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[pt] A teoria de Bardin, Cooper e Schrieffer (BSC) teve enorme sucesso na explicação das propriedades da maior parte dos materiais supercondutores. Esses materiais, onde a teoria BCS se aplica, são denominados supercondutores convencionais. A observação do aparecimento de supercondutividade não-convencional em diversos materiais reabriu as discussões sobre o fenômeno. Enquanto a transição para fase supercondutora em materiais convencionais envolve apenas a quebra da simetria de calibre, no caso dos materiais não-convencionais, a mesma é caracterizada pela quebra de diversas simetrias adicionais. O mecanismo microscópico da supercondutividade nessas novas classes de materiais ainda é uma questão em aberto. no entanto, muitas propriedades físicas podem ser extraídas apenas de conciderações sobre as simetrias do parâmetro de ordem supercondutor, que está intimamente ligadoá função de onda do par de Cooper. Neste trabalho são analisadas algumas propriedades destes novos supercondutores baseadas em critérios de simetria. Um enfoque especial é dado à classe dos supercondutores não-convencionais onde há uma quebra de simetria de reversão temporal. Para estes materiais são previstas algumas propriedade bem pouco usuais. Quando a estrutura cristalina tiver alta simetria, é possível o surgimento de uma polarização de um spin no condensado. Nestes casos, a magnetização intrínseca pode levar à formação de uma fase espontânea de vórtices. Ocorre também uma forte anisotropia na resposta do supercondutor frente à aplicação de campos magnéticos externos. / [en] The theory of Bardeen, Cooper and Schrieffer (BCS) had great success in explaining most properties of superconducting materials. These materials, where BCS applies, are denominated conventional superconductors. the experimental evidence of unconventional superconductivity in several materials reopened discussions about the phenomenon. While, in conventional materials, the superconducting phase involves only the breaking of gauge symmetry, in the unconventional materials the phase is characterized by several additional broken symmetries. The microscope mechanism of superconductivity in this new classes of materials is still an open question. However, many phisical properties can be understood considering only symmetries of the superconducting order parameter, which is intimately linked to Cooper pair wave function. In this work some properties of these new superconductors are analyzed based symmetry criteria. Special emphasis is given to the class of unconventional superconductors where time- reversal symmetry is broken. For these materials, some unusual properties are predicted. When the crystal structure has high symmetry, the appearence of a spin polarization in the condensate is possible. In these cases, an intrinsic magnetization can lead to the information of a spontanous vortex phase. A strong anisotropic response to an externally applied magnetic field also occurs.

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On considère généralement que la fonction d’onde macroscopique décrivant un condensat de paires de fermions possède une phase parfaitement définie et immuable. En réalité, il n’existe que des systèmes de taille finie, préparés à température non nulle ; le condensat possède alors un temps de cohérence fini, même lorsque le système est isolé. Cet effet fondamental, crucial pour les applications qui exploitent la cohérence macroscopique, restait très peu étudié.Dans cette thèse, nous relions le temps de cohérence à la dynamique de phase du condensat, et nous montrons par une approche microscopique que la dérivée temporelle de l’opérateur phase ˆθ0 est proportionnelle à un opérateur potentiel chimique qui inclut les deux branches d’excitations du gaz : celle, fermionique, de brisure des paires et celle, bosonique, de mise en mouvement de leur centre de masse. Pour une réalisation donnée de l’énergie E et du nombre de particules N, la phase évolue aux temps longs comme −2μmc(E,N)t/~ où μmc(E,N) est le potentiel chimique microcanonique ; les fluctuations de E et de N d’une réalisation à l’autre conduisent alors à un brouillage balistique de la phase, et à une décroissance gaussienne de la fonction de cohérence temporelle avec un temps caractéristique ∝ N1/2. En l’absence de telles fluctuations, la décroissance est au contraire exponentielle avec un temps de cohérence qui diverge linéairement en N à cause du mouvement diffusif de ˆθ0 dans l’environnement des modes excités. Nous donnons une expression explicite de ce temps caractéristique à bassetempérature dans le cas d’une branche d’excitation bosonique convexe lorsque les phonons interagissent via les processus 2 ↔ 1 de Beliaev-Landau. Enfin, nous proposons des méthodes permettant de mesurer avec un gaz d’atomes froids chaque contribution au temps de cohérence / It is generally assumed that a condensate of paired fermions at equilibrium is characterized by a macroscopic wavefunction with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature ; the condensate has then a finite coherence time, even when the system is isolated. This fundamental effect, crucial for applicationsusing macroscopic coherence, was scarcely studied. Here, we link the coherence time to the condensate phase dynamics, and show using a microscopic theory that the time derivative of the condensate phase operator ˆθ0 is proportional to a chemical potential operator which includes both the fermionic pair-breaking and the bosonic pair-motion excitation branches.For a given realization of the number of particle N and of the energy E, the phase evolves at long times as −2μmc(E,N)t/~ where μmc(E,N) is the microcanonical chemical potential ; fluctuations of N and E from one realization to the other then lead to a ballistic spreading of the phase and to a Gaussian decay of the temporal coherence function with a characteristictime ∝ N1/2. On the contrary, in the absence of energy and number fluctuations, the decay of the temporal coherence function is exponential with a characteristic time scaling as N due to the diffusive motion of ˆθ0 in the environnement created by the excited modes. We give an explict expression of this characteristic time at low temperature in the case where the bosonicbranch is convex and the phonons undergo 2 ↔ 1 Beliaev-Landau process. Finally, we propose methods to measure each contribution to the coherence time using ultracold atoms.