in the regime when the Ginzburg-Landau parameter is large. This shows in particular that the superconductivity persists in a thin boundary sheath of the normal material, near the boundary of the superconductor.ORSAY-PARIS 11-BU Sciences (914712101) / SudocORSAY-PARIS 11-Bib. Maths (914712203) / SudocSudocFranceF

Effect of molecular weight on the structure and crystallinity of poly(3-hexylthiophene)

Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites

Triplet-State and Singlet Oxygen Formation in Fluorene-Based Alternating Copolymers

Data are reported on the triplet states of a series of fluorene-based A-alt-B type alternating copolymers based on pulse radiolysis-energy transfer and flash photolysis experiments. From the pulse radiolysis experiments, spectra are given for eight copolymers involving phenylene, thiophene, benzothiadiazole, and oligothienylenevinylene groups. Quantum yields for triplet-state formation (ΦT) have been obtained by flash photolysis following laser excitation and in one case by photoacoustic calorimetry. In addition, yields of sensitized formation of singlet oxygen have been determined by time-resolved phosphorescence and are, in general, in excellent agreement with the ΦT values. In all cases, the presence of thiophene units is seen to increase intersystem-crossing quantum yields, probably because of the presence of the heavy sulfur atom. However, with the poly[2,7-(9,9-bis(2‘-ethylhexyl)fluorene)-alt-1,4-phenylene] (PFP), thiophene S,S-dioxide (PFTSO2) and benzothiadiazole (F8BT) copolymers, low yields of triplet formation are observed. With three of the copolymers, the energies of the triplet states have been determined. With PFP, the triplet energy is virtually identical to that of poly[2,7-(9,9-bis(2‘-ethylhexyl)fluorene)]. In contrast, with fluorene−thiophene copolymers PFaT and PF3T, the triplet energies are closer to those of thiophene oligomers, indicating that there is significant conjugation between fluorene and thiophene units but also that there is a more localized triplet state than with the homopolymers

Modélisation numérique du comportement d'une particule sous champ électrique dans les microsystèmes (de la déformation au déplacement)

Un nouvel outil numérique pour prédire le mouvement et la déformation de particules sous champ électrique dans les microsystèmes biologiques est proposé. Ce moyen permettra à terme l'optimisation de labopuces dans les premières phases de conception. Le phénomène utilisé pour manipuler les particules est la diélectrophorèse associée au mouvement dû à la polarisation des diélectriques sous champ électrique non uniforme. L'outil numérique utilisé pour le suivi des interfaces mobiles sous champ électrique est la Méthode Intégrale aux Frontières (MIF). Les écoulements sont supposés irrotationnels. Par cet outil, nous avons réussi à retrouver les résultats de Taylor sur l'instabilité des gouttes sous champ uniforme. Les résultats du déplacement des gouttes sous champ non uniforme montrent un bon accord également entre les simulations et des modèles analytiques approchés. L'état de développement de l'outil MIF nous permet d'envisager son extension vers des problèmes électrohydrodynamiques encore plus riches.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Alternating Binaphthyl−Thiophene Copolymers: Synthesis, Spectroscopy, and Photophysics and Their Relevance to the Question of Energy Migration versus Conformational Relaxation

The synthesis and a comprehensive spectroscopic and photophysical study are presented of four alternating binaphthyl−oligothiophene copolymers (DP: 10−15 repeat units) in solution at room and low temperature and in the solid state (thin films). Detailed results are presented on absorption, emission, and triplet−triplet absorption spectra together with all relevant quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation), excited-state lifetimes, and singlet and triplet energies. From these, several conclusions can be drawn. First, the main deactivation channels for the molecules in solution are the radiationless processes (S1 → S0 internal conversion and S1 → T1 intersystem crossing). Second, in the solid state the fluorescence quantum yields are smaller than those in solution. From time-resolved fluorescence decays in the picosecond time domain, three decay components are seen: a fast decay (40−60 ps) at short wavelengths, which becomes a rising component at longer wavelengths, an intermediate decay component (330−477 ps) associated with an ensemble of isolated segment-like units, which is dominant at the initial part of the emissive spectra and progressively decreases for longer emissions, and a third exponential related to the emission of the fully relaxed polymer. Together with steady-state anisotropy studies, this is discussed in terms of the possibilities of energy migration/transfer along the polymer chain and of the conformational (torsional) relaxation of the systems studied

Opto-Electronic Properties of Fluorene-Based Derivatives as Precursors for Light-Emitting Diodes

This paper reports optical absorption spectra of oxidized fluorene copolymers obtained by chemical oxidation with Ce(IV) and by pulse radiolysis experiments in chlorinated solvents. Comparison of the results observed by the two techniques is used to provide spectral data on the copolymer radical ions and information on stability of the oxidized species. In addition, a detailed quantum chemical characterization is presented, concerning the electronic and optical properties of three series of charged oligomers containing alternating fluorene and phenylene or thienylene or benzothiadiazole units, respectively. The introduction of the comonomer strongly influences the optical properties, leading to a red shift in the absorption spectra of the charged oligomers. This shift is more pronounced in the case of fluorene benzothiadiazole anions due to the strong electron-accepting character of the benzothiadiazole moieties. The charge distribution of the fluorene benzothiadiazole anion is different from that corresponding to fluorene phenylene and fluorene thienylene anions. The negative charge of the latter oligomers is evenly distributed over the fluorene units, while the former oligomer localizes the negative charge on the benzothiadiazole units. The charge distribution correlates with the optical absorption spectra. When the positive charge is localized on a different unit than the negative charge, the cation and anion spectra are different. Similar spectra are obtained if both the positive and negative charges are localized on the same unit

Blue-emitting poly(2,7-pyrenylene)s: Synthesis and optical properties

The first 2,7-linked conjugated polypyrene tethering four aryl groups has been synthesized, and the photophysical properties of the polymer have been characterized. Due to the unique substitution with bulky aryl groups at the 4,5,9,10-positions of pyrene, the polymer although comprising large ??-units is readily soluble in common organic solvents. The polymer has a blue fluorescence in solution with an emission band maximum at X = 429 nm, fulfilling the requirements for a blue-emitting polymer. To a first approximation, this can be attributed to electronically decoupled pyrene subunits due to the nodal properties of their frontier orbitals. Furthermore, in solution the intriguing blue fluorescence is combined with solvent-dependent features, which could be used as a probe for local solvent polarity. Finally, in the solid state, the red-shifted emission of the polymer could be strongly reduced by blending with a nonconjugated polymer such as polystyrene.close373