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Superconducting Quantum Interference Devices (SQUIDs) are currently being used as flux qubits and read-out detectors in a variety of solid-state quantum computer architectures. The main limitation of SQUID qubits is that they have a coherence time of the order of 10 us, due to the presence of intrinsic flux noise that is not yet fully understood. The origin of flux noise is currently believed to be related to spin impurities present in the materials and interfaces that form the device. Here we present a novel numerical method that enables calculations of the flux produced by spin impurities even when they are located quite close to the SQUID wire. We show that the SQUID will be particularly sensitive to spins located at its wire edges, generating flux shifts of up to 4 nano flux quanta, much higher than previous calculations based on the software package FastHenry. This shows that spin impurities in a particular region along the wire's surface play a much more important role in producing flux noise than other spin impurities located elsewhere in the device. / Graduate / 0611 / 0607 / 0753 / laforest@uvic.ca

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (p. [225]-236).

Thesis (Ph.D.)--Mississippi State University. Department of Physics and Astronomy. / Title from title screen. Includes bibliographical references.

Les récents progrès des supraconducteurs à haute température critique (SHTC) rendent ces matériaux très prometteurs pour les applications de grande envergure. Ils montrent des propriétés stupéfiantes, particulièrement à très basse température où ils sont capables de transporter des courants extrêmement élevés. Ces conducteurs sont construits sur un substrat en alliage de nickel qui leurs permettent de supporter des efforts mécaniques considérables. Pour ces raisons, les SHTC sont à présent au cœur de nombreux projets pour bâtir une nouvelle génération d’aimants produisant des champs intenses. Ce travail est mené dans le cadre du projet Nougat dont l’objectif est la conception d’un insert générant 10 T à l’intérieur d’un aimant plus grand de 20 T. Malgré leurs remarquables propriétés, les conducteurs SHTC montrent d’importantes inhomogénéités de performance ce qui, ajouté à leur forte capacité calorifique, peut provoquer des échauffements très localisés. La transition locale du matériau vers un état dissipatif est difficile à détecter notamment à cause de la lente propagation de l’évènement. Le signal de transition risque donc d’être noyé dans le bruit de mesure et être à l’origine de dommages sévères voir irréversibles du conducteur. Pour répondre à ce problème, des électro-aimants sont fabriqués en remplaçant l’isolation électrique par un ruban métallique. Ces aimants sont appelées Bobines Métal-Isolée (BMI). Dans cette configuration et dans le cas spécifique d’une inhomogénéité sur le conducteur, le courant court-circuite le défaut en passant sur la spire suivante ce qui permet d’empêcher la destruction du bobinage. Cependant, dans ce type de bobinage, la stabilité et la linéarité du champ magnétique généré peut être un problème et les dynamiques lentes de ce type de bobine les empêchent de prétendre à être utilisé pour certaines applications. Pour des raisons pratiques, le prototype final du projet Nougat sera construit avec un bobinage métalliquement isolé mais le travail mené ici se concentre sur les bobines électriquement isolées, plus particulièrement l’étude de la transition dans le bobinage afin d’identifier des possibilités de protection fiables. Quelques résultats d’échantillons BMI sont cependant présentés pour comparaison. Dans un premier temps, les contraintes générales de conception de l’insert NOUGAT sont définies, principalement les exigences pour le conducteur et les calculs préliminaires de performance. Cette étude souligne le besoin de caractériser le conducteur SHTC sous haut champ et à basse température, dans des conditions similaires à celles du fonctionnement final du prototype d’insert. Dans un second temps, cette caractérisation expérimentale d’échantillons courts est implémentée dans un modèle pour simuler le comportement de bobines pour la construction d’aimants. L’objectif de ce modèle est d’étudier le départ de transition et sa propagation dans le bobinage. Une des particularités du modèle est la simulation de l’entièreté du bobinage dans le but de prendre en compte les inhomogénéités de performance du conducteur, dont les mesures en continu sont fournies à 77 K en champ propre par les constructeurs. Le dernier axe de ce travail est l’étude de plusieurs échantillons bobinés pour permettre notamment la comparaison entre bobinage isolé électriquement et métalliquement. Leurs comportements électrique, mécanique et thermique sont examinés ainsi que les problèmes de couplage dus à la présence d’un aimant extérieur. Les résultats expérimentaux sont comparés aux calculs préliminaires ainsi qu’aux simulations de modèle. Un protocole expérimental est également proposé pour évaluer les performances d’une bobine sans risque et est testé avec succès. / Recent improvements in High Temperature Superconductors (HTS) make them promising for large scale applications. They show astonishing properties, especially at very low temperature where they are able to carry high amount of current. These conductors are also built on a nickel alloy substrate allowing them to face severe mechanical stresses. For these reasons, HTS are now placed at the heart of numerous projects for building a next generation of high field magnets. This work is conducted in the NOUGAT project, which intends to design and build a 10 T HTS insert working in a 20 T background field. Despite their outstanding properties, HTS conductors show strong inhomogeneities in their performance along their length. This added to their high heat capacity can be at the origin of local hot spots. The transition to a dissipative state on this local area is then difficult to detect; because of the low speed of its propagation. The transition signal is likely to be lost in the high noise level environment, which can lead the winding to severe or irreversible damages.One way of dealing with this problem is to create coils where the electric insulation is removed and replaced by a metallic layer, the so-called Metal-as-Insulation winding technique. In this configuration and in a case of an inhomogeneity, the current bypasses through the turn-to-turn contact resistance and prevents the winding from burning. However, in such kind of winding field stability and linearity can be an issue. The slower dynamics obtained with this method prevent its use in some applications.The main focus of this work is therefore insulated coils especially the study of the transition behaviour to explore the possibility of reliable protections. In the same extend, the present work also considers safe ways to evaluate the performance of a wound sample in order for it to work with appropriate margins when at nominal operating conditions. This would decrease the need of a strong detection/protection system: the high heat capacity of the conductor makes windings unlikely to be damaged by an outside event.For practical reasons, the NOUGAT project will be built using MI technique, and therefore some MI coils results are presented in this work for comparison purpose. In a first step, the general design constrains for the NOUGAT project insert are defined, especially tape requirements and performance calculations. This underlines the necessity of characterizing the HTS conductor under high field and at low temperature, under conditions similar to the expected operation of the final insert prototype.In a second step, this experimental short sample characterization is implemented in a model to simulate the behaviour of pancake coils designed to build magnet. The aim of the model is to investigate on the transition start and propagation inside the winding. One of the main specificities is the simulation of the whole winding taking as an input the continuous critical current density measurement given by the providers at 77 K, self-field.The last axis of this work is the study of several wound samples allowing comparison between both insulated and metallic insulated windings. Their electric, magnetic, mechanic and thermal behaviours are examined as well as the coupling issues while working inside an outer magnet. Experimental results are compared to preliminary calculations and modelling results. An experimental protocol to safely evaluate coil performance margins is also proposed and tested successfully.Conclusions are then drawn about the possibility to operate safely full scale HTS magnets with present day conductor performances.

At their discovery in 2008, iron pnictide superconductors (IPS) provoked tremendous scientific interest, comparable to the discovery of the cuprate superconductors. So far, IPS reached critical temperatures T c up to 56K. Typically, they show an antiferromagnetic (afm) spin density wave (SDW) which has to be suppressed by doping before superconductivity develops, which then is supported by further doping. Due to the close vicinity of the magnetic and the superconducting (sc) phase, magnetic fluctuations are discussed to be responsible for the sc pairing mechanism in IPS. A special member of the IPS is LiFeAs, because it does not need doping to become sc. It is a stoichiometric superconductor at a T c of 18K. In fact, doping is suppressing its T c . Also, there is no sign of an afm SDW present. Therefore, LiFeAs is a interesting material to study the properties of the IPS in an undisturbed material. In 2010, experiments of the Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden) revealed further surprising properties of LiFeAs. Samples with a Li deficiency undergo a ferromagnetic (fm) phase transition at 165K. Theoretical calculations suggest that fm fluctuations could induce triplet superconductivity in LiFeAs. This would cause a nonvanishing dynamic susceptibility below T c , which is supported by nuclear magnetic resonance (NMR) experiments. This thesis is discussing the results of the IFW Dresden experiments, and concludes that this ferromagnetism is of weak itinerant nature. The origin might be an increase of the density of states (DOS) at the Fermi level, which is causing an instability towards fm order, as proposed by the Stoner model. For further doping experiments, the synthesis procedure of polycrystalline LiFeAs was optimized to get samples with maximum T c and minimum impurities. Therefore, nuclear quadrupole resonance (NQR) was used. The NQR line width is a measure of impurities in the sample. By minimizing the NQR line width, optimal samples were synthesized. These samples are able to compete with the properties of single crystals. To investigate the doping behavior of LiFeAs, a scenario with four different kinds of impurities and deficiencies was performed with the optimized synthesis procedure. 24 different samples were analyzed, by means of NQR and electrical conductivity. It was found that in fact Fe excess is responsible for changing the physical properties of LiFeAs, and not Li deficiency. It is causing a shrinking of the unit cell volume, as seen by X-ray diffraction (XRD) measurements and it causes a decrease of T c . It also leads to a decrease of room temperature resistivity, which is supporting an increase of the DOS at the Fermi level. The NQR frequency is scaling with the amount of Fe excess and can be used to draw the sc and fm phase diagram of off-stoichiometric LiFeAs. At an amount between 3.2 and 3.6% o f Fe excess LiFeAs undergoes the fm transition.

A superfície (001) dos compostos III-V têm grande importância para as modernas tecnologias de crescimento de materiais, assim o estudo da formação dessa superfície, de suas propriedades e dos processos de adsorção de moléculas que ocorrem sobre ela é essencial para o desenvolvimento da ciência de materiais. Sabe-se que o InP, a exemplo de outros compostos III-V, apresenta uma variedade de padrões de reconstrução para superfície (001) que dependem das condições iniciais que prevalecem durante o seu crescimento. Neste trabalho estudamos os padrões da superfície InP(001) originados em ambientes com concentração máxima de átomos de índio e de fósforo e os processos envolvidos com a adsorção da molécula H2S sobre estas superfícies. Usamos cálculos de primeiros princípios dentro do formalismo da Teoria do Funcional da Densidade (DFT) associados à pseudopotenciais de norma conservada com aproximação generalizada do gradiente para o termo da energia de troca e correlação (DFT-GGA) e correlação não linear de caroço (NLCC). Fizemos a adsorção da molécula de H2S sobre os padrões mais estáveis da superfície InP(001), considerando os casos com a molécula dissociada e não dissociada. No padrão reconstrução 2x2 com dois dímeros, superfície originada em ambientes ricos em átomos de fósforo, fizemos o cálculo das barreiras de energia entre as configurações energeticamente mais favoráveis e encontramos um mecanismo de adsorção para a molécula de H2S dissociada Na superfície com padrão de reconstrução 2x4, crescida em ambientes ricos em átomos de índio, com formação de um dímero misto, verificamos que a molécula não dissociada adsorve sobre os sítios formados por átomos de fósforo e que adsorve sobre os sítios formados por átomos de índio. O átomo de enxofre da molécula dissociada adsorve sobre os sítios formados por átomos de fósforo e de índio e as moléculas de hidrogênio permanecem desorvidas, porém as estruturas são desfavoráveis energeticamente. / The (001) surface of the III-V compounds have great importance for modern technologies of materials growth thus the study of these surfaces formation, its properties and the absorption process of the molecules are essential for the development of materials science. It is well known that the InP, as well others III-V compounds, present a variety of reconstruction patterns for (001) surface that depend on the initial growth conditions. In this work we studied the patterns of InP(001) surface considering an environment with high concentration ln and P atoms and the interaction of the H2S molecule with these surfaces. Using first-principles calculations within the Density Functional Theory (DFT) formalism, norm-conserving pseudopotentials with the generalized gradient approximation for the exchange and correlation energies (GGA) and non-linear core-correction (NLCC). We have studied the H2S molecule adsorption on different reconstruction of the InP(001) surface, considering the cases in which the H2S molecule was dissociated and non-dissociated. In the 2x2 pattern with two dimmers, the energy barriers were calculated between the more energetically favorable configurations and a possible mechanism of adsorption of dissociated H2S molecule is proposed. The non-dissociated molecule does not bind over phosphorus sites but over indium sites, for all considered reconstructions. The sulfur atom of dissociated molecule adsorbs over phosphorus and indium sites and the hydrogen molecules stands not joint, however the structure are energetically unfavorable.

Orientador: Carlos Alberto dos Reis Filho / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-20T21:29:13Z (GMT). No. of bitstreams: 1 OtaloraBuitrago_DianaPatricia_M.pdf: 4654440 bytes, checksum: 441fc975cdf8f7e9af7978587263e135 (MD5) Previous issue date: 2012 / Resumo: Este trabalho descreve os procedimentos experimentais e as justificativas que levaram à determinação de uma relação entre a densidade de tensão de ruído em altas frequências e a temperatura da junção de um LED branco de alta eficiência. As medições da densidade de tensão de ruído foram realizadas polarizando o LED com correntes na faixa de 1'mi'A até 326mA e temperaturas de junção desde 24ºC até 120ºC. A realização deste trabalho, que é de cunho exploratório, foi motivada pela busca de mecanismos que permitam a avaliação contínua da temperatura da junção de um LED em operação, tendo em vista sua grande influência na durabilidade do LED. A crescente expansão do uso do LED em lugar das lâmpadas incandescentes e fluorescentes está mudando a indústria da iluminação, trazendo com isto novos desafios. Um destes desafios está na melhoria dos circuitos de acionamento (drivers e fontes de alimentação), que hoje respondem por 59% das falhas nos LEDs. Pesa na confiabilidade destes circuitos a falta do controle da temperatura de junção. Os resultados deste trabalho, apesar de restritos à caracterização de apenas duas amostras de um LED branco de alta eficiência, mostraram nitidamente a presença de um ruído de excesso cuja amplitude tem forte correlação com a temperatura da junção. Foi constatado, nas duas amostras caracterizadas, que a densidade de tensão de ruído medida é equivalente ao que produz um resistor de aproximadamente 13K'ômega' / Abstract: This paper describes the experimental procedures and the reasons that led to determining a relationship between the high-frequencies noise voltage density and the junction temperature of a highefficiency white LED. the measurements of noise voltage density were performed biasing the LED with currents in the range of 1'mi'A up to 326mA and temperatures junction from 24ºC to 120ºC. This work, which is exploratory, was motivated by the search for mechanisms that allow to continually assess the junction temperature of an LED in operation, given its great influence on the lifetime of the LED. The growing expansion of the use of LEDs instead of incandescent and fluorescent bulbs is changing the lighting industry, bringing with it new challenges. One of these challenges is the improvement of the power supplies and driver circuits, which now account for 59% of failures in LEDs. The lack of control of the LED junction temperature in these circuits is one of the issues that respond to their reduces reliability. The outcome of the herein presented work, although only restricted to the characterization of two samples of a high-efficiency white LED, clearly showed the presence of an excess noise whose amplitude has strong correlation with the junction temperature. It was found from the two tested samples, that the measure noise voltage density is equivalent to what is produced by a resistor of approximately 13K'omega' / Mestrado / Eletrônica, Microeletrônica e Optoeletrônica / Mestre em Engenharia Elétrica

Using time-resolved photo-induced reflectivity, we reported for the first time a systematic work on the ultrafast response of Bi₂Sr₂CaCu₂O_8+δ (BSCCO-2212) and Tl₂Ba₂CuO_6+δ (TBCO-2201), measurements of detwinned YBa₂Cu₃O_7-δ (YBCO-123) single crystal with the electric field ̲E parallel to the a and b-axis and high-resolution measurements of the rising edge dynamics of YBCO-123 thin films. We identified similar photo-induced responses for BSCCO, TBCO and for YBCO with ̲E ⊥ ̲b, which indicates that we observed a universal response of HTSC coming from the CuO2 superconducting planes. This latter dynamics is composed of three different components corresponding to the superconducting, pseudogap and normal state. A bi-molecular model has been put forward to explain the linear temperature dependence of the decay rate; the model implies that the re-formation of the condensate is limited by the rate at which quasiparticle interact. Moreover, we observed superconducting fluctuations up to 13K above T_c and a divergence of the long-lived component magnitude at very low temperature, which is explained by a cw heating model. In the pseudogap state, we have several indications that the negative peak observed between T_c and T* has a different origin from that of the superconducting signal below T_c. We argued that the probe mechanism of the pseudogap signal is electronic excitations of the pseudogap correlations. In the normal state, the observed dynamics is similar to that of simple metals. In the second part of this thesis, the rising edge dynamics of YBCO has been resolved in time. The model developed to interpret the results implies that the hot quasiparticles relaxation time down to the Fermi energy is 55fs. In this context, we proved that the Mazin model cannot explain both the oscillatory and the non-oscillatory part of the dynamics in YBCO. Finally, in YBCO-123, a new response has been observed with ̲E ∥ ̲b. We argued that the origin of this component is intraband transitions. This dynamics is solely responsive to the pseudogap, coming from the difference in scattering rate between pre-formed pairs and quasiparticles in the Drude reflectivity. The response with ̲E ⊥ ̲b exhibits a strong a-b plane anisotropy in its long-lived component, which can be interpreted as a d-wave gap symmetry using the thermally-activated model.

In this thesis, we study quantum phase transitions and topological phases in low dimensional fermionic systems. In the first part, we study quantum phase transitions and the nature of currents in one-dimensional systems, using eld theoretic techniques like bosonization and renormalization group. This involves the study of currents in Luttinger liquids, and the fate of a persistent current in a 1D system. In the second part of the thesis, we study the different types of Majorana edge modes in a 1D p-wave topological superconductor. Further we extend our analysis to the e ect of an additional s-wave pairing and a Zeeman field on the topological properties, and present a detailed phase diagram and symmetry classification for each of the cases. In the third part, we concentrate on the topological phases in two-dimensional systems. More specifically, we study the experimental realization of SU(3) topological phases in optical lattice experiments, which is characterized by the presence of gapless edge modes at the boundaries of the system. We discuss the specific characteristics required by a such a three component Hamiltonian to have a non-zero Chern number, and discuss a schematic lattice model for a possible experimental realization. The thesis is divided into three chapters, as discussed below: In the first chapter, we study the effect of a boost (Fermi sea displaced by a finite momentum) on one dimensional systems of lattice fermions with short-ranged interactions. In the absence of a boost such systems with attractive interactions possess algebraic superconducting order. Motivated by physics in higher dimensions, one might naively expect a boost to weaken and ultimately destroy superconductivity. However, we show that for one dimensional systems the e ect of the boost can be to strengthen the algebraic superconducting order by making correlation functions fall o more slowly with distance. This phenomenon can manifest in interesting ways, for example, a boost can produce a Luther-Emery phase in a system with both charge and spin gaps by engendering the destruction of the former. In the second chapter, we study the type of Majorana modes and the topological phases that can appear in a one-dimensional spinless p-wave superconductor. We have considered two types of p-wave pairing, 4"" = 4## and 4"" = 4##., and show that in both cases two types of Majorana bound states (MBS) with different spatial dependence emerge at the edges: one purely decaying and one damped oscillatory. Even in the presence of a Zeeman term B, this nature of the MBS persists in each case, where the value of chemical potential and magnetic field B decides which type will appear. We present a corresponding phase diagram, indicating the number and type of MBS in the -B space. Further, we identify the possible symmetry classes for the two cases (based on the ten-fold classification), and also in the presence of perturbations like a s-wave pairing and various terms involving magnetic field. It is seen that in the presence of a s-wave perturbation, the MBS will now have only one particular nature, the damped oscillating behaviour, unlike that for the unperturbed p-wave case. In the third chapter, we study SU(3) topological phases in two dimension. It is shown by Barnett et.al that N copies of the Hofstadter model with 2N Abelian ux per plaquette is equivalent to an N-component atom coupled to a homogeneous non-Abelian SU(N) gauge field in a square lattice. Such models have non-zero Chern number and for N = 3, can be written in terms of the SU(3) generators. In our work, we uncover two salient ingredients required to express a general three-component lattice Hamiltonian in a SU(3) format with non-trivial topological invariant. We nd that all three components must be coupled via a gauge eld, with opposite Bloch phase (in momentum space, if the NN hopping between two components is teik, then for the other two components, this should be te ik) between any two components, and there must be band inversion between all three components in a given eigenstate. For spinless particles, we show that such states can be obtained in a tripartite lattice with three inequivalent lattice sites, in which the Bloch phase associated with the nearest neighbor hopping acts as k-space gauge eld. The second criterion is the hopping amplitude t should have an opposite sign in the diagonal element for one of the two components, which can be introduced via a constant phase ei along the direction of hopping. The third and a more crucial criterion is that there must also be an odd-parity Zeeman-like term (as k ! k, the term changes sign), i.e. sin(k) z term, where z is the third Pauli matrix defined with any two components of the three component basis. In the presence of a constant vector potential, the kinetic energy of the electron gets modified when the vector potential causes a flux to be enclosed. This can generate the desired odd parity Zeeman term, via a site-selective polarization of the vector potential. This can be achieved in principle by suitable modifications of techniques used in Sisyphus cooling, and with a suitable arrangement of polarizer plates, etc. The topological phase is a firmed by edge state calculation, obeying the bulk-boundary correspondence.