U6Fe has the highest superconducting transition temperature TSC ~ 4 K out of all of the U-based compounds. Unusually, the Pauli limit (1:84TSC = 7:36 T) is less than the observed critical field for both the a and c axes in this tetragonal material. Neither Pauli or usual BCS orbital limit is apparently respected. In order to explain why superconductivity exceeds the Pauli limit, it must be considered that either the superconducting state is unaffected by paramagnetic effects, or there is a large amount of spin-orbit scattering. Superconductivity is in the dirty limit for typical samples of U6Fe, which means that the latter cannot be precluded. Another unusual property of the superconducting state of U6Fe is that TSC has a positive dependence on the applied pressure P, for P < 4 kbar. This combined with other subtle signals in various measurements have led to the suggestion that a Charge Density Wave (CDW) state may exist in U6Fe below 110 K. The CDW state is typically favoured by materials with low-dimensional structural features such as chains of atoms. Such materials, if superconductors, are also candidates to exhibit the sought-after Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, an unusual state in which the superconducting order parameter is modulated in real space. The FFLO is expected to be suppressed except in clean, Pauli limited materials. Therefore if U6Fe can be produced at high enough quality to bring the superconducting state into the clean limit, it would be a potential candidate for an FFLO state. Part of this project discusses apparatus and techniques applied with the goal of producing such quality samples of U6Fe. We have succeeded in the application of the Solid State Electrotransport (SSE) method to purifying samples, and have been able to replicate the highest Residual Resistivity Ratios (RRRs) achieved (~9, compared to 4 for typical samples), but for single crystals instead of the polycrystals produced in the past. In parallel with the progress made towards higher quality samples of U6Fe, a new X-ray scanner has been developed for grain mapping of samples. This has found application in the course of our synthesis studies. The best quality samples have been studied by X-ray diffraction on the XMaS beamline at the ESRF in Grenoble, France. Below TCDW ~ 10 K, satellites at (δH; δK; 0) = (±0:11;±0:11; 0) were observed that confirm a CDW state, albeit at much lower temperatures than anticipated. By examination of systematic satellite absences we have determined that the displacement vector → u is perpendicular to the modulation direction in k-space. Additionally it has been found that the symmetry of the lattice below TCDW is reduced from that of the room temperature I4=mcm structure. The appearance of additional Bragg peaks below ~110 K during these experiments were later cast into doubt by multiple scattering. We have, however, detected a signal in the form of a jump at ~110 K in specific heat measurements of our samples. These measurements also show a kink near to TCDW. We have additionally extended the investigation of the effect of pressure on the superconducting state. The maximum of TSC is confirmed in our samples, and the subsequent suppression of TSC and Hc2 is investigated up to 8 GPa. We have analysed our Hc2(T) curves at different pressures under a simple two-band model that fits the observed trends well and suggest that at the highest pressures U6Fe is approaching even more unusually enhanced Hc2 values.

The topological aspects of superconductivity on doped Weyl semimetals are investigated. / Topological phases of matter have sparked significant experimental and theoretical interest due to the topologically robust edge modes they host, as well as their classification through rich mathematics. An interesting example of a topological phase in three dimensions, the Weyl semimetal, can exhibit topological ordering through the existence of Fermi arcs on the surfaces of the material. For the doped Weyl semimetal, we investigate possible resulting Weyl superconducting states --- both the inter-Fermi surface pairing state following Li and Haldane, and the intra-Fermi surface pairing state following Burkov --- in this thesis, and study their topological properties by computing the gapless Weyl-Majorana edge modes they host. The results obtained in Ref. \cite{LH} for the inter-Fermi surface superconducting state are reproduced, and the bulk and edge properties of the intra-Fermi surface pairing superconducting state are studied in detail. / Thesis / Master of Science (MSc) / In this thesis, we study an interesting class of topological materials called the Weyl semimetal as well as its associated superconducting phases. A description of the Fermi arcs on Weyl semimetals are given, and the topological properties of the inter-Fermi surface and intra-Fermi surface pairing states are studied in detail.

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).

The understanding of new emerging unconventional ground states is a great challenge for experimental and theoretical solid-state physicists. New ground states are developing, where different energy scales compete, leading to a high sensitivity of the system to external tuning parameters like doping, pressure or magnetic field. The exploration of superconductivity proved to be a fascinating and challenging scientific undertaking. Discovered by H. Kammerlingh Onnes in 1911, prior to the development of the quantum theory of matter, superconductivity was defying a microscopic theory for more than four decades until the BCS theory was formulated in 1957 by J. Bardeen, L. N. Cooper and J. R. Schrieffer. Superconductivity of most of the simple metals or metallic alloys is well described within the frame of the BCS scenario, however, in the last thirty years numerous new superconducting materials were found to exhibit exotic properties not accounted for by the BCS theory. Among them are included the high-Tc compounds, the heavy-fermion superconductors and as well the organic superconductors. It was the purpose of this work to probe different facets of superconductivity in heavy-fermion and in low-dimensional metallic compounds. This dissertation is divided into six chapters. After this introduction, in Chapter 1 we will outline the basic theoretical concepts later needed for the analysis of the experimental results. In Chapter 2 we briefly introduce the experimental techniques with a special focus on the new pressure cells developed during this thesis and used for the measurements presented in Chapters 3 to 5. In Chapter 3 the possible realization of the inhomogeneous superconducting FFLO state in CeCoIn5 is studied by specific heat measurements under hydrostatic pressure, while in Chapter 4 the results of AC specific heat experiments on UBe13 under uniaxial pressure are presented. The ambient pressure properties as well as results obtained by resistivity measurements under hydrostatic pressure on the one-dimensional metallic compounds TlxV6S8 are discussed in Chapter 5. At the end, Chapter 6 summarizes and concludes this thesis.

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).

L'état supraconducteur étant un condensat de paires de Cooper constitué d'électrons de moments et de spins opposés, il peut être fortement influencé par des effets de spin. Au cours de cette thèse, nous étudions l'effet d'un fort champ d'échange et d'un effet spin-orbite de type Rashba sur les propriétés supraconductrices. Dans une première partie, on étudie les effets associés à l'interaction entre supraconductivité et fort champ d'échange, se caractérisant par une transition de phase vers un état supraconducteur inhomogène découvert par Fulde, Ferrell, Larkin et Ovchinnikov (FFLO). On étudie tout particulièrement les fluctuations supraconductrices à l'approche de la transition de phase. On montre que ces fluctuations peuvent servir de révélateur à cette phase. Notamment, la capacité calorifique et la paraconductivité divergent de façon caractéristique à la transition vers un état modulé. On décrit également comment les effets paramagnétiques modifient les fluctuations de l'aimantation, annulant la réponse diamagnétique ou produisant des oscillations entre réponse para- et dia-magnétique. La seconde partie est dévolue aux jonctions supraconducteur-ferromagnétique (S/F). Dans les jonctions Josephson S/F/S, le champ d'échange donne lieu à des oscillations du courant critique en fonction de la longueur de la jonction, charactérisées par une alternance des états 0 et . On prédit une transition entre les états 0 et induite par la température, même dans la limite ballistique. Dans cette limite ballistique, on montre également que le courant de Josephson s'atténu sous la forme de lois de puissance en fonction de la longueur de la jonction, alors que le cas diffusif présente une atténuation exponentielle. On étudie ensuite la seconde harmonique de la relation courant-phase en présence d'une faible quantité d'impuretés. La dernière partie traite des effets de proximité lorsque les deux effets paramagnétique et spin-orbite sont présents dans une jonction Josephson. On montre que l'association d'une interaction Rashba et d'un champ d'échange induit un couplage direct entre les ordres magnétique et supraconducteur. En particulier, ce couplage permet de générer toute la dynamique magnétique par l'application d'une simple tension électrique. / The superconducting state being a Cooper pair condensate built on opposite spin and momentum electrons, it can be strongly influenced by any spin effect. In this thesis, we investigate the roles of strong paramagnetic and spin-orbit effects on superconducting properties. In a first part, the interplay between paramagnetic effect and bulk superconductivity is studied, leading to the modulated Fulde, Ferrell, Larkin and Ovchinnikov phase (FFLO phase). We focus on superconducting fluctuations near to the FFLO state. We show that these fluctuations can serve as a smoking gun for this phase. Noticeably, the fluctuation heat capacity and paraconductivity diverge in a characteristic way when approaching the phase transition towards a modulated state. Moreover, the fluctuation induced magnetization is predicted to be drastically quenched or to oscillate between dia- and para-magnetic responses. The second part is devoted to superconductor-ferromagnetic (S/F) junctions. In S/F/S Josephson junctions, the exchange field is responsible for the critical current oscillation, characterized by alternative 0- and -states, with respect to the junction length. We predict a temperature induced (0-) state transition, even in the ballistic case. Moreover, the ballistic case exhibits some power law decays of the Josephson current, in contrast to the exponentially decaying current in dirty limit. The moderately dirty limit is then investigated, and the second harmonic of the current-phase relation is established. The last part deals with proximity effects when both paramagnetic and spin-orbit interactions are present in a Josephson junction. We show that the association of both Rashba interaction and exchange field induces a direct coupling between magnetic and superconducting orders. Particularly, this coupling generates the complete magnetization dynamics by applying an appropriate d.c. voltage.

Les progrès techniques réalisés dans le cadre des expériences sur les gaz de fermions ultrafroids ont engendré une émulation particulièrement importante ces dernières années. En effet, ces dispositifs expérimentaux permettent de produire des systèmes gazeux ≪ à la carte ≫, notamment grâce au phénomène de résonances de Feshbach qui permet de contrôler le signe de la longueur de diffusion a par application d'un champ magnétique extérieur. Il est alors possible de générer aussi bien une interaction attractive (a < 0) que répulsive (a > 0). La résonance de Feshbach en elle-même se trouve en a → ±∞, cette limite correspondant à un régime de fortes corrélations entre les particules. De plus, dans la région où a est positive, des états lies moléculaires (bosoniques car formés de deux fermions) peuvent se former. En-dessous d'une certaine température, une phase superfluide peut alors apparaitre, et une transition de phase continue entre l'état bosonique et l'état fermionique peut être observée (BEC-BCS crossover). En fonction de la position dans le diagramme de phases, les modes collectifs possèderont des caractéristiques (fréquence, amortissement) différentes. En ce sens, ils constituent une sonde de l'état de la matière et une connaissance précise de ces modes est par conséquent très importante. Le travail présenté dans cette thèse comporte une caractérisation détaillée de plusieurs modes collectifs dans la phase normale du système atomique. L'étude repose principalement sur l'équation de Boltzmann, que nous résolvons de deux façons différentes. La première consiste à utiliser une méthode des moments ≪ améliorée ≫ (c'est-à-dire d'ordre supérieur). La seconde est numérique et a nécessité l'écriture d'un programme de simulation permettant l'incorporation de tous les effets de milieu (potentiel de champ moyen et section efficace). Une attention toute particulière a été apportée à la mise en place des simulations afin de reproduire le plus fidèlement possible les conditions expérimentales. Les techniques expérimentales permettent également désormais la création de gaz polarisés. Nous présenterons donc dans ce travail une étude de ces gaz utilisant notre programme de simulation (mise en évidence des différents régimes de collision), puis une étude plus théorique ayant pour principal objectif d'établir le diagramme de phase encore méconnu de ces gaz particuliers, et enfin de proposer une méthode de calcul des effets de milieu, les techniques habituelles utilisées pour les gaz non polarisés n'étant plus valables / Technical progress on ultra-cold Fermi gases experiments induced numerous studies for the last few years. Using these experimental setups, it is effectively possible to generate ultra-cold gases with selected properties, in particular through the Feshbach resonances phenomenon. This allows us to set the sign of the scattering length a using an external magnetic field. It is then possible to have an attractive interaction (a < 0) as well as a repulsive one (a > 0). The Feshbach resonance itself is defined for infinite values of a (positive or negative), which corresponds to a strongly interacting regime. Moreover, when a > 0, molecular bound states (bosonic because they are made with two fermionic atoms) can appear. Thus, below a critical temperature, a superfluid phase can emerge and a crossover can be observed (from the BEC to BCS superfluid states). Depending on the position on the phase diagram, frequency and damping of collective modes will be different. This is why the collective modes are good probes of the system phase. A precise extensive knowledge of their characteristics is thus very important. This thesis presents a complete study of some of these collective modes in the normal phase. This work mainly relies on the Boltzmann equation which will be solved in two different ways: firstly, with an improved (higher order) version of the so-called moments method; secondly with a numerical solution that has required to write a numerical code in order to take into account the in-medium effects (mean field potential and in-medium cross section). Particular attention has been paid to numerical simulations in order to reproduce as closely as possible the experimental conditions. Moreover, experimental procedures now allow to create spin unbalanced gases. We have shown in this work a study of these systems using the numerical resolution of the Boltzmann equation. Moreover, we have developed a theoretical approach in order to build the phase diagram of these polarized gases, which is not fully described yet. Finally, we have suggested a method to determine the in-medium effects, with the aim to solve the problem emerging with the usual method used in the balanced case

The understanding of new emerging unconventional ground states is a great challenge for experimental and theoretical solid-state physicists. New ground states are developing, where different energy scales compete, leading to a high sensitivity of the system to external tuning parameters like doping, pressure or magnetic field. The exploration of superconductivity proved to be a fascinating and challenging scientific undertaking. Discovered by H. Kammerlingh Onnes in 1911, prior to the development of the quantum theory of matter, superconductivity was defying a microscopic theory for more than four decades until the BCS theory was formulated in 1957 by J. Bardeen, L. N. Cooper and J. R. Schrieffer. Superconductivity of most of the simple metals or metallic alloys is well described within the frame of the BCS scenario, however, in the last thirty years numerous new superconducting materials were found to exhibit exotic properties not accounted for by the BCS theory. Among them are included the high-Tc compounds, the heavy-fermion superconductors and as well the organic superconductors. It was the purpose of this work to probe different facets of superconductivity in heavy-fermion and in low-dimensional metallic compounds. This dissertation is divided into six chapters. After this introduction, in Chapter 1 we will outline the basic theoretical concepts later needed for the analysis of the experimental results. In Chapter 2 we briefly introduce the experimental techniques with a special focus on the new pressure cells developed during this thesis and used for the measurements presented in Chapters 3 to 5. In Chapter 3 the possible realization of the inhomogeneous superconducting FFLO state in CeCoIn5 is studied by specific heat measurements under hydrostatic pressure, while in Chapter 4 the results of AC specific heat experiments on UBe13 under uniaxial pressure are presented. The ambient pressure properties as well as results obtained by resistivity measurements under hydrostatic pressure on the one-dimensional metallic compounds TlxV6S8 are discussed in Chapter 5. At the end, Chapter 6 summarizes and concludes this thesis.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18059号 / 理博第3937号 / 新制||理||1567(附属図書館) / 30917 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 池田 隆介, 教授 松田 祐司, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Les hétérostructures de taille nanométrique comprenant des matériaux supraconducteurs (S) en contact avec des matériaux métalliques (N) ou ferromagnétiques (F) présentent des propriétés surprenantes. L'effet de proximité dans les structures F/S/F se manifeste par l'effet vanne de spin. Dans les jonctions S/F/S, on voit l'apparition d'une transition de phase 0-p. Ces propriétés dépendent des paramètres internes du ferromagnétiques. Dans la première partie, nous étudions l'influence d'un décalage énergétique et d'une différence de masse effective pour expliquer l'effet de vanne de spin inverse qui se manifeste dans certaines hétérostructures F/S/F. On étudie la transition de phase 0-p dans le cas de décalage énergétique et d'anisotropie des surfaces de Fermi.La seconde partie est consacré à l'étude de l'effet paramagnétique sur le diagramme (H,T) des bicouches S/N et S/S. On demontre qu'il se forme une phase de supraconductivité induite par champ magnétique à fort champ magnétique et faibles températures. Calculée en présence d'un phase supraconductrice inhomogène de type Fulde-Ferrell-Larkin-Ovchinikov (FFLO), on s'interesse également à l'influence des impuretés sur cette nouvelle phase supraconductrice à fort champ magnétique.La troisième partie est dévolue à l'étude des multicouches supraconducteur/métal normal(N). Le but de cette partie est d'étudier l'influence du nombre de couche et de décalage d'énergie sur la température critique, la densité d'état des multicouches S/N/.../N épaisses et de l'effet Josephson dans les multicouches S/N/.../N/S. / The atomic-scaled heterostructures with superconducting and ferromagnetic materials exhibit astonishing properties. For example, the proximity effect in the F/S/F sandwiches leads to the spin-valve effect. In the S/F/S junctions, one can observe 0-p phase transiton. These effects depend on the ferromagnetic properties.In the first part, we study the influence of energy shift and effective mass difference to explain the inverse spin valve effect. We also study the 0-p phase diagram and its dependence on the energy shifts and anisotropic sprectra in S/F/S junctions.The second part is devoted to the study of paramagnetic effet on the (H,T) phase diagram of the S/N and S/S bilayers. We demonstrate the formation of a superconducting field induced phase for high magnetic fields and low temperature. Calculated in presence of the superconducting inhomogeneous Fulde-Ferrell-Larkin-Ovchinikov (FFLO) state, we study the influence of the impurities on this new superconducting phase.The last part deals with the study of superconducting-normal metal(N) multilayers. We calculate the influence of the number of layers and energy shift on the density of state, the thermodynamical properties of the S/N/.../N thick multilayer and the Josephson current in the S/N/.../N/S thick junctions.