In this thesis, the electronic properties in superconducting LiFeAs single crystal are investigated using low temperature scanning tunneling microscopy and spectroscopy (STM/S) at various temperatures. For this purpose, the differential conductance (dI/dV) measured by STS which is directly proportional to the local density of states (LDOS) of the sample to the sub-atomic precision, is used together with the topography information. The dI/dV spectra within the ±1 V energy range reveal a characteristic feature at around -350 mV to -400 mV in stoichiometric LiFeAs. This feature seems to be a universal property among all the Fe-based high temperature superconductors, because it is also found in Fe0.965Se1.035 and NaFe0.975Co0.025As single crystals at the energy of -210 mV and -200 mV, respectively. The temperature dependent spectroscopy data averaged over a spatially fixed clean area of 2 nm × 2 nm are successfully executed between 5 K and 20 K. The two distinct superconducting phases with critical temperatures Tc = 16 K and 18 K are observed. In addition, the distance between the dip position outside the superconducting gap and the superconducting coherence peak in the spectra remains temperature independent which confirms that it is not connected to an antiferromagnetic (AFM) spin resonance. The temperature dependent spectra have been measured between 5 K and 61 K within the energy range of ±100 mV as well. The hump structure at 42 mV tends to disappear around 60 K from unknown origin. The temperature dependent quasiparticle interference (QPI) has been studied within the temperature range between 6.7 K and 25 K and analyzed by the Fourier transformation of the measured spectroscopic maps. The dispersion plots in momentum space as a function of temperature show an enhancement of QPI intensity (±5.5 mV) within the superconducting gap at the Fermi level at 6.7 K near q ~ 0. This is interpreted on the basis of Andreev bound state. In both polarities outside of this, a depletion of QPI intensity is noticed between 5.5 mV and around 9 mV. At positive energies, the QPI intensity becomes very rich above 9 mV. The size of the enhanced QPI intensity near the Fermi level, and the edge of the rich QPI intensity beyond 9 mV are found to behave like superconducting order parameter with rising of temperature. Furthermore, an energy mode peaked at around 14 mV appears in the integrated QPI intensity below superconducting Tc (6.7 K). This is consistent with the observed peak at 1st derivative of the dI/dV spectra. In both of these cases, such 14 mV peak is suppressed at normal state (25 K). This mode is therefore directly related to superconductivity in LiFeAs. The off-stoichiometric LiFeAs single crystal with superconducting Tc of 6.5 K has a 10 mV rigid band shift of the Fermi level towards electron doping. The absence of the rich QPI intensity between 9 mV and 17 mV is found compared to the stoichiometric LiFeAs, and hence the 14 mV mode is absent here. This brings us to conclude once more time that such 14 mV energy mode is relevant for superconductivity in LiFeAs.

In this thesis, the electronic properties in superconducting LiFeAs single crystal are investigated using low temperature scanning tunneling microscopy and spectroscopy (STM/S) at various temperatures. For this purpose, the differential conductance (dI/dV) measured by STS which is directly proportional to the local density of states (LDOS) of the sample to the sub-atomic precision, is used together with the topography information. The dI/dV spectra within the ±1 V energy range reveal a characteristic feature at around -350 mV to -400 mV in stoichiometric LiFeAs. This feature seems to be a universal property among all the Fe-based high temperature superconductors, because it is also found in Fe0.965Se1.035 and NaFe0.975Co0.025As single crystals at the energy of -210 mV and -200 mV, respectively. The temperature dependent spectroscopy data averaged over a spatially fixed clean area of 2 nm × 2 nm are successfully executed between 5 K and 20 K. The two distinct superconducting phases with critical temperatures Tc = 16 K and 18 K are observed. In addition, the distance between the dip position outside the superconducting gap and the superconducting coherence peak in the spectra remains temperature independent which confirms that it is not connected to an antiferromagnetic (AFM) spin resonance. The temperature dependent spectra have been measured between 5 K and 61 K within the energy range of ±100 mV as well. The hump structure at 42 mV tends to disappear around 60 K from unknown origin. The temperature dependent quasiparticle interference (QPI) has been studied within the temperature range between 6.7 K and 25 K and analyzed by the Fourier transformation of the measured spectroscopic maps. The dispersion plots in momentum space as a function of temperature show an enhancement of QPI intensity (±5.5 mV) within the superconducting gap at the Fermi level at 6.7 K near q ~ 0. This is interpreted on the basis of Andreev bound state. In both polarities outside of this, a depletion of QPI intensity is noticed between 5.5 mV and around 9 mV. At positive energies, the QPI intensity becomes very rich above 9 mV. The size of the enhanced QPI intensity near the Fermi level, and the edge of the rich QPI intensity beyond 9 mV are found to behave like superconducting order parameter with rising of temperature. Furthermore, an energy mode peaked at around 14 mV appears in the integrated QPI intensity below superconducting Tc (6.7 K). This is consistent with the observed peak at 1st derivative of the dI/dV spectra. In both of these cases, such 14 mV peak is suppressed at normal state (25 K). This mode is therefore directly related to superconductivity in LiFeAs. The off-stoichiometric LiFeAs single crystal with superconducting Tc of 6.5 K has a 10 mV rigid band shift of the Fermi level towards electron doping. The absence of the rich QPI intensity between 9 mV and 17 mV is found compared to the stoichiometric LiFeAs, and hence the 14 mV mode is absent here. This brings us to conclude once more time that such 14 mV energy mode is relevant for superconductivity in LiFeAs.

In this work we investigate some aspects of the physics of strongly correlated systems by taking into account both electron-electron and electron-phonon interactions as basic mechanisms for reproducing electronic correlations in real materials. The relevance of the electron-electron interactions is discussed in the first part of this thesis in the framework of a self-consistent theoretical approach, named Composite Operator Method (COM), which accounts for the relevant quasi-particle excitations in terms of a set of composite operators that appear as a result of the modification imposed by the interactions on the canonical electronic fields. We show that the COM allows the calculation of all the relevant Green s and correlation functions in terms of a number of unknown internal parameters to be determined self-consistently. Therefore, depending on the balance between unknown parameters and self-consistent equations, exact and approximate solutions can be obtained. By way of example, we discuss the application of the COM to the extended t-U-J-h model in the atomic limit, and to the two-dimensional single-band Hubbard model. In the former case, we show that the COM provides the exact solution of the model in one dimension. We study the effects of electronic correlations as responsible for the formation of a plethora of different charge and/or spin orderings. We report the phase diagram of the model, as well as a detailed analysis of both zero and finite temperature single-particle and thermodynamic properties. As far as the single-band Hubbard model is concerned, we illustrate an approximated self-consistent scheme based on the choice of a two-field basis. We report a detailed analysis of many unconventional features that arise in single-particle properties, thermodynamics and system's response functions. We emphasize that the accuracy of the COM in describing the effects of electronic correlations strongly relies on the choice of the basis, paving the way for possible multi-pole extensions to the two-field theory. To this purpose, we also study a three-field approach to the single-band Hubbard model, showing a significant step forward in the agreements with numerical data with respect to the two-pole results. The role of the electron-phonon interaction in the physics of strongly correlated systems is discussed in the second part of this thesis. We show that in highly polarizable lattices the competition between unscreened Coulomb and Fröhlich interactions results in a short-range polaronic exchange term Jp that favours the formation of local and light pairs of bosonic nature, named bipolarons, which condense with a critical temperature well in excess of hundred kelvins. These findings, discussed in the framework of the so-called polaronic t-Jp model, are further investigated in the presence of a finite on-site potential U, coming from the competition between on-site Coulomb and Fröhlich interactions. We discuss the role of U as the driving parameter for a small-to-large bipolaron transition, providing a possible explanation of the BEC-BCS crossover in terms of the properties of the bipolaronic ground state. Finally, we show that a hard-core bipolarons gas, studied as a charged Bose-Fermi mixture, allows for the description of many non Fermi liquid behaviours, allowing also for a microscopic explanation of pseudogap features in terms of a thermal-induced recombination of polarons and bipolarons, without any assumption on preexisting order or broken symmetries.

This thesis describes a portfolio of work aimed at the high field applications of superconductors and can be split into four main topics: The thermal stability of technical superconductors. This section investigates the effects of thermal perturbations on technical superconducting wire used in MRI scanner construction. The ultimate aim of this section is to predict how the architecture of the wire may affect its thermal stability. To this end a detailed finite element analysis model was constructed, verified by detailed experimental data, which could then be used to quickly and easily vary the wire’s parameters. Design of a high field pulsed electromagnetic coil for flux trapping in superconductors. This section details the design, construction and testing of a novel pulsed high field magnet. The design uses finite element analysis to predict the electromagnetic, thermal and structural properties of the coil. Explosive testing of high tensile fibres used in the construction of the high field coil. This section describes the refinement and use of a novel method for testing the mechanical properties of high tensile fibres in cylindrical geometries by using highly pressurized copper vessels. Pulsed field magnetization of bulk high temperature superconductors. This section discusses the process of magnetizing bulks of high temperature superconductors by using pulsed magnetic fields. It investigates how the trapped field varies with the magnitude and rise-time of the magnetizing field, sample temperature and time after magnetization.

Die vorliegende Arbeit befasst sich mit der elektronischen Struktur von Bi-Kupraten vom Normalzustand bis in den supraleitenden Zustand. Der Normalzustand von einschichtigen Bi-Kupraten wurde mittels polarisationsabhängiger XAS untersucht. Es konnte eine deutliche Polarisationsabhängigkeit der CuL3- und OK-Kante innerhalb der Kupferoxidebene beobachtet werden. Insbesondere in den Merkmalen, die den dotierten Löchern zugeordnet werden. Die Winkelabhängigkeit geht über die erwartete Hybridisierung von Cu3dx^2-y^2- und O2px,y-Orbitalen hinaus, und unterstützt somit Theorien, die auch Orbitale ausserhalb der Kupferoxidebene zur Beschreibung der elektronischen Struktur einbeziehen. Desweiteren wurde beobachtet, dass die Ladungs-Transfer-Lücke sich mit steigender Lochkonzentration vergrößert konform zu Theorien zum Zusammenbruch der Zhang-Rice-Singuletts im überdotierten Bereich. Mittels ARPES wurden die Anregungen nahe der Fermikante in antinodaler Richtung an zweischichtigen Bi-Kupraten untersucht. Die komplexe Linienform im zweischichtigen Bi-Kuprat, die aus Interlageneffekten resultiert, wurde durch die gezielte Ausnutzung von Matrixelementeffekten vereinfacht. Dadurch konnten, in Kombination mit der spezifischen Ausrichtung der Polarisation, vorherige, sich scheinbar widersprechende Beobachtungen am einschichtigen und zweischichtigen Bi-Kuprat in Einklang gebracht werden. Es konnte gezeigt werden, dass im zweischichtigen Bi-Kuprat eine Anregung zusätzlich zum bindenden und antibindenden Band existiert, welche mit dem antibindenden Band korreliert zu sein scheint. Außerdem zeigt es Gemeinsamkeiten mit dem scharfen Peak, der im einschichtigen Bi-Kuprat gefunden wurde. So besteht es über die supraleitende Sprungtemperatur Tc hinaus, und verschwindet vermutlich bei oder über der Pseudolücken-Temperatur T*. Die ARPES Messungen lassen sich am Besten innerhalb des Modells elektronischer Inhomogenitäten erklären, welches Hochtemperatursupraleitung aus Streifen ableitet. / The electronic structure of Bi cuprates from the normal state down to the superconducting state has been investigated. The normal state electronic structure is probed by polarization dependent XAS on single layer Bi cuprates. With the x-ray beam being incident normal to the CuO2 plane the azimuthal angle was varied to explore the polarization effects on orbitals within the plane. In the CuL3- as well as the OK-edge spectra, the spectral features related to the doped holes showed a distinct polarization dependence within the CuO2 plane. The revealed polarization dependence is more complex than expected from hybridization of Cu3dx^2-y^2 and O2px,y orbitals only. Thus, the results support the inclusion of out-of-plane orbitals into the description of the electronic structure as has been previously theoretically proposed. Furthermore, the charge transfer gap has been observed to rise with rising hole concentration supporting theories of the instability of Zhang-Rice-singlets in the overdoped regime. By ARPES the excitations close to the Fermi surface in the antinodal region of double layer Bi cuprates have been investigated. The complex lineshape in double layer Bi cuprates that results from interlayer effects has been disentangled by exploiting matrix element effects. In combination with distinct polarization settings this enabled to unify seemingly inconsistent observations made on single and double layer Bi cuprates. The existence of an excitation additional to antibonding and bonding band could be shown in the double layer Bi cuprate. This additional excitation is probably connected to the antibonding band. It furthermore shows similarities to the sharp peak observed in single layer Bi cuprates. It persists to temperatures above the superconducting temperature Tc, and presumably vanishes at or above the pseudogap temperature T*. The ARPES results could be best explained within the model of electronic inhomogeneity which derives superconductivity from stripes.

The thesis is focused on high-temperature superconducting (HTS) ceramics series of bismuth. Specifically, there is studied Bi2Sr2CaCu2O8+x phase, known as the 2212 phase. The theoretical part describes the basic properties of superconductivity and superconductors, the properties of cuprate ceramics and description of LBCO, YBCO and BSCCO structures. The experimental part deals with the preparation of the precursor own Bi-2212 phase. It is synthesized by a process called sol-gel. Feedstock Bi2O3, CaCO3, SrCO3 and CuSO4 was dissolved in HNO3 and transferred to a complex with ethylenediaminetetraacetic acid (EDTA, Chelaton II). With NH3 pH > 9 was maintained due to the stability of complex. The obtained gel was concentrated, calcinated in a furnace at 860 °C and crushed into powder. Pure powder was subjected to analysis dipping microscope, SEM, XRD, FT-IR, TG-DTA at different atmospheres argon, oxygen and air. Then 1 wt.% of the oxide (La, Y, Sc, Sm) was added to part of the powder precursor and the samples were examined using TG-DTA.

The work described in this thesis uses oxygen NMR to probe the electronic system of rare earth co-doped La_{2-x}Sr_xCuO_4, the prototypical high temperature superconducting cuprate (HTSC). Oxygen NMR turns out to be a powerful tool for this purpose. The nucleus is located directly inside the CuO_2 planes. It has a spin of 5/2 and a quadrupole moment and therefore can probe both, interactions with the magnetic hyperfine field as well as interactions through the electric field gradient of the crystal. Furthermore, the spin lattice relaxation time T_1 and the spin spin relaxation time T_2 contain information about the dynamics of these interactions. Such a link between the spin and charge structures in high temperature superconductors has been elusive until today. Instead, there are magnetic probes such as neutron scattering and muSR that provide evidence for a modulation of the spin structure and static magnetic moments, respectively, and charge probes like STM that reveal inhomogeneous doping distributions in the CuO_2 planes. In either case, inhomogeneities in the spin and charge system seem to be typical for HTSCs. Whereas the spin and charge modulations are believed to be dynamic in the superconducting compounds, they become static at low temperatures in Eu doped La_{2-x}Sr_xCuO_4, where superconductivity is suppressed. As could be demonstrated here, evidence for such a spin and charge separation, that often revealed stunning similarities to the spin ladder compounds, is apparent in almost all measured NMR parameters. / In dieser Arbeit werden Sauerstoff NMR Untersuchungen der elektronischen Struktur von Selten-Erd dotiertem La_{2-x}Sr_xCuO_4, dem prototypischen Hochtemperatursupraleiter (HTSL), vorgestellt. Sauerstoff NMR ist zu diesem Zweck besonders gut geeignet. Der Kern befindet sich innerhalb der Kupferoxid-Ebenen. Er hat einen Spin von 5/2 und ein Quadrupolmoment. Damit lassen sich Wechselwirkungen mit dem magnetischen Hyperfeinfeld der Cu-Atome sowie Wechselwirkungen mit dem elektrischen Feldgradienten des Kristalls untersuchen. Des Weiteren geben die Spin-Gitter-Relaxationszeit T_1 sowie die Spin-Spin-Relaxationszeit T_2 Aufschluss über die Dynamik dieser beiden Wechselwirkungen. Eine Verbindung zwischen der Spin- und Ladungsordnung gibt es in den HTSL bisher nicht. Statt dessen haben magnetische Messmethoden wie Neutronenstreuung oder muSR Aussagen über die magnetische Ordnung geliefert. Unabhängig davon liefern Messmethoden wie STM nur Informationen über eine Ladungsordnung oder inhomogene Ladungsverteilungen. Inhomogenitäten der Spins und Ladungen scheinen aber typisch für die HTSL zu sein. Man vermutet, dass diese Inhomogenitäten dynamisch in den supraleitenden Verbindungen sind, während sie in Eu dotiertem La_{2-x}Sr_xCuO_4 bei tiefen Temperaturen statisch werden und die Supraleitung unterdrücken. In dieser Arbeit wird gezeigt, dass sich diese Ladungs- und Spininhomogenitäten in vielen Parametern der NMR Spektren bemerkbar machen.

[pt] A geometria quântica na física da matéria condensada nos permite entender várias propriedades geométricas dos estados da zona de Brillouin, como a curvatura de Berry e a métrica quântica. Especialmente em relação a esta última, foram observados estudos que mostram sua relação com a supercondutividade. Motivados por estas investigações, esta dissertação visa investigar as propriedades geométricas quânticas de supercondutores singletos, como os tipos s-wave e d-wave, e identificar sua relação com várias respostas eletromagnéticas. Começamos mostrando a descrição desses supercondutores através da teoria do campo médio, posteriormente analisando sua métrica quântica, que é definida pela sobreposição de dois estados de quasihole em momentos ligeiramente diferentes. Subsequentemente, estudamos o número de fidelidade, que é definido como a integração de momento da métrica quântica e representa a distância média entre estados de quasihole vizinhos. Além disso, expressamos esse número de fidelidade como um marcador de fidelidade definido localmente em cada sítio da rede, o que nos permite observar o efeito de impurezas não magnéticas nesse marcador. Para supercondutores de tipo s-wave, mostramos que respostas eletromagnéticas como a absorção no infravermelho estão relacionadas à métrica quântica, enquanto, por outro lado, a corrente paramagnética e a função dielétrica estão relacionadas ao número de fidelidade, que por sua vez é determinado pelo comprimento de coerência. Por outro lado, para supercondutores de tipo d-wave, observamos que sua métrica quântica mostra um comportamento singular e que seu número de fidelidade diverge. O resultado mais relevante desta dissertação é que descobrimos que supercondutores singletos, descritos pela teoria do campo médio BCS, exibem uma métrica quântica não trivial, e que para supercondutores de tipo s-wave as respostas eletromagnéticas mencionadas estão diretamente relacionadas à geometria quântica, o que não havia sido encontrado anteriormente. / [en] Quantum geometry in condensed matter physics allows us to understand various geometric properties of the Brillouin zone states, such as the Berry curvature and the quantum metric. Especially in relation to the latter, studies have been observed that show its relationship with superconductivity. Motivated by these investigations, this dissertation aims to investigate the quantum geometric properties of singlet superconductors, such as s-wave and d-wave types, and identify their relation to various electromagnetic responses. We begin by showing the description of these superconductors through mean field theory, subsequently analyzing their quantum metric, which is defined by the overlap of two quasihole states at slightly different momenta. Subsequently, we study the fidelity number, which is defined as the momentum integration of the quantum metric and represents the average distance between neighboring quasihole states. Furthermore, we express this fidelity number as a fidelity marker defined locally at each lattice site, which allows us to observe the effect of non-magnetic impurities on this marker. For s-wave superconductors, we show that electromagnetic responses such as infrared absorption are related to the quantum metric, while on the other hand, the paramagnetic current and the dielectric function are related to the fidelity number, which in turn is determined by the coherence length. On the other hand, for d-wave super-conductors, we observe that their quantum metric shows a singular behavior and that their fidelity number diverges. The most relevant result of this dissertation is that we have discovered that singlet superconductors, described by the BCS mean field theory, exhibit a nontrivial quantum metric, and that for s-wave superconductors the aforementioned electromagnetic responses are directly related to the quantum geometry, which has not been found previous.

La découverte en 2008 de supraconductivité à relativement haute température (Tc,max = 56K) dans les pnictures de Fer a ravivé les questions fondamentales sur l’origine et la nature de la supraconductivité posés par les supraconducteurs non conventionnels. En particulier, la présence d’une phase antiferromagnétique à proximité de celle supraconductrice dans leur diagramme de phase pose la question du lien entre magnétisme et supraconductivité. Ces supraconducteurs à base de Fe présentent un diagramme de phase générique, mais quelques exceptions remettent en question une description qui se voudrait universelle. Nous avons choisi d’étudier ces cas particuliers grâce à une sonde locale, la résonance magnétique nucléaire (RMN). Nos observations nous ont non seulement permis de comprendre la raison de ces exceptions, mais aussi de s’en servir pour mieux sonder les corrélations magnétiques dans ces matériaux, un ingrédient clé pour la compréhension de la supraconductivité. Premier sujet, la coexistence de supraconductivité et de magnétisme : celle-ci a été observée dans la plupart des supraconducteurs à base de Fer de façon homogène ou inhomogène, mais toujours pour des états magnétiques à faible TN et faibles moments en accord avec des descriptions itinérantes à faibles corrélations. Pourtant un nouveau composé au Sélénium est venu remettre en cause ces conclusions en présentant une apparente coexistence homogène entre une forte supraconductivité macroscopique (Tc ≈ 30K) et un très fort antiferromagnétisme (TN ≈ 600K, moments magnétiques de valeur élevée de 3.3µB). Cette observation suggère donc une description ici plutôt en terme d’isolants de Mott contrairement aux autres supraconducteurs à base de Fer. Nos mesures RMN permettent de montrer en fait l’existence d’une séparation de phase et de statuer sur la stœchiométrie et les propriétés électroniques des différentes phases, pour finalement réconcilier ce composé et les autres familles. Deuxième exception : dans la famille archétype BaFe₂As₂, tous les dopages sur site Fer ou Arsenic ou même l’application de pression mènent à la supraconductivité, sauf dans le cas du dopage au Manganèse ou au Chrome en site Fer, qui ne provoquent pas l’apparition de la supraconductivité. Nos mesures RMN nous ont permis de sonder la nature de la transition magnétique, mais aussi l’état métallique de ces composés substitués. Nous montrons en particulier que le trou supplémentaire du Manganèse substitué à la place du Fer reste en fait localisé sur son site et se manifeste alors par un moment magnétique localisé. Cette étude du dopage par le Manganèse ouvre la voie à l’idée d’utiliser le Manganèse en faible concentration comme source de moments localisés qui polarisent magnétiquement leur environnement. Cette polarisation permet en effet de caractériser la nature même des corrélations de spin. Nous avons donc utilisé la RMN ainsi que la magnétométrie-SQUID pour mesurer cette polarisation dans des composés supraconducteurs pour sonder les corrélations de spins de ces systèmes. Nous concluons que ces corrélations sont plutôt faibles et indépendantes de la température dans les composés dopés en électrons. / The discovery in 2008 of superconductivity at a rather high temperature in the iron pnictides (Tc,max = 56K) has revived the fundamental questions about the existence and the nature of the superconducting phase raised by the unconventional superconductors. In particular, the existence of an antiferromagnetic phase that is in vicinity of the superconducting phase in the phase diagram raises questions about the link between magnetism and superconductivity. These Iron based superconductors have a generic phase diagram, but some exceptions are questioning a description that would be universal. We chose to study these cases through a local probe, nuclear magnetic resonance (NMR). Our observations have not only allowed us to understand the reasons for these exceptions, but also be used to better probe the magnetic correlations in these materials, a key ingredient for the understanding of superconductivity. First subject, the coexistence of superconductivity and magnetism: it was observed in most superconductors based on iron homogeneously or inhomogeneously, but always for magnetic states at low TN and low magnetic moments in accordance with nesting descriptions with low correlations. Yet a new compound Selenium came to question these conclusions with an apparent homogeneous coexistence between a strong macroscopic superconductivity (Tc ≈ 30K) and a very strong antiferromagnetism (TN ≈ 600K, magnetic moments of high value of 3.3μB). This observation suggests a description rather in terms of Mott insulators, unlike other iron-based superconductors. Our NMR measurements show the existence of an effective phase separation and determine the stoichiometry and the electronic properties of the different phases, eventually reconciling this compound and other families. Second exception : in the archetype family BaFe₂As₂, all iron or arsenic on-site doping or even application of pressure leads to superconductivity, except in the case of Chrome or Manganese doping in Iron site, which does not cause the onset of superconductivity. Our NMR measurements have allowed us to probe the nature of the magnetic transition, but also the metallic state of the substituted compounds. We show in particular that the extra hole Manganese substituted in place of the iron is actually located on its atom and then manifested by a localized magnetic moment. This study of Manganese doping opens up the idea of using Manganese in low concentrations as a source of localized moments which magnetically polarize their environment. This polarization makes it possible to characterize the nature of the spin correlations. We used NMR and SQUID magnetometry, to measure the polarization in superconducting compounds to probe the spin correlations of these systems. We conclude that these correlations are rather low and independent of temperature in electrons doped compounds.

The thesis is dealing with the investigation of charge and orbital order and their behaviour under external pressure. Therefore, a new pressure cell has been developed which allows the observation of superlattice reflections corresponding to the order phenomena under pressure using scattering of high-energy synchrotron radiation. The maximum pressure that can be reached is 1.25 GPa. Until today there has been no possibility to conduct such studies of charge and orbital order superlattice reflections under pressure using x-ray scattering. The intensities of the reflections of the single crystalline samples are quite weak compared to fundamental peaks. Therefore the measurements are strongly affected by the absorption of the radiation in the pressure cell itself. Further difficulties result from the facts that low temperatures are needed and the sample has to be oriented in reciprocal space after being mounted into the cell. Therefore, the design of a compact clamp-type piston pressure cell was chosen here. The cell is made from a copper-beryllium alloy with the wall thickness reduced in the height of the sample volume. This allows the usage inside a closed-cycle cryostat mounted on a three-axis-diffractometer. Absorption effects are minimized due to the combination of reduced wall thickness and the usage of high energy synchrotron radiation (E = 100 keV at the beamline BW5 at HASYLAB/DESY). The new experimental technique was established and used for a study of two representatives of the transition metal oxide compounds, i.e. doped cuprates and manganites, which belong to the class of strongly correlated electron systems. The 1/8-doped cuprate La_{2-x}Ba_{x}CuO_{4} reveals an ordered state at low temperatures. Inside the CuO_{2} planes a combined order of charge stripes and antiferromagnetic spin stripes is observed. The ordering results from the interaction between charge, spin and lattice degrees of freedom. Here the lattice degrees of freedom play a major role. Particularly, a structural transition from an orthorhombic to a tetragonal symmetry is prerequisite for the observation of the ordered state. The cell constructed in this work allows a more exact analysis of the coupling between the crystal lattice and the formation of the charge and spin ordered phase. The manganite system Pr_{0.7}(Ca_{0.9}Sr_{0.1})_{0.3}MnO_{3} shows a strong magnetoresistive effect, called colossal magnetoresistance (CMR). In this system, several ordered phases can be found, which exhibit charge, spin and - since the orbital degree of freedom is also present in the manganites - additionally orbital ordering phenomena. In particular, an antiferromagnetically spin ordered insulating phase, which is connected to a charge- and orbital ordered state competes with a ferromagnetic metallic phase. This competition leads to a phase separation, which determines the properties of the sample. Both phases are strongly coupled to the lattice degrees of freedom, so that application of external pressure drastically affects the interplay between the different phases and allows a detailed study of the relation between the charge and orbital ordered phase and the crystal structure. / Die vorliegende Arbeit befaßt sich mit dem Studium der Ordnungszustände von Ladungen und Orbitalen und deren Beeinflußung durch externen Druck. Als experimentelle Neuentwicklung wurde dafür eine Druckzelle entworfen, mit deren Hilfe die Beobachtung der jeweiligen Ordnungsphänomene unter Druck mittels der Streuung hochenergetischer Synchtrotronstrahlung möglich ist. Die Zelle erlaubt die Messung der orbitalen und Ladungsüberstrukturreflexe, welche aus den geordneten Zuständen resultieren, in einem Druckbereich bis 1.25 GPa. Die experimentelle Herausforderung ergibt sich hierbei aus der Tatsache, dass die Überstrukturreflexe im Vergleich zu den fundamentalen Reflexen der einkristallinen Proben sehr schwach sind und zusätzlich durch die Absorption im Mantelmaterial der Druckzelle stark beeinträchtigt werden. Darüber hinaus soll die Zelle bei tiefen Temperaturen einsetzbar und die Probe auch innerhalb der Zelle im reziproken Raum orientierbar sein. Bei dem hier realisierten Ansatz wurde für das Design daher der Typ einer kompakten Klemmdruckzelle aus einer Kupfer-Beryllium-Legierung gewählt, deren Zellwände im Bereich des Probenvolumens reduziert wurden. Dadurch ist der Einsatz der Zelle im Inneren eines Closed-Cycle-Kryostaten auf einem Einkristall-Diffraktometer möglich. Aufgrund der geringen Wandstärke der Zelle und der Nutzung von hochenergetischer Röntgenstrahlung (E = 100 keV am Messplatz BW5 des HASYLAB/DESY) werden Absorptionseffekte minimiert. Die neue Messmethode wurde im Rahmen der Arbeit etabliert und zur Untersuchung zweier wichtiger Übergangsmetalloxidverbindungen (dotierte Kuprate, Manganate), die zur Klasse der stark korrelierten Elektronensysteme gehören, eingesetzt. Das 1/8-dotierte Kupratsystem La_{2-x}Ba_{x}CuO_{4}, weist bei tiefen Temperaturen einen statisch geordneten Zustand auf. Innerhalb der CuO_{2}-Schichten des Kristalls ergibt sich eine Ordnung, bei der sich Streifen lokalisierter Löcher und antiferromagnetische Bereiche abwechseln. Ursache dieses Zustands ist das Wechselspiel von Ladungen, Spins und strukturellen Freiheitsgraden. Dabei spielen letztere eine herausgehobene Rolle. So ist insbesondere ein struktureller Übergang von einer orthorhombischen zu einer tetragonalen Phase Voraussetzung für die Beobachtung der Ordnung. Die in dieser Arbeit aufgebaute Druckzelle erlaubt eine genauere Analyse des Zusammenhangs zwischen Struktur des Kristalls und der Ausbildung der ladungs- und spingeordneten Phase. Das Manganatsystem Pr_{0.7}(Ca_{0.9}Sr_{0.1})_{0.3}MnO_{3}, zeichnet sich durch einen sehr starken magnetoresistiven Effekt aus, der auch als kolossaler Magnetowiderstand (CMR) bezeichnet wird. Auch hier kann bei tiefen Temperaturen eine geordnete Phase beobachtet werden. Allerdings spielt in diesem System zusätzlich der orbitale Freiheitsgrad der Elektronen eine entscheidende Rolle, so dass sich eine kombinierte Ladungs- und Orbitalordnung ergibt. Diese Phase, die isolierend und zusätzlich antiferromagnetisch geordnet ist, steht im direkten Wettbewerb zu einer ferromagnetischen Phase. Aus dieser Konkurrenz ergibt sich eine Tendenz zur Phasenseparation, deren Effekte die Eigenschaften des Kristalls dominieren. Da beide Phasen stark an die strukturellen Freiheitsgrade gekoppelt sind, läßt sich das Gleichgewicht zwischen ihnen durch externen Druck beeinflussen und die Abhängigkeit der ladungs- und orbitalgeordneten Phase von den strukturellen Eigenschaften des Kristalls im Detail untersuchen.