Cette thèse expérimentale explore les propriétés du point critique de la phase pseudogap dans le diagramme de phase des cuprates supraconducteurs. Dans une première partie, j’expose un état de l’art sur les connaissances du diagramme de phases température-dopage (T-p) de ces systèmes. Des études récentes montrent une chute importante de la densité de porteurs électroniques au voisinage du point critique suggérant une reconstruction de la surface Fermi. Pour comprendre la nature exacte de la transition de phases liée à cette reconstruction, j’ai réalisé des mesures complémentaires de transport thermique et de chaleur spécifique sous champ magnétique intense sur les familles La1.8-xSrxEu0.2CuO4 et La1.6-xSrxNd0.4CuO4.Dans une deuxième partie, après une introduction théorique sur la chaleur spécifique et le transport thermique, je détaille comment ces deux grandeurs ont été mesurées. En particulier, une technique originale de mesures de la chaleur spécifique a été mise au point pour combiner haute résolution et précision absolue en champ magnétique intense et basse température. Différents modèles thermiques et électroniques ont été développés pour comprendre et analyser les mesures et ont permis d’optimiser les différents montages de chaleur spécifique selon les gammes de température.Dans une troisième partie, je présente l’ensemble des résultats obtenus en transport thermique et chaleur spécifique. Le transport thermique confirme la chute de la densité de porteur dans l’état normal (sans supraconductivité) des cuprates déjà observée en transport électrique sous champ intense. Par ailleurs, j ‘ai montré que cette chute existe également au sein de la phase supraconductrice (à champ magnétique nul), montrant qu’elle n’est influencée ni par la présence de la supraconductivité ni par le champ magnétique. Dans l’état normal, la loi de Wiedemann-Franz est respectée prouvant le caractère métallique de la phase pseudogap.La chaleur spécifique électronique montre un comportement non classique à proximité du point critique. Ce comportement anormal est caractérisé par une dépendance logarithmique en fonction de la température au dopage critique p* correspondant à la chute du nombre de porteurs. De plus, ces mesures suggèrent une divergence de la masse effective à p* en fonction du dopage. Ces deux observations sont la signature d’un point critique quantique localisé à T = 0 et p = p* dont l’origine est discutée dans la dernière partie. Les différentes classes d’universalités possibles sont discutées et une comparaison avec d’autres composés (fermions lourds, pnictures) possédant un point critique quantique est présentée. / This experimental PhD thesis explores the properties of the pseudogap critical point in the phase diagram of superconducting cuprates. In a first part, I present a state of the art on the knowledge of the temperature-doping (T-p) phase diagram of these systems. Recent studies show a dramatic drop in the electronic carrier density near the critical point, suggesting a Fermi surface reconstruction. To understand the exact nature of the phase transition related to this reconstruction, I performed complementary high magnetic field measurements of thermal transport and specific heat on La1.8-xSrxEu0.2CuO4 and La1.6-xSrxNd0.4CuO4 cuprates.In a second part, after a theoretical introduction on specific heat and thermal transport, I detail how these two quantities were measured. In particular, an original technique for measuring specific heat has been developed to combine high resolution and absolute accuracy in high magnetic field and low temperature. Different thermal and electronic models have been developed to understand and analyze the measurements in order to optimize the different set-ups according to the temperature range.In a third part, I present the results obtained in thermal transport and specific heat. Thermal transport confirms the drop in carrier density in the normal state (without superconductivity) of cuprates, already observed in high magnetic field electrical transport. Moreover, this drop also exists within the superconducting phase (in zero magnetic field), showing that it is neither influenced by the presence of superconductivity nor by the magnetic field. In the normal state, the Wiedemann-Franz low is satisfied, proving the metallic character of the pseudogap phase.Electronic specific heat shows non-classical behavior in the vicinity of the critical point. This abnormal behavior is characterized by a logarithmic dependence as a function of temperature at the critical doping p *, corresponding to the drop in the carrier density. Moreover, these measurements suggest a divergence of the effective mass at p * as a function of doping. These two observations are the signature of a quantum critical point located at T = 0 and p = p *, whose origin is discussed in the last part. I discuss the possible universality classes, and I compare with others compounds (heavy fermions, pnictides) which present a quantum critical point.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Desde a descoberta do estado quasicristalino por Daniel Shechtman et al. em 1984 e da fabricação por Roberto Merlin et al. de uma superrede artificial de GaAs/ AlAs em 1985 com características da sequência de Fibonacci, um grande número de trabalhos teóricos e experimentais tem relatado uma variedade de propriedades interessantes no comportamento de sistemas aperiódicos. Do ponto de vista teórico, é bem sabido que a cadeia de Fibonacci em uma dimensão se constitui em um protótipo de sucesso para a descrição do estado quasicristalino de um sólido. Dependendo da regra de inflação, diferentes tipos de estruturas aperiódicas podem ser obtidas. Esta diversidade originou as chamadas regras metálicas e devido à possibilidade de tratamento analítico rigoroso este modelo tem sido amplamente estudado. Neste trabalho, propriedades de localização em uma dimensão são analisadas considerando-se um conjunto de regras metálicas e o modelo de ligações fortes de banda única. Considerando-se o Hamiltoniano de ligações fortes com um orbital por sítio obtemos um conjunto de transformações relativas aos parâmetros de dizimação, o que nos permitiu calcular as densidades de estados (DOS) para todas as configurações estudadas. O estudo detalhado da densidade de estados integrada (IDOS) para estes casos, mostra o surgimento de plateaux na curva do número de ocupação explicitando o aparecimento da chamada escada do diabo" e também o caráter fractal destas estruturas. Estudando o comportamento da variação da energia em função da variação da energia de hopping, construímos padrões do tipo borboletas de Hofstadter, que simulam o efeito de um campo magnético atuando sobre o sistema. A natureza eletrônica dos auto estados é analisada a partir do expoente de Lyapunov (γ), que está relacionado com a evolução da função de onda eletrônica ao longo da cadeia unidimensional. O expoente de Lyapunov está relacionado com o inverso do comprimento de localização (ξ= 1 /γ), sendo nulo para os estados estendidos e positivo para estados localizados. Isto define claramente as posições dos principais gaps de energia do sistema. Desta forma, foi possível analisar o comportamento autossimilar de cadeias com diferentes regras de formação. Analisando-se o espectro de energia em função do número de geração de cadeias que seguem as regras de ouro e prata foi feito, obtemos conjuntos do tipo-Cantor, que nos permitiu estudar o perfil do calor específico de uma cadeia e Fibonacci unidimensional para diversas gerações / Since the discovery of a quasicrystalline state by Daniel Shechtman et al. in 1984 and the growth of artificial GaAs/AlAs superlattices on nonperiodic Fibonacci sequence by Roberto Merlin et al., a number of theoretical and experimental works have reported a variety of interesting physical properties of aperiodic systems. Theoretically, it is well known that in one dimension, the Fibonacci chain is a successful prototype to describe a quasicrystalline state. Depending on the in ation rule, different kinds of aperiodic structures can be obtained. This diversity originates the called metallic means, and due to the possibility of analytical and rigorous mathematical treatments the Fibonacci model has been applied by several authors. In this work, electronic localization properties are studied, taking into account a set of metallic means in one dimension. Considering a single band tight-binding Hamiltonian, a set of decimation transformations is obtained allowing the calculation of the Density of States (DOS) for all configurations. The detailed study of the Integrated Density of States (IDOS), shows the appearance of plateaux in the occupation number curve exhibiting the so-called "devil's staircase"indicating the fractal nature of the structures. Studying the behavior of the energy as a function of the hopping we derive Hofstadter butter y type patters, which simulate the effect of a magnetic field acting on the system. The electronic nature of the eigenstate is analyzed by looking at the Lyapunov exponent which is related to the evolution of the electronic wave unction along the one dimensional chain. Since it is zero for an extended state and positive for a localized one, defining the main gaps positions, it is related to the inverse of the localization length. Through a careful analysis of the Lyapunov curves it was also possible to obtain the perfect self-similarity structures for all chains. In particular,for the chains that follow the golden and silver rules, the study of the energy behavior was done by analyzing the energy spectrum as a function of the generation number of each one of the chains. The results yield Cantor-like sets, which allowed us to calculate the specific heat profile for several generations of the one-dimensional Fibonacci chain.

Made available in DSpace on 2015-03-03T15:16:23Z (GMT). No. of bitstreams: 1 DarlanAM.pdf: 1538525 bytes, checksum: af051bdcc4894d0bf236b2e6f943baa9 (MD5) Previous issue date: 2008-09-29 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Esta tese apresenta um abrangente e atualizado estudo de algumas propriedades f?sicas da mol?cula do DNA, tais como seus aspectos termodin?micos (calor espec?fico) e eletr?nicos (transmissividade eletr?nica, fator de localiza??o, entre outros). A mol?cula do DNA pode ser considerada uma seq??ncia simb?lica de quatro letras: guanina (G), adenina (A), citosina (C) e timina (T). Ela ? usualmente descrita como uma cadeia bidimensional aleat?ria com correla??o de curto-alcance, mas n?o h? impedimentos para que a cadeia seja crescida seguindo seq??ncias quasi-peri?dicas, como por exemplo, as sequ?ncias de Fibonacci e Rudin-Shapiro. Com o intuito de investigar a relev?ncia das correla??es subjacentes nas distribui??oes dos nucleot?deos, comparamos os resultados para a sequ?ncia gen?mica do DNA (Ch22) com as duas seq??ncias artificiais citadas acima, que possuem correla??ao de longo alcance. A an?lise do calor espec?fico ? feita considerando-se formalismos apropriados; o cl?ssico, utilizando a distribui??o de Maxwell-Boltzmann; a descri??oo qu?ntica, utilizando a distribui??o de Fermi-Dirac; e o formalismo da n?o-extensividade, usando a entropia de Tsallis. Os espectros de energias s?o calculados utilizando-se a equa??o de Schrodinger unidimensional na aproxima??o de liga??o forte. N?s calculamos tamb?m a transmissividade eletr?nica, o comprimento de localiza??o, bem como I (corrente) vs V (potencial), curva que caracteriza as propriedades el?tricas de uma mol?cula de DNA dupla fita. O modelo te?rico considerado faz uso de um Hamiltoniano efetivo com aproxima??o de liga??o-forte descrevendo um el?tron movendo-se em uma cadeia com um simples orbital por s?tio e intera??es entre vizinhos mais pr?ximos, juntamente com a equa??o de Schrodinger, e a muito conveniente t?cnica da matriz de transfer?ncia

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Desde a descoberta do estado quasicristalino por Daniel Shechtman et al. em 1984 e da fabricação por Roberto Merlin et al. de uma superrede artificial de GaAs/ AlAs em 1985 com características da sequência de Fibonacci, um grande número de trabalhos teóricos e experimentais tem relatado uma variedade de propriedades interessantes no comportamento de sistemas aperiódicos. Do ponto de vista teórico, é bem sabido que a cadeia de Fibonacci em uma dimensão se constitui em um protótipo de sucesso para a descrição do estado quasicristalino de um sólido. Dependendo da regra de inflação, diferentes tipos de estruturas aperiódicas podem ser obtidas. Esta diversidade originou as chamadas regras metálicas e devido à possibilidade de tratamento analítico rigoroso este modelo tem sido amplamente estudado. Neste trabalho, propriedades de localização em uma dimensão são analisadas considerando-se um conjunto de regras metálicas e o modelo de ligações fortes de banda única. Considerando-se o Hamiltoniano de ligações fortes com um orbital por sítio obtemos um conjunto de transformações relativas aos parâmetros de dizimação, o que nos permitiu calcular as densidades de estados (DOS) para todas as configurações estudadas. O estudo detalhado da densidade de estados integrada (IDOS) para estes casos, mostra o surgimento de plateaux na curva do número de ocupação explicitando o aparecimento da chamada escada do diabo" e também o caráter fractal destas estruturas. Estudando o comportamento da variação da energia em função da variação da energia de hopping, construímos padrões do tipo borboletas de Hofstadter, que simulam o efeito de um campo magnético atuando sobre o sistema. A natureza eletrônica dos auto estados é analisada a partir do expoente de Lyapunov (γ), que está relacionado com a evolução da função de onda eletrônica ao longo da cadeia unidimensional. O expoente de Lyapunov está relacionado com o inverso do comprimento de localização (ξ= 1 /γ), sendo nulo para os estados estendidos e positivo para estados localizados. Isto define claramente as posições dos principais gaps de energia do sistema. Desta forma, foi possível analisar o comportamento autossimilar de cadeias com diferentes regras de formação. Analisando-se o espectro de energia em função do número de geração de cadeias que seguem as regras de ouro e prata foi feito, obtemos conjuntos do tipo-Cantor, que nos permitiu estudar o perfil do calor específico de uma cadeia e Fibonacci unidimensional para diversas gerações / Since the discovery of a quasicrystalline state by Daniel Shechtman et al. in 1984 and the growth of artificial GaAs/AlAs superlattices on nonperiodic Fibonacci sequence by Roberto Merlin et al., a number of theoretical and experimental works have reported a variety of interesting physical properties of aperiodic systems. Theoretically, it is well known that in one dimension, the Fibonacci chain is a successful prototype to describe a quasicrystalline state. Depending on the in ation rule, different kinds of aperiodic structures can be obtained. This diversity originates the called metallic means, and due to the possibility of analytical and rigorous mathematical treatments the Fibonacci model has been applied by several authors. In this work, electronic localization properties are studied, taking into account a set of metallic means in one dimension. Considering a single band tight-binding Hamiltonian, a set of decimation transformations is obtained allowing the calculation of the Density of States (DOS) for all configurations. The detailed study of the Integrated Density of States (IDOS), shows the appearance of plateaux in the occupation number curve exhibiting the so-called "devil's staircase"indicating the fractal nature of the structures. Studying the behavior of the energy as a function of the hopping we derive Hofstadter butter y type patters, which simulate the effect of a magnetic field acting on the system. The electronic nature of the eigenstate is analyzed by looking at the Lyapunov exponent which is related to the evolution of the electronic wave unction along the one dimensional chain. Since it is zero for an extended state and positive for a localized one, defining the main gaps positions, it is related to the inverse of the localization length. Through a careful analysis of the Lyapunov curves it was also possible to obtain the perfect self-similarity structures for all chains. In particular,for the chains that follow the golden and silver rules, the study of the energy behavior was done by analyzing the energy spectrum as a function of the generation number of each one of the chains. The results yield Cantor-like sets, which allowed us to calculate the specific heat profile for several generations of the one-dimensional Fibonacci chain.

L'usage des matériaux composites dans les différents domaines technologiques (microélectronique, aéronautique, transports…) ne cesse de croître. Une telle augmentation vient du fait qu'il est possible de développer de nouveaux matériaux avec des propriétés adaptées à une application bien préc ise en combinant les propriétés physiques des différents constituants. Dans le travail de thèse, nous nous intéressons à l'étude des propriétés thermophysiques, électriques et diélectriques de composites à base de matrice polymère chargée avec des fibres naturelles et/ou de particules minérales. L'objectif final étant d'une part d'accroître notre connaissance sur le mécanisme de transfert (thermique, électrique et diélectrique) au sein des matériaux composites et d'autre part, de développer une méthode de mesure des propriétés thermophysiques des matériaux à différentes températures (-20°C etlt; T etlt; 180°C) / The use of composite materials in various fields of technology (microelectronics, aerospace, transportation ...) continues to grow. Such an increase is that it is possible to develop new materials with properties tailored to a specific application by combining the physical properties of different constituents.In the thesis, we focus on the study of thermophysical properties, electrical and dielectric of composites based on polymer matrix loaded with natural fibers and/or mineral particles.The final goal is to increase our knowledge on the mechanism of transfer (thermal, electrical and dielectric) in composite materials and secondly, to develop a method for measuring thermophysical properties of materials at different temperatures (-20°C etlt; Tetlt; 180°C)

Hybrid composites forms an important field of research in the area of composite science and engineering as it gives the advantage of avoiding complex lay-up designs and provides better tuning compatibility to get desired properties in comparison with their mono-fiber counterpart. Further, utilization of composites for low temperature structures has been hindered by inconsistency of material property data and not much is reported on thermal characteristics of hybrid composites at cryogenic temperatures. This research work is focused on development of carbon-glass epoxy hybrid composite and to study the thermal behavior of these materials in comparison to its mono-fiber counterparts especially at cryogenic temperatures. The objectives are classified into the following three parts: Development of a hybrid composite with urethane modified epoxy matrix system (toughening agent used is Propyltrimethoxysilane (PTMO) and Toluene Di-Isocyanate (TDI) is added to get the polyurethane structure), for cryogenic applications. Study and understand the limitations and complexities of the experimental methodologies involved in evaluating the thermal properties of these materials namely thermal conductivity, coefficient of thermal expansion and specific heat. Finally to look into the appropriate theoretical calculations and experimental results to understand the variations, if any, for these materials. Specifically the following contributions are reported in this thesis: Evaluated the modified matrix system for its physical and mechanical properties at 20K. Specimens were prepared with D638 ASTM standard, modified to suit pin loading configuration in the cryostat/Instron machine. After assessing the suitability of the matrix system, mono fibre composites of different types were made and evaluated their thermal properties viz, coefficient of thermal expansion, thermal conductivity and specific heat down to 20K. Based on the results of the above, a hybrid composite configuration was evolved which exhibits optimal thermal characteristics at low temperatures and its characterization for various thermal properties at cryogenic temperatures was carried out. Comparisons of the experimental results were made with macro-mechanical model and micro-mechanical model (rule of mixtures) of composite materials. The present work throws light to the fact that hybrid polymer matrix composites can very well be considered for cryogenic applications where the combination or trade off between properties like strength to conductivity ratio, modulus to conductivity ratio and low cost is to be made. The mechanical properties of hybrid composites also need to be studied to complement the study on thermal properties reported in this thesis. It is essential to have a complete understanding of behaviour of these materials at cryogenic temperatures with respect to both thermal and mechanical properties as it is evident from the available literature that the emerging demands are multi-disciplinary in nature. The present research work is aimed at highlighting the use of hybrid composites to achieve the desirable thermal properties and thereby inviting the attention of scientists and engineers who are engaged in the design of cost effective structures and appliances for cryogenic environments to focus on further research to develop

Im Jahr 1964 wurde eine Theorie der Supraleitung vorgestellt, welche Cooper-Paarbindungen mit nichtverschwindendem Gesamtimpuls berücksichtigt. Sie wird nach den maßgeblich beteiligten Physikern P. Fulde, R. A. Ferrell, A. I. Larkin und Y. N. Ovchinnikov als FFLO-Supraleitung bezeichnet [1, 2]. Aufgrund recht anspruchsvoller Voraussetzungen kommen nur wenige Festkörper-Systeme in Frage, die eine FFLO-Phase ausbilden könnten. Im Jahr 2007 konnte R. Lortz durch Messungen der spezifischen Wärme an dem organischen Supraleiter kappa-(ET)2Cu(NCS)2 einen soliden Nachweis für eine weitere thermodynamische Supraleitungs-Phase in hohen Magnetfeldern erbringen [3]. ET steht hierbei für Bis-(ethylen-dithiolo)-tetrathiafulvalen. Die Hochfeld-Phase von kappa-(ET)2Cu(NCS)2 erfüllt alle bekannten Bedingungen für einen FFLO-Zustand. Diese Arbeit befasst sich mit der Erbringung eines gleichwertigen Beweises einer thermodynamischen Hochfeld-Supraleitungs-Phase in dem quasi-zweidimensionalen und vollständig organischen Supraleiter beta\'\'-(ET)2SF5CH2CF2SO3 durch hochauflösende Messungen der spezifischen Wärme. Darüber hinaus sollte durch eine präzise Ausrichtung der Probe zum Magnetfeldvektor die Feldorientierungsabhängigkeit der spezifischen Wärme und damit der supraleitenden Phasen bestimmt werden. [1] - P. Fulde and R.A. Ferrell, Phys. Rev., 135:A550, (1964). [2] - A.I. Larkin and Y.N. Ovchinnikov, Zh. Eksp. Teor. Fiz., 47:1136,(1964). [3] - R. Lortz et al., Phys. Rev. Lett., 99:187002, (2007).

In this dissertation, intermetallic phases Fe2Al5, Fe5Al8, Al3FeSi2 have been studied using density functional theory (DFT). The theoretical methods allow for parameter-free predictions without any experimental input of thermodynamical data like specific heat or phase stabilities. Such information is of great interest to thermodynamical modelling. Another strength of DFT is the possibility to investigate chemical reactions as demonstrated in case of the formation of hercynite (Fe2AlO4) in a solid-state reaction of corundum (α - Al2O3) and iron. Further, the Al2O3 surface energies of various planes were investigated. In addition, interfaces, which are of experimental interest within CRC 920, were modelled: α - Al2O3 (0001) || Al(111), TiO2 (rutile) || MgTiO3 (geikielite) and α - Al2O3 (corundum).

Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.

The search for novel quantum states is a fundamental theme in condensed matter physics. The almost boundless number of possible materials and complexity of the theory of electrons in solids make this both an experimentally and theoretically exciting and challenging research field. Particularly, the concept of quantum criticality resulted in a range of discoveries of novel quantum phases, which can become thermodynamically stable in the vicinity of a second order phase transition at zero temperature due to the existence of quantum critical fluctuations. One of the materials in which a novel quantum phase is believed to form close to a proposed quantum critical point is Sr₃Ru₂O₇. In this quasi-two-dimensional metal, the critical end point of a line of metamagnetic first order phase transitions can be suppressed towards zero temperature, theoretically leading to a quantum critical end point. Before reaching absolute zero, one experimentally observes the formation of an anomalous phase region, which has unusual ‘nematic-like’ transport properties. In this thesis magnetocaloric effect and specific heat measurements are used to systematically study the entropy of Sr₃Ru₂O₇ as a function of both magnetic field and temperature. It is shown that the boundaries of the anomalous phase region are consistent with true thermodynamic equilibrium phase transitions, separating the novel quantum phase from the surrounding ‘normal’ states. The anomalous phase is found to have a higher entropy than the low and high field states as well as a temperature dependence of the specific heat which deviates from standard Fermi liquid predictions. Furthermore, it is shown that the entropy in the surrounding ‘normal’ states increases significantly towards the metamagnetic region. In combination with data from other experiments it is concluded that these changes in entropy are most likely caused by many body effects related to the underlying quantum phase transition.