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Since the discovery of cuprate superconductors, PrBa2Cu3O7 (Pr123) has attracted much attention due to its nonsuperconductivity and other anomalies. The very recent reports on superconducting Pr123 instead of putting an end of this issue, virtually stimulate more controversies. One of the proposed explanations for the recently observed superconductivity in Pr123 is that the samples could be Ba-rich Pr123 to investigate this possibility, we prepared Pr1-xBa2+xCu3O7 (x=1~0.3) and Pr1-xCaxBa2Cu3O7 (x=0~0.6) as comparison. X-ray absorption near edge spectroscopy (XANES) indicates an increase in carrier concentration with Ba and Ca doping, which is consistent with resistivity and thermoelectric power(TEP) data. However, carriers are introduced in a peculiar why rather than simply add into CuO2 plane.

Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.

Orientador: Dayse Iara dos Santos / Banca: Adilson Jesus Aparecido de Oliveira / Banca: Paulo Noronha Lisboa Filho / Resumo: O intermetálico diboreto de magnésio passou a ser um material intensamente estudado a partir de 2001, quando nele encontrou-se a propriedade de supercondição de corrente elétrica. Embora sua transição seja em temperatura relativamente baixa, próximo de 40 K, é grande o interesse comercial neste material devido à provável simplicidade da cinética de reação e à maneira direta e barata de processamento. Partindo de pós precursores ultrafinos de boa qualidade a formação da fase Mg'B IND. 2' parece ocorrer em apenas alguns minutos em temperatura relativamente baixa (650ºC). Sob este ponto de vista, este material é considerado bastante atrativo comercialmente, quando comparado aos supercondutores cerâmicos cujo processamento é muito mais complexo. Entretanto, suas propriedades supercondutoras, assim como nas cerâmicas, dependem da sua microestrutura e, portanto da cinética de formação da fase supercondutora. Além disso, com o objetivo de ampliar a densidade de corrente elétrica sob campos magnéticos cada vez mais altos, tem-se utilizado artifícios tais como a adição de diversos compostos geradores de centros de aprisionamento de fluxo magnético. Entre eles figuram o SiC e o silicone, formados por elementos que podem substituir um dos átomos do composto supercondutor, ou que apenas geram defeitos intersticiais. Neste trabalho, investigou-se a influência das adições de alguns compostos de carbono na formação da fase Mg'B IND. 2', assim como a determinação de alguns aspectos das microestruturas obtidas. Estudou-se também as mudanças na cinética de formação da fase devido à diferente granulação do precursor de magnésio. Os dados obtidos por DRX, ATD/TG, BET e técnicas de medidas magnéticas indicaram que um novo composto testado (gasolina azul) poderá apresentar características vantajosas com relação a outras adições mais conhecidas. / Abstract: The intermetallic magnesium diboride has become the object of many studies since was found to be a superconducting material, at 2001. Although it shows relatively low critical temperature, it presents very interesting points, like the simplicity of the formation of a binary compound and non expensive precursors powders. Starting from ultrafine quality precursor powders the formation of Mg'B IND. 2' phase seems to occur in a few minutes at low temperature (650ºC), what is considered commercially quite attractive when compared to complicated ceramic superconductors. Nevertheless, its superconducting properties, as well as the ceramics, depend on its microsstructure and, consequently on the kinetics of formation of the superconducting phase. Besides that, with the aim of enlarging the critical current density under higher magnetic fields, the addition of several compounds has been used in order to generate pinning centers. Silicon carbide and silicon oil are cited among the most used additions, which act like substitutional or interstitial impurities and defects generators. In this work, the influence of the addition of a few carbon compounds on the Mg'B IND. 2' phase formation was investigated, as well as the determination of a few structural parameters of the obtained materials. It was also studied the changes in the phase formation kinetics due to different magnesium precursor granulation. The XRD, DTA/TGA and magnetic measurements results indicated that the addition of a new compound (petrol) may present advantages to the additions already found in literature. / Mestre

Made available in DSpace on 2014-06-11T19:25:33Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-03-25Bitstream added on 2014-06-13T18:07:48Z : No. of bitstreams: 1 souza_ej_me_ilha.pdf: 1547635 bytes, checksum: a172b140a906aaf73e717b9702550ed0 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Com o surgimento da supercondutividade as pesquisas envolvendo aplicações desta, em especial no transporte de energia, obtiveram um avanço promissor ao longo dos anos. Assim, este trabalho objetivou a fabricação de fios supercondutores utilizando o método Powder-In-Tube (PIT), no qual foi inserido dentro de um tubo de prata o pó da cerâmica supercondutora do sistema BSCCO. Estudos preliminares foram feitos em pastilhas feitas com o mesmo material supercondutor e após alguns resultados foram confeccionados os fios supercondutores. Medidas de caracterização foram feitas dentre elas a difratometria de raios-X, medidas elétricas pelo método de quatro pontas e microscopia do tipo (FEG-MEV) equipado com EDS para determinar a morfologia e composição química dos elementos na amostra. Os resultados de difratometria de raios-X apresentaram a formação das fases desejadas do sistema BSCCO de diferentes concentrações dentre elas as fases Bi-2212 Bi- 2223 e Bi-2234 contidas nas amostras sintetizadas. As medidas elétricas detectaram uma queda abrupta na resistência elétrica dos fios mostrando a faixa de transição supercondutora das amostras estudadas. As medidas de densidade de corrente com base no critério de corrente de 1μV/cm, utilizadas nos fios confeccionados, foram obtidos valores da ordem de 1,30 – 3,90 (KA/m2). Nas imagens de microscopia vale ressaltar uma melhora na interação entre os grãos supercondutores nas amostras submetidas a um maior tempo de sinterização e também foi possível verificar e analisar através da extrusão a compactação do pó cerâmico dentro do tubo de prata. O EDS detectou a composição química dos elementos nas amostras bem como a concentração das fases em determinadas regiões analisadas / With the emergence of superconductivity research involving applications of this, particularly in the transport of energy, had a promising advance over the years. This study aims to manufacture superconducting wires using the method Powder-In-Tube (PIT), which was inserted inside a tube of silver powder of superconducting ceramic BSCCO system. Preliminary studies were done on pellets made with the same superconducting material, and after some results were prepared the superconducting wires. Characterization measurements were made among them the X-ray diffraction, electrical measurements by the method of four points and type of microscopy (FEG-SEM) equipped with EDX to determine the morphology and chemical composition of the elements in the sample. The results of X-ray diffraction showed the formation of the desired phases for this type of superconductor, ie the BSCCO system between them was possible to determine the Bi-2212 phase, Bi-2223 and Bi-2234 contained in the synthesized samples. The electrical measurements have detected a sudden drop in electrical resistance of the wires showing the range of superconducting transition with good accuracy. It was possible to perform the calculation of current density in the wires made based on a criterion of 1 μV/cm were obtained and values of around 1,30– 3,90 (KA/m2). The values are consistent with the specifications of the samples and within the limits of our research. In microscopy images is noteworthy improvement in the interaction between the superconducting grains in samples submitted to a higher sintering time and it was possible to verify and analyze the compaction of ceramic powder into silver tube through the extrusion process. The EDX detected the chemical composition of the elements in the samples in a qualitative way

Made available in DSpace on 2014-06-11T19:23:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-08-20Bitstream added on 2014-06-13T19:09:11Z : No. of bitstreams: 1 rocha_alt_me_bauru.pdf: 1916841 bytes, checksum: 67b11ea574b0bd9ed867651085e40896 (MD5) / O intermetálico diboreto de magnésio passou a ser um material intensamente estudado a partir de 2001, quando nele encontrou-se a propriedade de supercondição de corrente elétrica. Embora sua transição seja em temperatura relativamente baixa, próximo de 40 K, é grande o interesse comercial neste material devido à provável simplicidade da cinética de reação e à maneira direta e barata de processamento. Partindo de pós precursores ultrafinos de boa qualidade a formação da fase Mg'B IND. 2' parece ocorrer em apenas alguns minutos em temperatura relativamente baixa (650ºC). Sob este ponto de vista, este material é considerado bastante atrativo comercialmente, quando comparado aos supercondutores cerâmicos cujo processamento é muito mais complexo. Entretanto, suas propriedades supercondutoras, assim como nas cerâmicas, dependem da sua microestrutura e, portanto da cinética de formação da fase supercondutora. Além disso, com o objetivo de ampliar a densidade de corrente elétrica sob campos magnéticos cada vez mais altos, tem-se utilizado artifícios tais como a adição de diversos compostos geradores de centros de aprisionamento de fluxo magnético. Entre eles figuram o SiC e o silicone, formados por elementos que podem substituir um dos átomos do composto supercondutor, ou que apenas geram defeitos intersticiais. Neste trabalho, investigou-se a influência das adições de alguns compostos de carbono na formação da fase Mg'B IND. 2', assim como a determinação de alguns aspectos das microestruturas obtidas. Estudou-se também as mudanças na cinética de formação da fase devido à diferente granulação do precursor de magnésio. Os dados obtidos por DRX, ATD/TG, BET e técnicas de medidas magnéticas indicaram que um novo composto testado (gasolina azul) poderá apresentar características vantajosas com relação a outras adições mais conhecidas. / The intermetallic magnesium diboride has become the object of many studies since was found to be a superconducting material, at 2001. Although it shows relatively low critical temperature, it presents very interesting points, like the simplicity of the formation of a binary compound and non expensive precursors powders. Starting from ultrafine quality precursor powders the formation of Mg'B IND. 2' phase seems to occur in a few minutes at low temperature (650ºC), what is considered commercially quite attractive when compared to complicated ceramic superconductors. Nevertheless, its superconducting properties, as well as the ceramics, depend on its microsstructure and, consequently on the kinetics of formation of the superconducting phase. Besides that, with the aim of enlarging the critical current density under higher magnetic fields, the addition of several compounds has been used in order to generate pinning centers. Silicon carbide and silicon oil are cited among the most used additions, which act like substitutional or interstitial impurities and defects generators. In this work, the influence of the addition of a few carbon compounds on the Mg'B IND. 2' phase formation was investigated, as well as the determination of a few structural parameters of the obtained materials. It was also studied the changes in the phase formation kinetics due to different magnesium precursor granulation. The XRD, DTA/TGA and magnetic measurements results indicated that the addition of a new compound (petrol) may present advantages to the additions already found in literature.

Cette thèse présente le travail de conception, de fabrication et de test d'un processor à 4 qubits Josephson, avec un souci d’évolutivité. Les qubits ont une fréquence réglable et sont tous couplés à un unique bus de couplage, afin d’implémenter la porte à deux qubits iSWAP, sur n’importe quelle paire d'entre eux. Chaque qubit est aussi équipé d’un amplificateur Josephson à bifurcation (JBA). Le principe du processeur, le choix des paramètres, le design micro-onde ainsi que la fabrication sont décrits. Une première expérience montre la lecture simultanée, haute-fidélité et en un coup de tous les qubits, par une technique de multiplexage fréquentiel des signaux de lecture. Une seconde teste la fidélité de la porte à deux qubits iSWAP, qui apparait limitée par la décohérence intrinsèque des qubits. / This thesis presents our effort to design, fabricate and test a simple 4-Josephson qubit processor with scalability potential. The qubits are frequency tunable and are coupled to a shared coupling bus able to implement iSwap two-qubit gates on any pair of qubits. Each qubit is fitted with its own readout made of a Josephson bifurcation amplifier (JBA). The operation principle of the processor, the choice of parameters, the microwave layout design, as well as the fabrication processes are described. A first experiment demonstrates the simultaneous high-fidelity readout of all the qubits by frequency multiplexing of the JBA signals. A second one tests the two-qubit iSwap gate of the processor, the fidelity of which happens to be limited by the intrinsic qubit decoherence.

The focus of this report is the derivation of the Bogoliubov-de Gennes equations for superconductors from a tight-binding model, restricting ourselves to the case of s-wave superconductors.

Dans cette thèse, nous étudions d'un point de vue théorique différents aspects de la matière topologique. Ces systèmes présentent des propriétés résistantes aux éventuelles perturbations grâce à une topologie non-triviale de leur structure de bandes. En particulier, des excitations exotiques, par exemple des fermions de Majorana, peuvent apparaitre à leurs bords.L'entropie d'intrication, ainsi que le spectre d'intrication ont été fondamentaux dans l'étude théorique de ces systèmes, et plus généralement des phases libres. Il est cependant difficile de les mesurer expérimentalement. L'étude des fluctuations de charge bipartites a été proposée afin de remédier à ce problème, et celles-ci permettent une mesure faible de l'intrication, en particulier pour des modèles unidimensionnels libres. Nous généralisons les précédents travaux sur les Liquides de Luttinger à des familles génériques de supraconducteurs et isolants topologiques en une et deux dimensions, systèmes dans lesquels la charge observée n'est plus conservée. Nous montrons que les transitions de phases topologiques sont caractérisées par certains coefficients universels dans les fluctuations et les fonctions de corrélations. Les systèmes bidimensionnels que nous étudions présentent des cônes de Dirac, et ces coefficients dépendent de leur enroulement. Cela nous permet de caractériser la topologie de ces points critiques. Dans tous les cas, les fluctuations suivent une loi de volume, qui a un comportement non-analytique aux transition de phase.Dans un second temps, nous nous intéressons aux systèmes en interactions. Nous montrons tout d'abord que certaines des signatures des transitions topologiques survivent en leur présence, dans les supraconducteurs topologiques. Nous étudions ensuite le diagramme de phase de deux fils supraconducteurs couplés par une interaction Coulombienne. Celle-ci mène à la création de phases exotiques grâce à la compétition avec la supraconductivité non-conventionnelle. Nous montrons en particulier l'apparition de phases de Mott brisant spontanément la symétrie de renversement du temps et présentant des courant orbitaux non-triviaux, ainsi que celle d'une phase de fermions libres, qui est l'extension de deux chaînes de Majorana critiques en interaction.Enfin, nous nous intéressons aux effets de la présence de fermions de Majorana sur le transport électronique. Nous étudions un îlot supraconducteur où plusieurs de ces fermions existent. Ce système pourrait être l'un des composants élémentaires d'un éventuel ordinateur quantique. Les fermions de Majorana changent les statistiques d'échange des porteurs de charges, ce qui se traduit par une fractionnalisation de la conductance. Celle-ci se révèle très robuste face aux anisotropies et autres perturbations. Nous étendons les études précédentes au cas où le nombre d'électrons dans la boîte peut fluctuer, et montrons l'équivalence de ce problème avec le modèle Kondo à plusieurs canaux. Nous réinterprétons alors ce modèle en terme du déplacement d'une particule dans un réseau fictif dissipatif. / In this thesis, we study theoretically different aspects of topological systems. These models present resilient properties due to a non-trivial topology of their band structures, and in particular exotic edge excitations such as Majorana fermions.Entanglement entropy and entanglement spectrum have been fundamental to the study of these systems and of gapless systems in general, but are difficult to measure experimentally. Bipartite charge fluctuations were proposed as a weak measurement of this entanglement, in particular for one-dimensional gapless phases. We extend previous results on standard Luttinger Liquids to generic families of one- and two-dimensional non-interacting topological systems. Through exact computations, we show that their critical points are characterized by universal coefficients that reveal the topological aspect of the transitions. In two dimensions, the Dirac cones give quantized contributions to the fluctuations and various correlation functions. These contributions depend on their winding numbers, allowing for a precise determination of the topological structure of the gapless points. A volume law is also present and linked to the Quantum Fisher information, with characteristic non-analyticities at the phase transitions.In a second time, we include interactions and show that some of these signatures are preserved in topological superconductors even in their presence. Through analytical (bosonization, renormalization group) and numerical (exact diagonalization and DMRG) methods, we study the phase diagram of two Coulomb-coupled topological superconducting wires. We are interested in their behavior when the interactions are strong enough to break the topological protection: the interplay between unconventional superconductivity and interactions leads to exotic phases. We show the appearance of phases spontaneously breaking the time-reversal symmetry, with non-trivial orbital currents, and of an unusual gapless phase that is the extension of two critical interacting Majorana modes.Finally, we are interested in electronic transport mediated by Majorana fermions. We study a floating superconducting island carrying several such impurities. This device is thought to be a potential building block for a quantum computer. The Majorana fermions affect the statistics of the charge carriers, which leads to very resilient fractionalized transport. We extend previous studies to the charge degenerate case, where the total number of fermions in the island is not fixed, and map it to the well-known Multi-Channel Kondo model at large interaction. We reinterpret this standard model in terms of a particle moving in a highly dimensional, dissipative lattice.

Indiana University-Purdue University Indianapolis (IUPUI) / As the search for ever-higher-speed, greater-density, and lower-power technologies accelerates, so does the quest for devices and methodologies to fulfill the increasingly-difficult requirements for these technologies. A possible means by which this may be accomplished is to utilize superconducting devices and graphene nanoribbon nanotechnologies. This is because superconductors are ultra-low-power devices capable of generating extremely high frequency (EHF) signals, and graphene nanoribbons are nanoscale devices capable of extremely high-speed and low-power signal amplification due to their high-mobility/low-resistance channels and geometry-dependent bandgap structure. While such a hybrid co-integrated system seems possible, no process by which this may be accomplished has yet been proposed. In this thesis, the system limitations are explored in-depth, and several possible means by which superconducting and graphene nanotechnological systems may be united are proposed, with the focus being placed on the simplest method by which the technologies may be hybridized and integrated together, while maintaining control over the intended system behavior. This is accomplished in three parts. First, via circuit-level simulation, a semi-optimized, low-power (~0.21 mW/stage) graphene-based amplifier is developed using ideal and simplified transmission line properties. This system is theoretically capable of 159-269 GHz bandwidth with a Stern stability K >> 1 and low noise figure 2.97 <= F <= 4.33 dB for all appropriate frequencies at temperatures between 77 and 90 K. Second, an investigation of the behavior of several types of possible interconnect methodologies is performed, utilizing hybrid substrates and material interfaces/junctions, demonstrating that an Ohmic-contact superconducting-normal transmission line is optimal for a hybrid system with self-reflections at less than -25 dB over an operating range of 300 GHz. Finally, a unified layout and lithography construction process is proposed by which such a hybrid system could be developed in a monolithic physical system on a hybrid substrate while maintaining material and layout integrity under varying process temperatures.