by Dimitri P. Bertsekas. / Caption title. / Includes bibliographical references (p. 12-13). / Supported by the NSF. CCR-9103804

Graphite is the strongest diamagnet of all known materials to date. Recent studies of the thickness dependence of the resistance of graphite have demonstrated the heterogeneity of the charge distribution in bulk graphite and prompted the study of its magnetic properties. The studies of the thickness dependence of the magnetic susceptibility of graphite, done in this work, showed that two-dimensional interfaces between the crystalline (Bernal or rhombohedral stacking order) blocks in graphite make a dominant contribution to its diamagnetic susceptibility. Previously proposed models of diamagnetism in graphite are not suitable for explaining its magnetic properties, and therefore new concepts should be considered. Additionally, the studies of the transport and magnetic properties of graphite and multilayer graphene indicated the existence of superconductivity at the interfaces in well-ordered graphite. The possibility of creating permanent circulating currents around artificial holes in highly oriented graphite was studied by highly sensitive magnetization measurements. The obtained results provide hints for the possible existence of superconducting regions inside the bulk highly ordered graphite. In the present thesis, a further thickness dependent phenomenon on the depinning line (DL) of the flux line lattice of the high-Tc superconductor Bi2Sr2CaCu2O8+δ was studied. This geometrical effect shifts to notably lower temperatures in micrometer ring, compared with bulk crystals and thin flakes. The shift is related to a decrease in the overall pinning potential as a result of size effects, caused by: a) the thickness of the sample being smaller than the pinning correlation length, and b) the increase in the effective London penetration depth of the vortices (Pearl vortices). The large shift of the DL to lower temperatures may significantly influence the suitability of such elements for device applications in microstrip antennas and THz emitters.

The Bi2Sr2CaCu2O7+d, i.e. Bi2212 phase of bismuth derived high temperature superconductors (HTS), powder precursor were synthesized via sol – gel technique using ethylendiamintetraacetic acid (Chelaton II) as a chelating agent. Metal nitrate’s solutions were mixed with EDTA suspension. The pH value was adjusted to 9 by NH4OH by reason forming of stable metal’s complexes. The mixture was heated to 80 °C for gelation. Solution taken before solid gel was form is used for preparation of Bi2212 layer on to a-Al2O3 surface via spin coating deposition’s technique. Reactive powder used for bulk sample preparation was made by calcination (800 °C) of pyrolyzed xerogel (500 °C). Sintering in oxygen atmosphere was proceeding at temperature from 850 to 880 °C. Bulk sample properties were compared with sample prepared by common ceramic method. Simultaneous TG-DTA, IR spectroscopy and heating microscopy were used for sample characterization. Furthermore, construction of furnace for sintering in O2 atmosphere was described.

In dieser Doktorarbeit wird der Hochtemperatur-Supraleiter Bi-2212 auf seine elektronische Struktur hin untersucht. Für diesen Zweck wurden Röntgen- Absorptionsspektroskopie (XAS) und winkelaufgelöste Photoemissionsspektroskopie (ARPES) verwendet. Zusätzlich wurden mittels Dichtefunktionaltheorie theoretische Trends in der elektronischen Struktur aufgezeigt. Zu Beginn wurde die Temperaturabhängigkeit der Löcher-Konzentration (nH) von nahezu optimal dotierten und geringfügig unterdotierten Einkristallen mittels XAS untersucht. Die Messungen der Temperaturabhängigkeit von nH mit XAS zeigen ein komplett anderes Verhalten als das, welches aus dem Hall-Effekt hergeleitet wurde. Hinzu kommt, dass es unmöglich ist, die Formel von Gor''kov und Teitel''baum, d.h. mit einem konstanten und einem Aktivierungsterm, an die Daten anzupassen. Als Lösungsansatz kommen Magnonen in Frage. Zusätzlich wurde die Polarisationsabhängigkeit der Löcher- Konzentration mittels XAS gemessen, die zeigt, dass in den Kupferoxidschichten die Löcher offenbar offenbar inhomogen verteilt sind. Solch ein Verhalten wird für die isotrope Struktur der Bi-2212-Kristalle nicht erwartet und kann nur schwer erklärt werden. Möglicherweise sind die, die Symmetrie brechenden magnetischen Eigenschaften wie magnetische Streifen die Antwort. Um zusätzliche experimentelle Informationen zu erhalten, wurde darüber hinaus noch die Temperatur- und Polarisationsabhängigkeit der Bi-2212-Einkristalle mittels ARPES studiert. Insbesondere die ''Peak-Dip-Hump''-Emissionsstruktur an der Fermi-Energie wurde gemesssen, die sich am M-Punkt der Brillouin-Zone befindet. Die Ergebnisse zeigen, dass der scharfe Peak nahe der Fermi-Kante eine deutliche Polarisations- und Temperaturabhängigkeit aufweist, welche bei der Pseudolücken-Temperatur T* und nicht bei der Sprungtemperatur TC verschwindet. Die Polarisationsabhängigkeit an den vier M-Punkten der Brillouin-Zone hat gezeigt, dass eine Symmetrieachse im 45°-Winkel zur Cu-O-Cu-Richtung existiert. Dies könnte ebenfalls mit magnetischer Streifenbildung erklärt werden. Des Weiteren wurde das beobachtete Versagen der Dipol-Näherung zur Beschreibung der Spektren bei einer Polarisation senkrecht zur Spiegelebene erörtert. Andererseits ist die Berechnung von Materieleigenschaften mittels der Dichtefunktionaltheorie ein sehr aktiver Bereich der Festkörperphysik geworden. Nachdem meine DFT-Rechnungen auf der Basis des MBJ-Potentials sehr gute Übereinstimmung mit den experimentellen Ergebnissen des ternären Halbleiters ZrSexS2-x und die LiZrSexS2-x –Verbindung erzielt haben, habe ich diese auch zur Unterstützung der experimentellen Supraleiterdaten angewendet. Die elektronischen Eigenschaften von CaCuO2 und des Bi-2212-Kuprats wurden mittels DFT auf der Basis von GGA und GGA+U berechnet, wobei der Hubbard-U-Term mit einer „abinitio“-Methode berechnet wurde. Die Ergebnisse ergaben, dass nur Rechnungen auf der Basis von MBJ+U einen antiferromagnetischen Grundzustand für die CaCuO2- Verbindung lieferten, während alle Funktionale versagten, den antiferromagnetischen Grundzustand für das Bi-2212-Kuprat zu finden. / In this PhD work the electronic structure of high-TC Bi-2212 cuprates is investigated. For this purpose x-ray absorption spectroscopy (XAS) and angle resolved photoemission spectroscopy (ARPES) are used. Additionally, density functional theory is applied for making theoretical trends of the electronic structure evident. At first, the temperature dependence of the hole density (nH) by XAS on nearly optimum and slightly underdoped single crystals is studied. The measurements of the temperature dependence of nH by XAS show completely different behavior as that derived from Hall effect. Moreover, fitting our data by the Gor’kov and Teitel’baum formula, i.e. assuming a constant term and an activation term, was impossible. For solving the problem a contribution of magnons is suggested. Additionally, the polarization dependence of the hole density has been measured by XAS showing that an inhomogeneity of holes in the copper oxide planes may exist. Such a behavior is not expected for the isotropic structure of the Bi(Pb)-2212 crystals and was only hardly to be explained. Possibly, magnetic symmetry breaking properties like e.g. magnetic stripes might give an answer. In order to derive additional experimental information, the temperature and polarization dependence of the electronic structure of the CuO2 planes of Bi(Pb)-2212 single crystals has been studied by ARPES. In particular, the so-called peak-dip-hump emission structure close to the Fermi energy has been measured, which is located at the M point of the Brillouin zone. The results show that the sharp emission structure close to the Fermi edge reveals a distinct polarization dependence and it vanishes at the pseudogap temperature T* and not at the critical temperature TC. The polarization dependence at the four M points of the Brillouin zone has revealed that there exists a symmetry line along to 45?? from the Cu-O-Cu direction. This could also be due to stripe formation. Additionally, the observed failure of the dipole approximation to describe the spectra at normal polarization with respect to the mirror plane is discussed. On the other hand, the calculation of material properties by density functional theory has become a very active area of research in condensed matter physics. Therefore, after my calculations based on DFT by the MBJ potential have revealed good agreement with experimental data for the ternary semiconductors ZrSexS2-x and the LiZrSexS2-x compound I applied it for a support of the experimental superconductor data. The electronic properties of CaCuO2 and the Bi-2212 cuprate have been calculated by DFT based on GGA and GGA+U where the Hubbard U term has been calculated by an ab initio method. The results reveal that only calculations based on MBJ+U lead to an anti-ferromagnetic ground state for the CaCuO2 compound while all functionals fail to find an antiferromagnetic ground state for Bi-2212.

High-temperature superconductors belong to the group of strongly correlated materials. In these compounds, complex repulsive electron interactions and a large number of degrees of freedom lead to a rich variety of states of matter. Exotic phases like the pseudogap, charge-, spin- and pair-density waves, but also the remarkable phenomenon of superconductivity emerge, depending on doping level and temperature. However, up to now it is unclear what exactly causes these states, to what extent they are coexisting or competing, and where their borders in the phase diagram lie. A better understanding could help in finding the mechanism behind high-temperature superconductivity, but would also provide a better insight into the puzzling behavior of strongly correlated materials. This thesis tries to resolve some of these questions with focus on the underdoped pseudogap regime. Mesa structures of bismuth-based cuprate superconductors were studied using intrinsic tunneling, which allows spectroscopic characterizations of electronic density of states inside the material. A micro/nano fabrication method was developed to further reduce mesa areas into the sub square-micrometer range, in order to minimize the effect of crystal defects and measurement artifacts caused by heating induced by the measurement current. The comparison of energy scales in Bi-2201 and Bi-2212 cuprates shows that the pseudogap phenomenon is not connected to superconductivity, but possibly represents a competing spin-singlet order that is universal to all cuprates. The analysis of the upper critical field in Bi-2201 reveals a low anisotropy, which gives evidence of paramagnetically limited superconductivity. Furthermore, a new electrical doping method is demonstrated, which enables the reversible tuning the doping level of Bi-2212 and study a broad doping range upon a single sample. Using this method, two distinct critical points were observed under the superconducting dome in the phase diagram: one at the overdoped side, associated with the onset of the pseudogap and a metal to insulator transition, and one at optimal doping, associated with an enhanced "dressed" electron energy. Finally, a novel angular-dependent magnetotunneling technique is introduced, which allows for the separation of the superconducting and non-superconducting contributions to the pseudogap phenomenon. The method reveals that after an abrupt decay of the energy gap for T→Tc, weak superconducting correlations persist up to several tens of degrees above Tc.

Dans le contexte d'un contrat de la Délégation Générale pour l'Armement (DGA) avec la société Nexans, le CRTBT-CNRS a développé une bobine supraconductrice de stockage d'énergie, ou SMES Superconducting Magnetic Energy Storage), avec des rubans PIT Bi-2212 pour un fonctionnement à 20 K. L'utilisation d'un bobinage supraconducteur permet de stocker l'énergie électrique sous forme magnétique sans conversion d'énergie, ceci pendant des temps très longs. Le bobinage stocke 800!kJ à décharger en 1 s pour atteindre une puissance de 500 kW sur la charge, ce qui génère une tension maximale de 5 kV. La cryogénie est réalisée avec des pièces en cuivre qui relient les cryoréfrigérateurs et le bobinage, avec différence de température de 2 K au maximum. L'interface HT (Haute Tension) entre les drains et le bobinage a une tenue diélectrique de 5 kV et permet de refroidir efficacement les amenées de courant et les 26 galettes, soit 40 km de ruban. L'énergie dissipée dans le cuivre et le bobinage pendant la décharge représente 1 l'énergie stockée. Des mesures thermiques à 20 K ont été réalisées sur des échantillons pour mesurer l'interface!HT par exemple, puis sur un bobinage de dimensions réduites pour valider les solutions retenues. Le procédé de coétamage des rubans supraconducteurs développés par Nexans permet d'adapter la géométrie du conducteur à sa situation dans le bobinage. Les essais des dix premières galettes bobinées a validé la cryogénie développée. L'étude d'extrapolation pour un SMES de 20 MJ présente une géométrie torique adaptée à un refroidissement par thermosiphon avec un câble bi-étagé Rutherford / 6+1 en fils ronds de Bi-2212.

Dans ce mémoire, nous avons approché l'étude du diagramme de phase (T,p) des cuprates à travers l'étude des effets du dopage sur les propriétés électroniques de deux systèmes La2-xSrxCuO4 et Bi2Sr2Ca1-xYxCu2-yZnyO8+d.  Dans un premier temps, la conductivité optique de deux échantillons La2-xSrxCuO4 a été analysée, dans les plans et perpendiculairement aux plans : un composé sous-dopé (x = 0,08) et le composé optimalement dopé (x = 0,15). Suivant l'axe c et à faibles fréquences, nous avons observé une diminution de la conductivité optique en diminuant la température pour l'échantillon sous-dopé. Dans les plans et pour le même composé, nous avons montré que le taux de diffusion diminuait et que la masse effective augmentait, lorsque la température diminue et à faibles fréquences. Puisque ces résultats ne sont obtenus que sur l'échantillon sous-dopé, ils peuvent correspondre à une signature de la phase de pseudo-gap.<br /> Dans le but de comparer et d'étudier le comportement du cuprate Bi-2212, nous avons synthétisé des échantillons de composition Bi2Sr2Ca1-xYxCu2-yZnyO8+d. La substitution à l'yttrium modifie la concentration des porteurs et la substitution au zinc conduit à la suppression de la supraconductivité. Nous avons d'abord analysé des échantillons céramiques. Deux voies d'élaboration ont été confrontées : la voie solide et la voie citrate modifiée. Cette dernière a montré une meilleure cristallinité et un caractère monophasé des poudres, caractéristiques importantes pour la croissance cristalline. Les mesures de transport ont montré que les céramiques évoluaient d'un régime métallique vers un comportement semi-conducteur, à haute température, en fonction de la substitution à l'yttrium mais également au zinc.<br /> Finalement, nous nous sommes intéressés à la croissance de monocristaux de Bi2Sr2Ca1-xYxCu2-yZnyO8+d dans un four à image. Les expériences ont montré que la croissance était délicate. Après avoir optimisé les paramètres de la fusion pour la composition non-dopée, nous avons fait croître des monocristaux de taille 15 x 5 x 0,2 mm3. Pour les substitutions, l'yttrium empêche la croissance de larges monocristaux qui n'excèdent pas la taille de 3 x 1,5 x 0,2 mm3. La limite de solubilité du zinc sur site cuivre a été définie à 0,03. A partir de ces résultats, des monocristaux de composition Bi1,99Sr2,01Ca0,76Y0,3Cu1,9Zn0,03O8+d ont été synthétisés et caractérisés.

In this thesis, the tunneling between individual atomic layers in structures of Bi2Sr2CaCu2O8+x based high-temperature superconductors are experimentally studied employing the intrinsic Josephson effect. A special attention is paid to the fabrication of small mesa structures using micro and nanofabrication techniques. In the first part of the thesis, the periodic Fraunhofer-like modulation of the critical current of the junctions as a function of in-plane magnetic field is investigated. A transition from a modulation with a half flux quantum to a flux quantum periodicity is demonstrated with increasing field and decreasing junction length. It is interpreted in terms of the transformation of the static fluxon lattice of stacked, strongly coupled intrinsic Josephson junctions and compared with theoretical predictions. A fluxon phase diagram is constructed.Numerical simulations have been carried out to complement the experimental data. In the second part of the thesis, different resonant phenomena are studied in the dynamic flux-flow state at high magnetic fields, including Eck-resonances and Fiske steps. Different resonant modes and their velocities, including superluminal modes, are identified. In the third part, different experiments attempting to detect radiation from small mesa structures using different setups based on hot-electron bolometer mixers and calorimeters are described. No distinct radiation with emission powers higher than about 500pW could be detected. Furthermore, the interaction with external GHz-radiation is studied. Resonances attributed to an induced flux-flow are observed, and the reflectivity of the sample can be tuned by switching mesas between the superconducting and quasiparticle state. In the last part, the resistive switching of mesas at high bias is studied. It is attributed to a persistent electrical doping of the crystal. Superconducting properties such as the critical current and temperature and the tunneling spectra are analyzed at different doping states of the same sample. The dynamics of the doping is studied, and attributed to two mechanisms; a charge-transfer effect and oxygen reordering

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.

In einer initialen Studie verglichen wir das XQi Revolution, welches auf indirektem CsI (Cäsium Iodit) /a: Si (amorphes Silizium) basiert mit einem direkten Digitalröntgengerät a: SE (amorphes Selen) an einem CDRAD-Phantom bei vier unterschiedlichen Eintrittsdosen und an einem TRG-Phantom bei zwei unterschiedlichen Eintrittsdosen. Mittels des berechneten Bildqualitätsfaktors des CDRAD-Phantoms konnten wir zeigen, daß das indirekte im Vergleich zum direkten System bei niedrigeren Dosen eine bessere Detailerkennungsrate aufweist. Ein positiver Trend läßt sich auch beim TRG-Phantom darstellen. In einer weiteren Studie untersuchten wir anhand von Patientenbildern die Bildqualität des Dual Energy Systems bei zwei unterschiedlichen Dosisniveaus, der Standarddosis sowie einer doppelten Dosis, was einem Speed-Äquivalent von 400/1000 bzw. 200/500 entspricht. Bei hoher Dosis konnten wir eine signifikante Reduktion des Rauschens im Knochen- und Weichteilbild feststellen, gleichzeitig nahmen die Störungen durch Bewegungsartefakte signifikant zu. Im Anschluß verglichen wir die Erkennbarkeit verkalkter Lungenpathologien im Standard P/A Bild mit zusätzlichem Einsatz von Dual Energy. Als Goldstandard erfolgte der sichere Nachweis der Pathologien im CT. Bei zusätzlichem Einsatz von Dual Energy konnten wir eine signifikante Steigerung der Sensitivität erkennen. Dieses Ergebnis wurde durch den Qualitätsfaktor, der die Bildeigenschaften kumulativ beschreibt, bestätigt. Weiterhin untersuchten wir analog die Erkennbarkeit von nichtverkalkten Lungenrundherden. Auch bei diesen Pathologien ließ sich ein positiver Trend der Sensitivität und Spezifität bei zusätzlichem Einsatz von Dual Energy erkennen. Zusätzlich stieg die durchschnittliche Entscheidungssicherheit der Gutachter signifikant an. Damit bietet die Dual Energy Subtraktionstechnik eine wertvolle Ergänzung in der Diagnostik verkalkter und nichtverkalkter Lungenpathologien eine wertvolle Ergänzung zum Standardröntgen. / First study assess and quantify the image quality at two dose levels for an amorphous Silicon (a:Si) Cesium Iodide (CsI) flat panel system compared with a direct amorphous Selenium (a:Se) digital radiography system. Image quality of a:Si flat panel digital radiography proved to be superior to a:Se drum digital radiography using low-dose settings. Second study assess the image quality of subtracted soft tissue and bone images of a CsIdetector-based dual-energy system for chest radiography at varying dose levels. Radiation dose did not significantly influence the perception of dual-energy image quality. Next study assess the value of dual-energy chest radiography obtained using a cesium iodide flat-panel detector in addition to standard posteroanterior chest radiography for the detection of calcified chest abnormalities. When dual-energy images were added, sensitivity increased significantly. Brunner and Langer’s test revealed a highly significant difference between posteroanterior chest radiography and dual-energy imaging in the detection of calcified chest abnormalities. Dual-energy images added to standard posteroanterior chest radiographs significantly improve the detection of calcified chest lesions. Last study compare the sensitivity and specificity of digital chest radiography alone with digital chest radiography combined with dual-energy chest radiography in the detection of small non-calcified pulmonary nodules. Standard and dual-energy radiographs were obtained with a flat-panel digital chest system. The increase of nodule detection overall as well as for different size categories was significant. The increase of the confidence level rating was also significant. Dual energy added to standard posteroanterior chest radiography significantly improves the sensitivity, specificity, and confidence in detection of small non-calcified pulmonary nodules. Dual-energy subtraction has the potential to become a future routine application in chest radiography.