There was a fame time for silicon in condensed matter physics, then the graphene era came and now topological insulators are gaining lot of attention, but magnetism in condensed matter physics has remained always fascinating starting from the ancient days up to now and it will remain as one of the core topic in basic or applied physics. The improvement in the modern techniques allows one to explore magnetism in different length scales as well as in different time scales. As an effect of the improvement in experimental techniques, different magnetic anomalies are unearthed. As a result theories are getting refined and the area of magnetism progresses. From the material point of view, oxides carry the most diverse nature in condensed matter starting from high temperature superconductivity (HTS), colossal magnetoresistance, metal insulator transition etc. to ferromagnetism (FM), anti-ferromagnetism (AFM), spin glass (SG) and so on. Among this list, SG and HTS are one of the least understood topics in magnetism till today. A large research community is involved in understanding the underlying physics behind these two, especially in transition metal oxides. It has drawn attention not only due to fundamental aspects but also due to various applications in day to day life. This thesis is an attempt to understand these two phenomena in transition metal oxides. As the title of this thesis suggest, it is all about magnetic and magneto-transport properties of certain transition metal oxide (crystalline) addressing the interplay between two competing order parameters to understand the underlying physics behind it from an experimental point of view. We have studied two different kinds of competing interactions: (i) the FM/AFM interplay either in bulk or at the interface of the two layers in thin films; (ii) the interplay between FM and superconductivity (SC) in superconductor (S)/ferromagnet (F) heterostructures. Basically both of these two kinds lead to non-equilibrium phenomena in these oxides. One of such competition is between FM and AFM leading to slow dynamics (glassy physics). Disorder and frustrations are the key ingredients for such slow dynamics. The spin frustration arises either due to geometry or due to competing interactions. For example, in a triangular antiferromagnet due to the triangular geometry spins gets frustrated. Now, if it prevails spin disorder as well then it satisfies both the criteria for a spin glass and hence it gives birth to glassiness. Another kind of competition is the interplay among SC and FM. It is known that SC and FM are two antagonistic quantum phenomena thus in a single material SC (singlet pairing) and FM does not co-exist. However one can realize this by making F/S heterostructures and observe the battle between these two competing order parameters. The spin polarized quasiparticle injection from F creates non equilibrium spin density inside S and thereby suppressing the order parameter of S. Also by choosing an appropriate ferromagnet the vortex motion inside S can be arrested to certain extent which can enhance the critical current density of S. Thus FM/SC interplay has become an alternative way to look at the high temperature superconductivity. This thesis is categorized into nine chapters. The summary of each chapter is as follows: Chapter: 1 contains certain concepts of magnetism and superconductivity which is useful to understand the topics and experiments described in this thesis. Chapter: 2 gives the underlying principles of the various experimental techniques used in this thesis. Chapter: 3 describes the magnetic properties of successfully synthesized five compositions of LixNi(2-x)O2 (0.67<x<0.99) which has five distinct ground states namely antiferromagnet (AF), spin glass (SG), cluster glass (CG), re-entrant spin glass (RSG) and ferromagnet (FM). The SG and CG ground state has been well described by the frequency dependent peak shift. From the power-law divergence of critical slowing down the estimated value of relaxation time indicates the presence of interacting macro spins (spin cluster) rather than individual spins in certain LixNi(2-x)O2 samples possessing CG ground state which is also supported by the Arrhenius law. The shift in the spin freezing temperature with the application of dc field obeys Almeida-Thouless line. It also exhibits memory effect which is generic to the slow dynamics. The remnant magnetization relaxation follows logarithmic decay. Interestingly, the sample having RSG ground state shows memory effect up-to 50K and behaves like a FM above that temperature. FC-ZFC M(T) curve shows a splitting at the ordering temperature. The critical analysis across the ferromagnetic-paramagnetic phase transition yields a self-consistent γ, β and δ value and the spin-spin interaction in this material follows long range mean field model. The critical exponents obey Widom scaling law: δ = 1 + γ β −1. The universality class of the scaling relations is also verified where the scaled m and scaled h collapses into two branches. Finally the magnetic phase diagram illustrates a vivid picture of the gradual evolution of ferromagnetism in LixNi(2-x)O2 through a glassy state. As a concluding remark, we think, the present study of glassy physics in magnetic insulator/semiconductor sets an example to compare them with the conventional metallic spin glass system. Chapter: 4 exhibits the results of the structural, magnetic and transport measurements to elucidate some of the most striking unusual physical responses of bulk SrRuO3. Two set of polycrystalline SrRuO3 samples with unique ordering temperature have been synthesized. In one case, we have taken the stoichiometric weight ratio of precursors that eventually resulted in Ru-deficient SrRuO3(SROD). In the other case, we have taken extra 2% wt. RuO2 deliberately to form stoichiometric SrRuO3(SRO). Both the samples are found to crystallize in orthorhombic crystal structure with Pnma space group. The low temperature magnetization is found to be well described by the Bloch T3/2 law and the magnetization near Tc is found to follow the scaling law; M~(Tc-T)β with β=0.35 and β=0.30 for SRO and SROD respectively, apparently showing the 3D Ising behaviour. This aspect will be elaborated in the next chapter. The magnetic ac susceptibility study exhibits a broad hump far below the ferromagnetic ordering temperature and the frequency dependence of this hump position exhibits the characteristics of multiple relaxations. Most strikingly, we notice a low temperature glassy magnetic behaviour clearly demonstrated by the time dependent memory effect. This is very surprising and unlikely to happen in systems, which have itinerant ferromagnetic character. However, we conjecture that slow domain growth and spin canting could be the cause for such effect. The transport study evidences a crossover from Fermi liquid (FL) to non-Fermi liquid (NFL) behaviour around 40 K and a slope change in dρ/dT vs. T plot in the vicinity of that temperature. Astonishingly, we observe two distinct dips (one around ferromagnetic ordering temperature and the other far below the ferromagnetic ordering temperature) in the temperature dependent MR response. In addition, we also observe the signature of an unusual dip in the temperature dependent coercive field towards low temperature side. The emergence of such unusual magnetic and transport response is strongly believed to be connected with hidden magnetic interactions. Our effort on neutron diffraction study has been able to trace the cause of such cryptic magnetic interaction. The findings of neutron diffraction study evidence the change in the unit cell lattice parameters around 75 K and that could be the central cause behind such anomalous low temperature magnetic responses. It also demonstrates that the octahedral tilt freezes around the FM transition and has a minimum around the low temperature glass transition temperature. Most remarkably we observe a decline in the total integrated magnetic intensity towards the low temperature side indicating the presence of antiferromagnetic like interaction in SrRuO3. Chapter: 5 resolves the ambiguity in determining the crritical exponents in SrRuO3. Most remarkably, the application of scaling law in the FC magnetization leads a systematic change in the values of critical exponent with the measuring field in SRO. The β value changes from 0 to o.44 to to 0.29 (corresponds to mean field to Ising) with the increase in the measurement field from 10 to 2500 Oe. However, the H→0 extrapolation fields β=0.5. In order to substantiate the actual nature, the critical behavior is studied across the phase transition from the M-H isotherms. The critical analysis yields a self-consistent β, γ and δ values and the spin-spin interaction follows long range mean field δ=γ β model 1+. The critical exponents also obey Widom scaling law: δ = 1 + γ β-1 The universality class of the scaling relations is verified where the scaled m and scaled h collapses into two branches. We have also found that Ru deficiency does not affect the nature of the spin-spin interaction (though ferromagnetism gets reduced). Further the directional dependence of the critical exponent reflects the isotropic nature of the magnetic interaction. In other words the spin-spin interaction found to be: i) three dimensional, ii) long range, iii) mean field type and iv) isotropic in SrRuO3. We have also found magnetocaloric effect (calculated from the M-H isotherms) that across the phase transition. The specific heat measurements find sharp jump at the ferromagnetic transition due to the magnetic contribution of the specific heat. Chapter: 6 describes the magnetism at the SrRuO3 (SRO)/LaAlO3 (LAO) interface where SRO is an itinerant ferromagnet (FM) and LAO is non-magnetic (NM) (rather diamagnetic). Most surprisingly SRO/LAO exhibits pronounced exchange bias (EB) effect realized by observing a shift in the field cooled M-H hysteresis. Further investigation results an increasing trend of the strength of the EB with the decreases in the thickness of ferromagnetic layer. This system also displays the training effect which essentially confirms that this effect is due to EB. EB arises due to the uncompensated spins at the FM/AFM interface hence the EB effect in SRO/LAO system is unconventional. However, the origin of such AFM interaction (responsible for EB effect in FM/NM system) at SRO/LAO interface is realized and explained through the temperature dependence of the EB effect. Further, we have extensively investigated EB effect in other analogous ferromagnets, FM/FM bilayers and FM/FM superlattices. We found that La0.7Sr0.3MnO3 (LSMO) grown on LAO exhibits the signature of EB. In contrast to that La0.5Sr0.5CoO3 (LSCO) does not show any signature of EB. All the bilayers (LSMO/SRO, LSMO/LSCO and LSCO/SRO) exhibit EB and have similar kind of temperature dependence. In order to gain more insight we have grown a (LSMO/SRO)8 superlattice and observed a complex magnetic behaviour. It exhibits partial inverted magnetic hysteresis. But the system shows EB effect characterized by the shift in the FC hysteresis and training effect. All these observations essentially demonstrate that the magnetic nature of various ferromagnetisms at the interfaces can be changed by choosing a proper partner (acts like adding perturbations into one of those system which lies close to the instability region). Chapter: 7 presents the magneto-transport properties of three SRO films grown on LAO (100) of thicknesses of 12, 24 and 48 nm are studied extensively. For a one to one comparison one of the sample is also grown on STO(100). The coercivity vs. temperature in SRO(48 nm)/LAO exhibits a plateau at ~40 K. The dR/dT exhibits the low temperature hump in all the samples which very much replicates with the bulk scenario that we observed in SRO. Most strikingly the 12 nm SRO sample exhibits NFL behaviour throughout the temperature range of measurement (10-150 K). Our careful investigation reveals a cross-over from FL to NFL in all SRO thin films. The cross-over temperature increases with the increase in thickness and eventually shifts towards the bulk cross-over value. It is apt to remind that in bulk SRO we have demonstrated (by employing temperature dependent neutron diffraction) that there is a presence of antiferromagnetic like interaction at low temperature giving birth to glassiness in bulk SRO. Further, an attempt is made to understand the low temperature magneto-transport anomaly by looking into the spin fluctuation through the low frequency 1/f noise measurements. It conveys a message that there are two types of magnetic ordering present in SRO giving rise to two peaks in the temperature dependence of the relative variance. Application of magnetic field suppresses both the peaks in the relative variance. This certainly indicates that the origin of such peak is caused by the spin fluctuations and thereby it is of magnetic origin. Further we have looked into the Hall effect of a structured (Hall patterned) SRO thin film and observed regular Hall effect (RHE) as well as anomalous Hall effect (AHE) in it. Most remarkably the temperature dependence of the RHE coefficient changes its sign close to the ferromagnetic transition temperature of SRO. This implies a change of the type of the carrier as the temperature is varied. Based on these results, the carrier concentration of SRO as a function of temperature is determined. Chapter: 8 is about the magnetic and magnetotransport studies on the successfully grown high quality S/F heterostructures. The oxygen content plays a vital role in superconductivity of oxide materials thus for studying FM/SC interplay in oxides we have discussed how to achieve a high quality sample (oxygen stoichiometric). We have observed a great influence of a FM in suppressing the superconductivity in YBa2Cu3O(7-δ) (YBCO) in FM/SC heterostructures. The analysis of the out of plane M-H hysteresis reveals a significant reduction of the critical fields (HC1 and HC2) of the SC (in SRO/YBCO bilayer) which might have a great significance to understand the superconductivity in a better way (from both the perspectives: theory and experiments). Most remarkably we have found 40% enhancement of the critical current density of YBCO in SRO/YBCO bilayer. We have demonstrated that in order to see the effect of spin polarizes quasiparticle (SPQP) injection into YBCO, one should not apply more than 20mA current since Joule heating contribution wins over pair breaking effect. The SPQP injection from SRO into YBCO exhibits pair breaking effect as the TC (of the SC) shift follows I2/3 law. The resistive transitions under various applied magnetic fields and the field dependence of the activation energy confirms that the vortices are in the 2D regimes (it follows power law, U0~Hα withα=0.5) in SRO/YBCO. To get a better insight into the FM/SC interplay we have looked into two of the FM/YBCO combinations (LSCO/YBCO and LSMO/YBCO). We observe that the degree of the spin polarizations of the FMs scales with the suppression of superconductivity in YBCO which means more the spin polarization more is the suppression. We have also found out that spin polarization is not the sole parameter in suppressing superconductivity in SRO/YBCO bilayers. It also depends upon the state of magnetization of the ferromagnet. Further, we observed a significant reduction (one order) of the activation energy in LSCO/YBCO compared to SRO/YBCO which clearly indicates that the vortex dynamics might depend on other aspects as well (of the FM). It also reveals the formation of decoupled pancake vortices (pure 2D regime) in LSCO/YBCO and LSMO/YBCO bilayers whereas in case of YBCO and SRO/YBCO it is of 2D coupled type. Chapter: 9 summarizes the whole work presented in this thesis. It also discusses about few research problems which one need to look at in future.

Les limiteurs de courants de court-circuit supraconducteurs sont des appareillages à fort potentiel pour les réseaux électriques. En effet, ils offrent une limitation efficace dès les premiers instants du court-circuit. On peut qualifier la limitation de "naturelle", c’est-à-dire qu’elle est intrinsèquement liée aux caractéristiques du matériau et ne nécessite pas de commande particulière. Afin de faciliter la conception et l’intégration des limiteurs de courant de court-circuit supraconducteurs résistifs (rSFCL) destinés aux réseaux électriques, il est nécessaire de disposer de modèles de simulation précis. Ces derniers doivent prendre en compte et simuler correctement (et le plus précisément possible) les phénomènes électriques et thermiques du rSFCL en présence de surintensités de courant, qu’il s’agisse d’un court-circuit franc ou d’un phénomène temporaire de plus faible amplitude. Il est difficile d’envisager la planification de l’intégration d’un rSFCL sans passer par des outils numériques qui permettent la simulation d’un tel dispositif dans un réseau électrique en régime transitoire. Il est alors plus facile d’appréhender et de prédire le comportement transitoire du limiteur dans des conditions de stress réalistes, qui peuvent comprendre une grande variété de surintensités, tant en durée qu’en amplitude. Néanmoins, les rSFCL sont des dispositifs fortement non-linéaires caractérisés par un couplage électrique et thermique très fort. L’implémentation d’un tel modèle dans un logiciel de simulation de type “circuits électriques” en régime transitoire présente un certain défi. Bien que des modèles de rSFCL existent déjà, des améliorations doivent être apportées pour prendre en compte i) l’ensemble des phénomènes physiques liés à la limitation (thermiques et électriques), ii) les propriétés géométriques des rubans supraconducteurs utilisés et iii) la possibilité de réaliser des études globales (impact du limiteur sur le réseau) et iv) l’influence de l’architecture du ruban en présence de phénomènes locaux (points chauds). Cette thèse se concentre donc sur le développement d’un modèle de rSFCL basé sur des rubans supraconducteurs de deuxième génération. Ce modèle est développé dans le logiciel EMTP-RV, qui est un outil utilisé par un grand nombre de compagnies d’électricité dans le monde. Le modèle proposé dans cette thèse repose sur une analogie qui fait le lien entre les phénomènes électriques et thermiques, et qui permet une modélisation entièrement basée sur des éléments de circuits électriques. Le modèle permet de prendre également en compte les propriétés non linéaires des matériaux, tant au niveau électrique qu’au niveau thermique, avec l’utilisation de dipôles non-linéaires. Le modèle a été développé pour offrir un niveau de généricité intéressant pour la modélisation des rubans supraconducteurs. Il permet un fonctionnement avec une excitation AC ou DC en tension ou en courant et tient compte de la non-uniformité de courant critique, qui est typiquement observée dans la longueur des rubans disponibles commercialement. Il est également possible de représenter des variantes d’architectures (géométries et matériaux), avec une souplesse de modélisation qui est basée sur un assemblage de blocs “élémentaires” dont les dimensions peuvent être différentes. Cela permet alors d’évaluer, dans une même simulation, l’architecture du limiteur à une échelle submillimétrique (points chauds) et à une échelle “systémique”, tel que le comportement de plusieurs centaines de mètres de ruban. Des comparaisons ont permis de vérifier que le modèle circuit avait un comportement similaire à son équivalent en éléments finis, seulement si la taille des éléments électrothermiques de base (dans EMTP-RV) est adéquate. Le modèle équivalent circuit permet de réaliser des simulations de différentes architectures de rubans supraconducteurs, avec ou sans résistance d’interface, entre les couches tampons et la couche de (RE)BCO par exemple [...] / Superconducting fault current limiters (SFCL) are a promising technology for power systems, i.e. they provide efficient current limitation from the very beginning of the fault without requiring any control system. In fact, the current limiting characteristics are directly connected to the physical properties of superconducting materials. There is a need for accurate models to help designing resistive-type SFCLs (rSFCL) and planning their integration into electrical networks. Such models have to take into account the physics involved for simulating (as accurately as possible) the electrical and thermal behaviours for a wide range of fault conditions, i.e. high and low short-circuit currents that can be of various durations. It is difficult to see how the planning and integration of SFCLs can be realized without using numerical tools, especially tools that allow realizing power system transient simulations, such as EMTP-RV. In fact, such software packages support engineers in predicting the behaviour of SFCLs in realistic network conditions, which may comprise a wide variety of overcurrent or fault situations. However, rSFCLs exhibit highly non-linear behaviours with a strong coupling between thermal and electrical phenomena. The implementation of such a model in power systems simulation tools is therefore challenging. Although some models have been already developed over the years, improvements are needed to take into account i) all the phenomena linked to the current limitation (electrical and thermal), ii) geometric properties of superconducting tapes that are used in rSFCLs, and iii) the possibility to perform simulations at the system level, and iv) the influence of the tape architecture in relationship to local phenomena (hot spots). This thesis hence focuses on the development of a models for resistive-type SFCLs based on second generation high temperature superconducting coated conductors (2G HTS CCs), i.e. (RE)BCO tapes. The models are implemented in EMTP-RV, a tool that is used by many utilities around the world. However, the modeling technique can be adapted to other simulation tools as well. The model proposed in this thesis is based on an electro-thermal analogy, which allows modeling thermal effects with non-linear electrical circuit elements such as resistors and capacitors. The model has been developed with the aim of providing flexibility. Hence, it can be used with an AC or DC excitation, and can also take into account non-uniformity in critical critical current density typically observed along length of the conductors (i.e. tapes). It also allows modeling virtually any tape architecture using modular and flexible electrical and thermal basic building blocks that can be different in size. This in turn also allows modeling SFCLs with different level of discretization, i.e. from hot spot modeling with local heat transfer to several meters of (RE)BCO tape. It therefore becomes possible to analyze in the same simulation phenomena happening at the sub-millimetric scale, such as hot-spot phenomena, and at the system-scale, such as the impact on the network of several hundred meters of superconducting tape. In order to validate the EMTP-RV circuit model, comparisons with results obtained with finite elements have been carried out. A similar behavior could be observed, as long as the discretization size of the electro-thermal elements were appropriate. The EMTP-RV circuit model allows performing optimizations of the tape architecture for various thicknesses of stabilizer, in presence or not of an interfacial resistance layer, e.g. between the superconductor and the substrate. While the circuit model was developed to allow representing heat transfer and current distribution in 3D, simulations are still limited to 2D cases because the size of the nodal matrix is otherwise exceeded in EMTP-RV. Simulation results also show that neglecting heat transfer along the thickness of the tape can be risky, [...]

Diese Diplomarbeit untersucht den Casimir-Effekt zwischen normal- und supraleitenden Platten über einen weiten Temperaturbereich, sowie die Casimir-Polder-Wechselwirkung zwischen einem Atom und einer solchen Oberfläche. Hierzu wurden vorwiegend numerische und asymptotische Rechnungen durchgeführt. Die optischen Eigenschaften der Oberflächen werden dann aus dielektrischen Funktionen oder optischen Leitfähigkeiten erhalten. Wichtige Modellen werden vorgestellt und insbesondere im Hinblick auf ihre analytischen und kausalen Eigenschaften untersucht. Es wird vorgestellt, wie sich die Casimir-Energie zwischen zwei normalleitenden Platten berechnen lässt. Frühere Arbeiten über den in allen metallischen Kavitäten vorhandenen Beitrag von Oberflächenplasmonen zur Casimir-Wechselwirkung wurden zum ersten mal auf endliche Temperaturen erweitert. Für Supraleiter wird eine analytische Fortsetzung der BCS-Leitfähigkeiten zu rein imaginären Frequenzen, sowohl innerhalb wie außerhalb des schmutzigen Grenzfalles verschwindender mittlerer freier Weglänge vorgestellt. Es wird gezeigt, dass die aus dieser neuen Beschreibung erhaltene freie Casimir-Energie in bestimmten Bereichen der Materialparameter hervorragend mit der im Rahmen des Zwei-Fluid-Modells für den Supraleiter berechneten übereinstimmt. Die Casimir-Entropie einer supraleitenden Kavität erfüllt den Nernstschen Wärmesatz und weist einen charakteristischen Sprung beim Erreichen des supraleitenden Phasenübergangs auf. Diese Effekte treten ebenfalls in der magnetischen Casimir-Polder-Wechselwirkung eines Atoms mit einer supraleitenden Oberfläche auf. Es wird ferner gezeigt, dass die magnetische Dipol-Wechselwirkung eines Atomes mit einem Metall sehr stark von den dissipativen Eigenschaften und insbesondere von den Oberflächenströmen abhängt. Dies führt zu einer starken Unterdrückung der magnetischen Casimir-Polder-Energie bei endlichen Temperaturen und Abständen oberhalb der thermischen Wellenlänge. Die Casimir-Polder-Entropie verletzt in einigen Modellen den Nernstschen Wärmesatz.Ähnliche Effekte werden für den Casimir-Effekt zwischen Platten kontrovers diskutiert. In den entsprechenden elektrischen Dipol-Wechselwirkungen tritt keiner dieser Effekte auf. Die Ergebnisse dieser Arbeit legen nahe, das bekannte Plasma-Modells als Grenzfall eines Supraleiters bei niedrigen Temperaturen (bekannt als London-Theorie) zu betrachten, statt als Beschreibung eines normales Metalles. Supraleiter bieten die Möglichkeit, die Dissipation der Oberflächenströme in hohem Maße zu steuern. Dies könnte einen experimentellen Zugang zu den optischen Eigenschaften von Metallen bei niedrigen Frequenzen erlauben, die eng mit dem thermischen Casimir-Effekt verknüpft sind. Anders als in entsprechenden Mikrowellen-Experimenten sind hierbei die Energien und Impulse unabhängige Größen. Die Messung der Oberflächenwechselwirkung zwischen Atomen und Supraleitern ist mit den heute verfügbaren Atomfallen auf Mikrochips möglich und der magnetische Anteil der Wechselwirkung sollte spektroskopischen Techniken zugänglich sein / This thesis investigates the Casimir effect between plates made of normal and superconducting metals over a broad range of temperatures, as well as the Casimir-Polder interaction of an atom to such a surface. Numerical and asymptotical calculations have been the main tools in order to do so. The optical properties of the surfaces are described by dielectric functions or optical conductivities, which are reviewed for common models and have been analyzed with special weight on distributional properties and causality. The calculation of the Casimir energy between two normally conducting plates (cavity) is reviewed and previous work on the contribution to the Casimir energy due to the surface plasmons, present in all metallic cavities, has been generalized to finite temperatures for the first time. In the field of superconductivity, a new analytical continuation of the BCS conductivity to to purely imaginary frequencies has been obtained both inside and outside the extremely dirty limit of vanishing mean free path. The Casimir free energy calculated from this description was shown to coincide well with the values obtained from the two fluid model of superconductivity in certain regimes of the material parameters. The Casimir entropy in a superconducting cavity fulfills the third law of thermodynamics and features a characteristic discontinuity at the phase transition temperature. These effects were equally encountered in the Casimir-Polder interaction of an atom with a superconducting wall. The magnetic dipole coupling of an atom to a metal was shown to be highly sensible to dissipation and especially to the surface currents. This leads to a strong quenching of the magnetic Casimir-Polder energy at finite temperature. Violations of the third law of thermodynamics are encountered in special models, similar to phenomena in the Casimir-effect between two plates, that are debated controversely. None of these effects occurs in the analog electric dipole interaction. The results of this work suggest to reestablish the well-known plasma model as the low temperature limit of a superconductor as in London theory rather than use it for the description of normal metals. Superconductors offer the opportunity to control the dissipation of surface currents to a great extent. This could be used to access experimentally the low frequency optical response of metals, which is strongly connected to the thermal Casimir-effect. Here, differently from corresponding microwave experiments, energy and momentum are independent quantities. A measurement of the total Casimir-Polder interaction of atoms with superconductors seems to be in reach in today’s microchip-based atom-traps and the contribution due to magnetic coupling might be accessed by spectroscopic techniques

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).

This thesis presents theoretical investigations of triplet superfluidity (triplet superconductivity) in quasi-one-dimensional organic conductors and ultra-cold Fermi gases. Triplet superfluidity is different from its s-wave singlet counterpart since the order parameter is a complex vector and the interaction between fermions is in general anisotropic. Because of these distinctions, triplet superfluids have different physical properties in comparison to the s-wave case. The author discusses in this thesis the interplay between triplet superconductivity and spin density waves in quasi-one-dimensional organic conductors, and proposes a coexistence region of the two orders. Within the coexistence region, the interaction between the two order parameters acquires a vector structure, and induces an anomalous magnetic field effect. Furthermore, the author analyzes the matter-wave interference between two p-wave Fermi condensates, and proposes a polarization effect. For a single harmonically trapped p-wave Fermi condensate, the author also shows that the expansion upon release from the trap can be anisotropic, which reflects the anisotropy of the p-wave interaction.

1979 wurde mit CeCu2Si2 erstmalig ein Schwere-Fermionen-Supraleiter entdeckt. Diese Verbindung, entdeckt von Steglich und Mitarbeitern, befindet sich nahe an einem quantenkritischen Punkt, an dem die magnetische Ordnung gerade unterdrückt wird. Der Abstand zu diesem Punkt kann sowohl durch Druck als auch durch Germaniumsubstitution auf dem Siliziumplatz variiert werden. Dabei treten neben der Supraleitung in CeCu2Si2 auch verschiedene magnetische Phasen bei höherem Germaniumgehalt auf. CeCu2Si2 ordnet magnetisch unterhalb von TN = 0.8 K in einer Spindichtewelle, während das Schwere-Fermionen-System CeCu2Ge2 unterhalb von TN = 4.1 K antiferromagnetisch ordnet. In dieser Arbeit wurde die Substitutionsreihe CeCu2(Si1-xGex)2 mittels Neutronendiffraktion untersucht. Ausgehend von Proben mit hohem Germaniumgehalt von x = 0.45, deren magnetische Struktur detailliert untersucht wurde, wurden schrittweise die Eigenschaften von Proben mit kleinerem x erschlossen, um schließlich die (bis dato unbekannte) magnetische Struktur in CeCu2Si2 aufzuklären. Weiterhin wurden Untersuchungen zumWechselspiel zwischenMagnetismus und Supraleitung durchgeführt. Hierzu wurde mit einem selbstentwickelten Aufbau dieWechselfeldsuszeptibilität simultan zu den Diffraktionsexperimenten aufgezeichnet. Durch die direkte Korrelation konnte nachgewiesen werden, dass in CeCu2Si2 keine mikroskopische Koexistenz von Supraleitung und magnetischer Ordnung vorliegt, sondern mikroskopische Phasenseparation. - Die Arbeit ist auch über den Cuvillier-Verlag; Nonnenstieg 8; 37075 Göttingen mit der ISBN 978-3-86727-587-3 erhältlich. / In 1979 the first heavy-fermion superconductor CeCu2Si2 was discovered by Steglich et al. The system is near a quantum critical point (QCP), where the magnetic order is just suppressed. The distance to the QCP can be variied with hydrostatic pressure as well as by germanium substitution on the silicon site. Next to the superconductivity in CeCu2Si2 one finds distinct magnetic phases while increasing the germanium content. CeCu2Si2 shows a magnetic order of a spin-density-type below TN = 0.8 K, whereas the heavy fermion system CeCu2Ge2 orders below TN = 4.1 K as an antiferromagnet. The focus of this thesis is on neutron-diffraction in the system CeCu2(Si1-xGex)2. Starting with a sample with a high germanium content of x = 0.45, the magnetic structures are investigated in detail. Following a step-by-step approach, samples with reduced x are investigated subsequently to figure out the properties of pure CeCu2Si2, which were not accessible before. Furthermore, the complex interaction between magnetism and superconductivity is investigated in detail. Using a specially designed setup, the ac-susceptibility could be recorded simultaneously during the neutron diffraction experiments. Due to the direct correlation between antiferromagnetic signals and diamagnetic features, the microscopic coexistence of superconductivity and magnetic order can be ruled out. Instead, a phase separation on the microscopic scale is found. - The thesis is also available from the publisher Cuvillier-Verlag; Nonnenstieg 8; 37075 Göttingen under the ISBN 978-3-86727-587-3.

Glassy Bi-Sr-Ca-Cu-O (BSCCO) precursors were prepared by different melt-quenching methods to investigate the melt properties of the BSCCO system before the crystallization investigations were started. In order to fabricate superconductors having high critical temperature and current density using the glass-ceramic route, it is necessary to clarify the total chemical composition of the quenched precursor. For the first time the total chemical composition of such precursors has been directly measured by the direct element analysis and correlated with the taken process steps. The results from the element analysis demonstrated significant chemical deviations in composition with respect to the starting composition and strong chemical inhomogeneities of the sample. The crystallization dependence was investigated on numerous parameters for the BSCCO system such as initial composition, atmosphere, Sr:Ca ratio, average valence state of the glassy precursor and the dependence of Bi substitution by Pb. It could be demonstrated that the copper valence dependence on the phase formation and crystallization of the high-TC phase plays an important role in the BSCCO system. It could also be demonstrated that the smallest chemical deviation could strongly influence the phase formation in dependence of melt temperature, influencing not only the average copper valence but also the different cation concentrations. From literature there are barely any results or conclusions drawn of the chemical composition of the quenched glassy precursors that however is critical to control the crystallization behavior and understanding the influences on the superconductive properties as demonstrated in this work. / Amorphe Precursoren von dem Bi-Sr-Ca-Cu-O (BSCCO) System wurden durch verschiedene Methoden des Rascherstarrens hergestellt, um deren Schmelzeigenschaften vor dem Prozess der Kristallisation zu untersuchen. Um Supraleiter mit hoher kritischer Temperatur und Stromdichte mit der glas-keramischen Route anfertigen zu können, ist es notwendig, die chemische Zusammensetzung dieser amorphen Precursoren zu kennen. Erstmalig wurde die totale chemische Zusammensetzung der Precursoren durch die direkte Elementanalytik im Zusammenhang mit den jeweiligen Prozessschritten gemessen. Bei den Probeuntersuchungen zeigten sich wesentliche chemische Abweichungen von der nominalen Zusammensetzung und starke chemische Inhomogenitäten. In Abhängigkeit der Parameter nominale Zusammensetzung, Atmosphäre, Sr:Ca-Verhältnis, mittlerer Kupfervalenzzustand (für die Percursoren) und Bi Substitution mit Pb, ist die Kristallation ermittelt wurden. Es konnte gezeigt werden, dass der Kupfervalenzzustand eine wichtige Rolle in dem BSCCO System bei der Kristallisation von der Hoch-TC Phase spielt. Es hat sich auch herausgestellt, dass die kleinste chemische Abweichung stark die Phasenbildung beeinflussen kann. Diese Abweichung ist abhängig von der Schmelztemperatur, welche nicht nur den Kupfervalenzzustand sondern auch die Kationenkonzentrationen beeinflusst. In der Literatur finden sich wenig Veröffentlichungen oder Schlussfolgerungen zu dieser Thematik obwohl es die Kristallisationseigenschaften der Precursoren stark beeinflussen wird, wie es durch die vorliegende Arbeit bestätigt wurde.

Orientador: Cláudio Luiz Carvalho / Banca: Rafael Zadorosny / Banca: Carlos Yujiro Shigue / Resumo: Com a descoberta dos supercondutores de alta temperatura crítica na década de 80, as propriedades elétricas desses materiais passaram a ser intensamente estudadas, principalmente por meio do processo de dopagem. Desta forma, este trabalho teve por objetivo estudar os efeitos da dopagem nas propriedades elétricas do sistema supercondutor BSCCO com fórmula estequiométrica Bi1,6Pb0,4Sr2-xRExCa2Cu3O10+δ, por meio da substituição do elemento terra rara (RE), lantânio (La), em sítios de Sr, onde 0≤x≤2,0, em intervalos de 0,5. As soluções precursoras foram preparadas baseadas no método de Pechini, obtendo-se uma resina polimérica submetida a tratamento térmico de 200oC/10h, resultando em um material na forma de pó, o qual foi levado novamente a tratamento térmico entre 400oC a 810oC. Com o pó, foram preparadas pastilhas, submetendo-as a um novo tratamento térmico de 810oC/+31h. Para se conhecer as principais características das amostras, foram feitas a caracterização estrutural por meio da técnica de Difratometria de Raios X (DRX) para todos os tratamentos térmicos realizados; a caracterização elétrica, pelo método de quatro pontas dc; para a caracterização morfológica e química, a técnica de microscopia eletrônica de varredura (MEV), juntamente com a técnica de Espectroscopia por Energia Dispersiva de Raios X (EDX); e medidas magnéticas ac e dc para a caracterização magnética das amostras. Os resultados apontaram uma degradação das propriedades de supercondução, além de mudanças estruturais, morfológicas e magnéticas com o aumento da concentração de dopante / Abstract: With the discovery of high critical temperature superconductors in the 80, the electrical properties of these materials became intensively studied, mainly through the doping process. Thus, the objective of this work was the study of the effects of doping on the electrical properties of BSCCO superconducting system with stoichiometric formula Bi1,6Pb0,4Sr2-xRExCa2Cu3O10+δ, by the substitution of rare earth element (RE), lanthanum (La) in the sites of Sr, where 0 ≤ x ≤ 2.0, in interval of 0.5. The precursor solutions were prepared based by Pechini method, obtaining a polymer resin subjected to heat treatment at 200° 10h, which results in a material in C/ a powder. In sequence, futher heat treatments between 400oC to 810oC were made. With the powder were prepared bulks by subjecting them to a new heat treatment at 810° C/+31h. To know the main characteristics of the samples, structural characterization were made by the technique of X-Ray Diffraction (XRD) for all heat treated samples; the electrical characterization was made by the dc four probe method; for the morphological and chemical characterization by the technique of Scanning Electron Microscopy (SEM), together with the technique of Energy Dispersive X-ray Spectroscopy (EDX); and ac and dc magnetic measurements to magnetic characterization of the sample. The results shown a degradation of superconducting properties, as a consequence of structural, morphology and magnetic changes with the dopant concentration increase / Mestre

Les travaux de cette thèse portent sur un insert dipolaire de supraconducteur à haute TC (SHT) en utilisant un câble multi-rubans torsadé pour un accélérateur des particules dans le cadre du projet EuCARD2 au CERN. L’insert dipolaire SHT est la seule possibilité aujourd'hui pour aller au-dessus de 16 T pour le futur accélérateur des particules à haute énergie. Deux spécifications de cet insert SHT sont les grands courants de fonctionnement (> 10 kA) et les champs de fond élevés (> 13 T) conduisant à des conditions de fonctionnement sévères. Pour répondre à ces attentes, un premier insert SHT de multi-rubans torsadé/ type de bloc est proposé sur la base des approches analytiques, numériques et expérimentales. Les travaux sont principalement classés dans le design d’insert dipolaire et le design de câble multi-rubans torsadé. Cette thèse se terminée avec la meilleure solution pour l’insert SHT de multi-rubans torsadé/ type de bloc en utilisant le câble partiellement isolé. / This Ph.D. deals with a high Tc superconducting (HTS) dipole insert using a twisted stacked cable for a particle accelerator in the framework of EuCARD2 project in CERN. The HTS dipole insert is the only possibility today to go above 16 T for the future high-energy particle accelerator. Two specifications of these HTS insert are large operating currents (> 10 kA) and high background fields (> 13 T) leading to severe operating conditions. To meet these expectations, a first Twisted Stacked/ Block-type HTS insert is proposed based on analytical, numerical and experimental approaches. The works are mainly classified into dipole insert design and twisted stacked cable design. This Ph.D. ends with the best solution for Twisted Stacked/ Block-type HTS insert using partially-insulated cable.

Cette étude traite du dépiégeage et de la dynamique des systèmes élastiques désordonnés. Ce cadre regroupe une large classe de systèmes allant des interfaces (tel les que les parois de domaines dans les systèmes magnétiques ou ferroélectriques) aux systèmes périodiques(comme les réseaux de vortex dans les supraconducteurs de type II, les colloïdes ou encore les cristaux de Wigner). Dans ces systèmes, la compétition entre l'élasticité de la structure qui veut imposer un ordre parfait et le désordre induit une grande richesse dans le diagramme de phase. L'étude est menée par simulations numériques à grande échelle, dans lesquelles nous nous intéresserons spécifiquement aux réseaux 2D de vortex supraconducteurs.Deux types de dépiégeage sont observés lorsque l'on met en mouvement ces réseaux à l'aide d'une force extérieure : un dépiégeage plastique et un dépiégeage élastique. Nous porterons notre attention sur le dépiégeage élastique obtenu dans le cas d'un piégeage faible. A travers une analyse en loi d'échelle à température nulle et à température finie nous montrons le caractère continu de la transition de dépiégeage au seuil de dépiégeage. Divers exposants critiques sont déterminés dont l'exposant ß et δ caractérisant la dépendance en force et en température de la vitesse ou bien l'exposant ν caractérisant la divergence de la longueur de corrélation du système. Un modèle visco-élastique simple permettant de décrire la plasticité dans les systèmes périodiques évoluant sur un potentiel de piégeage en présence de désordre fort est également développé. Une grande variété de comportementsdynamiques, similaires à ceux observés à plus grande échelle dans des systèmes périodiques, peuvent être extraits d'un tel modèle. Un dépiégeage élastique ou plastique est observé, de l'hystérésis est mesurée dans le cas du dépiégeage élastique, et du chaos est détecté pour le dépiégeage plastique. / In this work we study the depinning and the dynamics of disordered elastic systems. This framework encompasses a broad class of systems from interfaces (like magnetic or ferroelectric domains walls) to periodic structures (like vortex lattices in type II superconductor, colloids or Wigner crystals). In these systems, the competition between the elasticity of the structure that wants to impose a perfect order and disorder produces a great variety of phases and transitions. The study is conducted by large-scale numerical simulations, in which we focus specifically on 2D superconductor vortex lattices. Two types of depinning are observed when we set in motion these lattices using an external force : a plastic depinning and an elastic depinning. We will focus on the elastic depinning obtained when the pinning is weak. Using a scaling law analysis at both zero and nonzero temperature we show that the depinning transition is continuous near the depinning threshold. Various critical exponent are evaluated such as the ß and δ exponents characterizing the force and temperature dependances of the velocity or the ν exponent characterizing the divergence of the correlation length of the system. A simple viscoelastic model allowing to describe plasticity in periodic structures driven over a strong disordered medium is also developed. A wide variety of dynamical behaviors, similar to those observed on a larger scale in periodic systems, can be extracted from such a model. An elastic or plastic depinning is observed, hysteresis is measured in the case of elastic depinning, while chaos is detected for plastic depinning.