Submitted by Izabel Franco (izabel-franco@ufscar.br) on 2016-09-19T18:31:46Z No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-20T14:01:52Z (GMT) No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-20T14:01:58Z (GMT) No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) / Made available in DSpace on 2016-09-20T14:02:06Z (GMT). No. of bitstreams: 1 TeseMGS.pdf: 2043252 bytes, checksum: a70365d13804598ad8c94cb75631f81d (MD5) Previous issue date: 2015-02-11 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Superconductivity one of the most intriguing physical phenomena has been extensively studied due to their potential for technological applications. Despite advances in research in physics there is still no complete theory to explain the phenomenon. We study the behavior of a superconducting system described by a multiband model. We consider the interband interaction, i.e., the interaction is between the fermions with dierent masses at dierent bands, under the inuence hybridization. Also, we study the intraband interaction with hybridization and external magnetic eld. Using the method of Green's functions of Zubarev, we calculate the superconducting order parameters with intra-and interbanda interaction at zero temperature, ie, T = 0, and for dierent temperatures of zero. Interband interaction obtained in the superconducting order parameter as a function of temperature and hybridization V. The phase diagram T versus V was also obtained. The superconducting properties, such as free energy, entropy and specic heat, were also determined in the intraband interaction. Initially, we calculate the superconducting properties in zero eld, ie, for H = 0, then we consider the presence of external magnetic eld H nonzero. obtained an implicit equation that relates the magnetic field, temperature and hybridization, which allowed us to obtain the phase diagram T versus T and T versus V . Our results show that both the hybridization V , as the external magnetic field H acts at the expense of superconductivity by suppressing it. We noticed also, and conrmed by other studies, that the intraband interaction, we have only second order transition, since the interaction interbanda, we obtained a superconducting tricritico point where we encounter a line first with a second order. Based on these results, we conclude that left a signicant contribution to research in superconductivity. / A supercondutividade, um dos fenômenos mais intrigantes da fisica, vem sendo extensivamente estudada, devido ao seu potencial para aplicações tecnológicas. Apesar do avanço nessa área de pesquisa, como a teoria BCS, teoria microscópica que explica os supercondutores convencionais, ainda não há uma teoria completa que explique os não convencionais. Neste trabalho estudamos o comportamento de um sistema supercondutor descrito por um modelo multibandas, pois é uma extensão da teoria BCS a qual descreve bem os supercondutores de baixas temperaturas. Consideramos a interação interbanda, isto é, a interação se da entre férmions que apresentam propriedades diferentes, tais como, massas distintas, ou mesma massa, mas comportamentos diferentes que se encontram em diferentes bandas, sob a influência da hibridização. Tambem, estudamos a interação intrabanda com hibridização e campo magnético externo. Usando o método das funções de Green de Zubarev, calculamos os parâmetros de ordem supercondutor para as interações intra e interbanda a temperatura nula, isto é, T = 0 (transições quânticas), e para temperaturas finitas (transições térmicas ou clássicas) afim de obtermos um estudo completo do sistema. Na interação interbanda obtivemos o parâmetro de ordem supercondutor em função da temperatura e da hibridização V . O diagrama de fase T versus V também foi obtido. As propriedades supercondutoras, tais como energia livre, entropia e calor especifico, também foram determinadas na interação intrabanda. Inicialmente, calculamos as propriedades Supercondutoras a campo nulo, isto é, para H = 0, em seguida, consideramos a presença do campo magnético externo H diferente de zero. Obtivemos uma equação implícita que relaciona o campo magnético, temperatura e hibridização, o que nos permitiu obter o diagrama de fase T versus H e T versus V . Nossos resultados mostram que tanto a hibridização V , quanto o campo magnético externo H atuam em detrimento da supercondutividade, suprimindo-a, concordando com outros trabalhos, apesar de não ser isso geral, uma vez que alguns trabalhos mostram que V contribui para a supercondutividade até um determinado valor e para outros valores, suprime-a. Percebemos também que tanto na interação interbanda, quanto na interação intrabanda temos transições de primeira e segunda ordem e um ponto tricritico, no qual temos o encontro de uma linha de primeira ordem com uma de segunda ordem.

Thesis submitted in fulfilment of the requirements for the degree Master of Technology: Electrical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology, 2012 / In order to effectively determine the occurrences of space weather anomalies in near Earth orbit, a highly sensitive space-grade magnetometer system is needed for measuring changes in the Earth’s magnetic field, which is the aftermath of space weather storms. This research is a foundational work, aimed at evaluating a commercial-off-the-shelf (COTS) high temperature DC SQUID (superconducting quantum interference device) magnetometer, and establishing the possibility of using it for space weather applications. A SQUID magnetometer is a magnetic field measuring in strument that produces an electrical signal relative to the sensed external magnetic field intensity.

Orientador: Iakov Veniaminovitch Kopelevitch / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-13T05:43:34Z (GMT). No. of bitstreams: 1 GutierrezYatacue_DiegoFernando_M.pdf: 13276293 bytes, checksum: 707c38ac7116b9830e881ac37cc37b9e (MD5) Previous issue date: 2009 / Resumo: No intuito de verificar a existência de correlações supercondutoras em grafite, estudamos a possível existência do fenômeno conhecido como efeito de proximidade gigante em amostras de grafite pirolítica altamente orientada (HOPG). Medidas de magneto-transporte realizadas em amostras de HOPG com eletrodos supercondutores de In ou In-Pb revelaram a ocorrência de efeito de proximidade em uma escala muito maior que o comprimento de coerência dos eletrodos supercondutores, o que indica que a grafite pode ser considerada um supercondutor com flutuações de fase. Além disso, nossos estudos revelaram uma supressão do efeito de proximidade para campos magnéticos da ordem de 1 kOe aplicado perpendicularmente aos planos de grafite. Adicionalmente, realizamos estudos comparativos do efeito de proximidade em bismuto metálico. Discutimos os resultados obtidos em termos de modelos teóricos propostos para este assunto. / Abstract: In order to verify the existence of superconducting correlations in graphite, in this work we studied the possibility of the so-called giant proximity effect in highly oriented pyrolytic graphite (HOPG) samples. Magnetoresistance measurements performed on various thoroughly characterized HOPG samples with attached superconducting In or Pb-In electrodes revealed the occurrence of proximity effect on a scale much bigger than a coherence length of superconducting electrodes, indicating that graphite can be considered as a phase-fluctuating superconductor, indeed. Besides, our studies revealed a suppression of the proximity effect in magnetic field H ~ 1 kOe applied perpendicularly to graphene planes. Additionally, we performed comparative studies of the proximity effect in semimetallic bismuth. We discuss the obtained results in terms of available theoretical models. / Mestrado / Física da Matéria Condensada / Mestre em Física

In der vorliegenden Arbeit wurden Schichten des Hochtemperatursupraleiters YBa2Cu3O7-δ (YBCO) untersucht, die epitaktisch mittels gepulster Laserabscheidung auf texturierten metallischen Templaten abgeschieden wurden. Dabei kamen ionenstrahltexturiertes ABAD-YSZ-Band und walztexturiertes NiW-Band zum Einsatz. Für Anwendungen solcher sogenannten Bandleiter ist die Fähigkeit zum Transport möglichst hoher kritischer Ströme Ic essentiell. Dies kann durch das Wachstum möglichst dicker Schichten, eine Verbesserung der Flussschlauchverankerung (Pinning), sowie die Reduzierung der durch Korngrenzen verursachten Stromlimitierung erreicht werden. Im ersten Teil der Arbeit wurden strukturelle und supraleitende Eigenschaften in bis zu 5 µm dicken YBCO-Schichten auf ABAD-YSZ-Band untersucht. Dazu wurden neben nicht-fremdphasendotierten (undotierten) YBCO-Schichten auch BaHfO3- und BaY(NbTa)O6-dotierte YBCO-Schichten herangezogen. Die Untersuchungen erfolgten mittels Röntgenbeugungs-methoden (XRD), Rasterelektronen- und Rasterkraftmikroskopie (REM, AFM), sowie resistiver und induktiver Methoden zur Bestimmung der Sprungtemperatur Tc und der kritischen Stromdichte Jc. Schichtdickenabhängige Messungen konnten u. a. zeigen, dass Ic in undotierten YBCO-Schichten bei einer Dicke oberhalb von 2,8 µm nicht weiter anstieg, während Ic in den dotierten Schichten bis zu einer Dicke von 5 µm nicht limitiert war. Darüber hinaus konnte in temperatur- und feldabhängigen Jc-Messungen ein verbessertes Pinningverhalten bei tiefen Temperaturen und in hohen äußeren Magnetfeldern gefunden werden. Mit Hilfe von transmissionselektronenmikroskopischen Untersuchungen (TEM) und Messungen der Jc-Anisotropie wurde zusätzlich ein Strukturschema herausgearbeitet, das eine Verknüpfung von Herstellungsparametern, Mikrostruktur und Pinningeigenschaften ermöglicht. Der zweite Teil befasste sich mit der Analyse der Mikrostruktur und der lokalen Textur von undotierten YBCO-Schichten auf ABAD-YSZ- und NiW-Band. Die lokale Textur wurde dabei mittels Rückstreuelektronenbeugung (EBSD) aufgelöst. Die Ergebnisse wurden mit orts-aufgelösten Magnetisierungsmessungen (Raster-Hall-Sonden-Mikroskopie) korreliert, mit denen der lokale Stromfluss untersucht werden kann. Auf beiden Templaten zeigte sich eine granulare Struktur, die durch gegeneinander verkippte bzw. verdrehte YBCO-Körner gekennzeichnet war. Während die granulare Struktur auf ABAD-YSZ-Band durch hauptsächlich in-plane missorientierte YBCO-Bereiche mit einem Durchmesser < 1 µm charakterisiert war, konnten auf NiW-Band hauptsächlich out-of-plane missorientierte Bereiche mit einer typischen Größe von 20-50 µm nachgewiesen werden. Auf NiW-Band hing die YBCO-Mikrostruktur und die Schärfe der lokalen Textur von der individuellen out-of-plane Missorientierung des darunter-liegenden NiW-Korns ab, was auf eine bereits im unbeschichteten NiW-Band vorhandene Facettierung zurückgeführt werden konnte. Zu deren Beschreibung wurde ein Facettenmodel entwickelt, das durch TEM-Untersuchungen bestätigt werden konnte. Abschließend wurde auf einkristallinen Substraten und auf ABAD-YSZ-Band ein homogener Stromfluss nachgewiesen, während die supraleitenden Eigenschaften in YBCO-Schichten auf NiW-Band von der lokalen Mikrostruktur und Textur bestimmt wurden. Damit konnte gezeigt werden, dass die globalen Eigenschaften der Bandleiter vom Zusammenspiel eines über Korngrenzen miteinander verbundenen Ensembles von Körnern mit individuellen Eigenschaften bestimmt werden.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Currently, several research groups have been devoted to the production of superconducting films. This interest lies on the facility of their manipulation, on the understanding of basic concepts of the superconductivity and, especially, in the possibility of reducing them to dimensions compatible to their application as devices. Therefore, there is a great technological and academic interest in this issue. The most used material in superconducting devices is the niobium (Nb) and its alloys. Although Nb has been known as superconductor since 1930, the interest in excellent films of this material still remains. In this work, using a DC magnetron sputtering technique, we have developed an experimental method that allows one to produce superconducting Nb films of good quality - transition temperature of 9.1 K and transition width of 0.2 K. Therefore, we performed a systematic study of the setup parameters that influences the structural and superconducting properties of the films. In particular, we have analyzed the evolution of the lattice parameter, average grain size crystalline and critical temperature. Such values have been compared in a diagram. Nb films exhibit flux avalanches at certain values of field and temperature. We have detected and visualized the occurrence of avalanches of magnetic flux in Nb films and, in order to suppress the abrupt flux penetration, we have coupled to the films of thickness of 200 nm, thick metal layers of aluminum, silver and copper of different thicknesses, so called hybrid systems - Superconductor/Normal Metal. In this work piece, two experimental techniques have been employed: magnetometry and magneto-optical imaging. A partial suppression of avalanches in the hybrid systems has been observed. / Atualmente, diversos grupos de pesquisa têm se dedicado à fabricação de filmes supercondutores. O interesse reside na facilidade de manipulação, no entendimento de conceitos básicos do fenômeno da supercondutividade e, principalmente, na possibilidade de reduzi-los a dimensões compatíveis à aplicação como dispositivos. Portanto, há um grande interesse acadêmico e tecnológico nesta área. O material mais utilizado na construção de dispositivos supercondutores é o nióbio (Nb) e suas ligas. Apesar do Nb ser conhecido como supercondutor desde 1930, o interesse em excelentes filmes desse material ainda persiste. Neste trabalho, utilizamos a técnica de magnetron sputtering DC, com a qual estabelecemos uma rotina experimental que possibilitou produzir filmes supercondutores de Nb de boa qualidade - temperatura crítica de 9,1 K e largura de transição de 0,2 K. Para tanto, realizamos um estudo sistemático de como os valores dos parâmetros experimentais influenciam nas propriedades estruturais e supercondutoras dos filmes. Em particular, analisamos a evolução do parâmetro de rede, do tamanho médio dos grãos cristalinos e da temperatura crítica com os valores das variáveis de deposição e, por conseguinte, comparamos esses valores em um diagrama. Filmes de Nb exibem avalanches de fluxo magnético quando submetidos a certos valores de campo e temperatura. Detectamos e visualizamos a ocorrência de avalanches de fluxo magnético nos filmes de Nb produzidos e, com o intuito de suprimir as penetrações abruptas de fluxo, acoplamos a filmes de 200 nm de espessura camadas metálicas de alumínio, prata e cobre de diferentes espessuras, denominados sistemas híbridos - Supercondutor/Metal Normal. Este estudo foi conduzido por meio de duas técnicas experimentais: magnetometria volumétrica e imageameto magneto-ótico. Observamos uma supressão parcial das avalanches de fluxo magnético nos sistemas híbridos.

Electronic systems are an indivisible part of modern life. Every day, new materials, devices, passive components, antennas for wireless communication are needed to be designed and developed. In particular, flexible and biocompatible wearable devices are urgent required for medical and industrial applications. The great hope lies in the materials with high crystalline quality and flexibility such as graphene and other 2D semiconductors and insulators. Doping is a conventional tool for tailoring of the electronic properties of the functional materials. Here we examine application of the widely used the electron donor species to the graphene and hexagonal boron nitride monolayer (h-BN). For each we determine surface-interface properties and the full electronic band structure using the combination of the surface science methods such as angle-integrated and angle resolved photoemission (XPS, ARPES), electron diffraction (LEED) and photo absorption (XAS). As the result we provided insight into mechanisms underlying the doping gating of the graphene h-BN monolayer by the alkali metals. We fully characterized their surface and interface structure. Finally we studied the interplay between electrons and phonons in the doped graphene and we demonstrated that Ca-doped graphene is the promising candidate for realizing superconductivity in graphene.

L'un des buts de la physique de la matière condensée est à l'heure actuelle de fournir de nouveaux systèmes topologiques en particulier dans le domaine de la supraconductivité. L'une des manières envisagée pour générer des supraconducteurs topologiques est d'utiliser une interaction magnétique locale sous la forme de chaînes d'impuretés magnétiques, de vortex ou de clusters d'impuretés magnétiques ordonnées. Dans cette thèse nous avons étudié un ensemble d'effets en partant de l'étude d'impuretés individuelles en allant jusqu'aux clusters organisés en interaction avec un supraconducteur bidimensionnel. En utilisant la microscopie et la spectroscopie tunnel nous avons étudié des monocrystaux de 2H-NbSe$_{2}$ ainsi que les monocouches de Pb/Si(111). En raison du caractère électronique bi-dimensionnel de ces deux systèmes nous avons pu montrer que l'étendue spatiale des états liés induits par des impuretés magnétiques était considérablement augmentée en comparaison avec les supraconducteurs tridimensionnels. En combinant ces atomes magnétiques par auto-assemblage nous sommes parvenus à réaliser des clusters ferromagnétiques qui génèrent une supraconductivité topologique dans la monocouche de Pb. Nous présentons en particulier ici la mesure d'états de bords topologiques à l'interface entre Pb/Si(111) et Pb/Co/Si(111). Nous présentons également la mesure d'états liés à zéro énergie au centre de clusters magnétiques signant la présence de fermions de Majorana dans ces systèmes. / One of the present days goals of condensed matter physics is to create new systems with topological properties, especially in the field of superconductivity. One of the ways envisioned to create topological superconductors is to locally induce a magnetic interaction in the form of chains of magnetic impurities, vortices or magnetic clusters of ordered magnetic impurities. In this thesis we studied a set of effects from individual impurities to organized clusters interacting with two-dimensional superconductors. Using scanning tunneling microscopy and spectroscopy we considered two systems, monocrystals of 2H-NbSe$_{2}$ and monolayers of Pb/Si(111). Thanks to the two-dimensional electronic behavior of these two systems we show how the spatial extent of the bound states induced by magnetic impurities is considerably enhanced compared to the case of a three-dimensional superconductor. By combining these magnetic atoms using a self-assembly method we were able to create ferromagnetic clusters that lead to a topological superconductivity in Pb monolayers. In particular we present here measurement of topological edge states at the interface Pb/Si(111) and Pb/Co/Si(111). We also present the measurement of zero bias peaks in the center of larger magnetic clusters that sign the presence of Majorana fermions in these systems. Our results show that an adequate patterning of surfaces could realize topological patches and call for a pursuit of the efforts in the subject in order to be able to control Majorana fermions that could eventually lead to breakthrough in quantum computation.

No description available.

Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures. In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.

Orientador: Eduardo Miranda / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-01T08:41:01Z (GMT). No. of bitstreams: 1 Almeida_DalsonEloy_D.pdf: 2470369 bytes, checksum: 93d3b945f62f374cfd686217575dda95 (MD5) Previous issue date: 2017 / Resumo: Nesta tese estudamos a relação entre antiferromagnetismo e supercondutividade em pnictídeos à base de ferro. Este estudo será feito através da análise de uma energia livre de Ginzburg-Landau de parâmetros de ordem acoplados que será derivada de um modelo microscópico. Em particular, estamos interessados em saber se a transição entre os estados ordenados é de primeira ordem ou se as duas ordens podem coexistir. Para o caso de supercondutividade convencional as duas fases puras nunca coexistem. Entretanto, quando a supercondutividade é não-convencional e a condição de nesting perfeito não é satisfeita, pode haver um regime intermediário de coexistência microscópica das duas ordens. Nesta nova fase termodinâmica, as simetrias de rotação no espaço de spins, de reversão temporal e U(1) são quebradas simultânea e localmente. Logo, os canais de supercondutividade singleto e tripleto se misturam quanticamente. Em outras palavras, uma componente tripleto secundária do estado supercondutor é gerada. Os diagramas de fases do sistema são apresentados e analisamos também como flutuações magnéticas, acima da temperatura de Néel pura, afetam a temperatura de transição tripleto. Investigamos também o efeito da magnetização alternada no efeito Josephson, i.e., na supercorrente que flui através de uma junção entre dois supercondutores na fase de coexistência. Por fim, mas não menos importante, estudamos o efeito de proximidade em uma interface entre um supercondutor e um antiferromagneto. Veremos que os pares de Cooper podem penetrar a região magnética e em consequência, uma componente tripleto é induzida próximo da interface / Abstract: In this thesis, we study the interplay between antiferromagnetism and superconductivity in iron pnictides. This study will be done analyzing a free energy of coupled order parameters which will be derived from a microscopic model. In particular, we are interested if the phase transition between the ordered states is first order or if the two orders can coexist. For the case of conventional superconductivity, the two phases cannot coexist. However, when superconductivity is unconventional and the perfect nesting condition is not satisfied, there can exist an intermediary state of microscopic coexistence of the two orders. In this new thermodynamic phase, spin rotation, time reversal and U(1) symmetries are simultaneously and locally broken. Therefore, the singlet and triplet superconductivity channels are quantum mechanically mixed. In other words, a secondary triplet component is generated. The phase diagrams of the system are presented and we also analyze the effect of magnetic fluctuations above the pure Néel temperature on the triplet temperature transition. We also investigate the effects of the staggered magnetization on the Josephson effect, i.e., on the supercurrent that flows through a junction of two superconductors in the coexistence phase. Last, but not least, we study the proximity effect at an interface between a superconductor and an antiferromagnet. We will see that the Cooper pairs can penetrate the magnetic region and consequently a triplet component is induced near the interface / Doutorado / Física / Doutor em Ciências / 140834/2013-3 / 2342/15-4 / CNPQ / CAPES / BEX