Submitted by Nicolas Porto Vizarim (nicolasvizarim@gmail.com) on 2018-03-12T18:32:53Z No. of bitstreams: 1 Dissertação - Nicolas - Versão Final.pdf: 7101906 bytes, checksum: 80f0204c2d349c83b7ed130e1147502f (MD5) / Rejected by Maria Marlene Zaniboni null (zaniboni@bauru.unesp.br), reason: Solicitamos que realize correções na submissão seguindo as orientações abaixo: 1) Falta ficha catalográfica. Obs: Inserir a ficha catalográfica após a folha de rosto. Modelo disponível em: http://www.biblioteca.bauru.unesp.br/#!/servicos/ficha-catalografica/ Agradecemos a compreensão. on 2018-03-14T13:03:07Z (GMT) / Submitted by Nicolas Porto Vizarim (nicolasvizarim@gmail.com) on 2018-03-14T13:20:53Z No. of bitstreams: 1 Dissertação - Nicolas - Repositório.pdf: 7208823 bytes, checksum: 9075fa2e8a261a4f2a51a2946604cacc (MD5) / Approved for entry into archive by Maria Marlene Zaniboni null (zaniboni@bauru.unesp.br) on 2018-03-14T17:06:41Z (GMT) No. of bitstreams: 1 vizarim_np_me_bauru.pdf: 7208823 bytes, checksum: 9075fa2e8a261a4f2a51a2946604cacc (MD5) / Made available in DSpace on 2018-03-14T17:06:41Z (GMT). No. of bitstreams: 1 vizarim_np_me_bauru.pdf: 7208823 bytes, checksum: 9075fa2e8a261a4f2a51a2946604cacc (MD5) Previous issue date: 2018-02-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O estudo do comportamento de vórtices em supercondutores de tipo II com centros de aprisionamento (CAs) vem atraindo muita atenção nas últimas décadas devido a possibilidade de aumento da corrente crítica, assim como pela variedade de comportamentos que o sistema pode apresentar. Muito foi feito nos últimos anos na tentativa de compreender os mecanismos de aprisionamento, incluindo estudos de filmes infinitos sob influência de arranjos periódicos e quase-periódicos focando nos efeitos de comensurabilidade, correntes críticas e fases dinâmicas. No entanto, quando se trata de fitas supercondutoras muito trabalho ainda é necessário, especialmente para verificar como os CAs influenciam o sistema. Neste trabalho, foi simulado computacionalmente, utilizando técnicas de Dinâmica Molecular, o comportamento dinâmico de vórtices no interior de uma fita supercondutora do tipo II sob a influência de CAs com distribuição aleatória, hexagonal e dois tipos de arranjos conformes. A simulação foi feita considerando fitas imersas no plano xy, finita em x e infinita em y, com um campo magnético aplicado perpendicularmente ao plano das fitas e temperatura nula. Como resultado, encontramos que a fita com aprisionamento aleatório apresentou, na média, um decaimento suave da força de depinning (força necessária para libertar os vórtices dos CAs) em função do campo aplicado. Além disso, foi observada a presença de vórtices intersticiais fracamente ancorados, resultando em baixos valores de força de depinning. Já a fita com arranjo hexagonal apresentou efeitos significativos de comensurabilidade apenas no primeiro matching field, onde há um grande aumento da força de depinning, diferentemente do observado em filmes supercondutores infinitos, onde vários picos de comensurabilidade na corrente crítica são observados. Para os arranjos conformes, a força de depinning muda muito pouco em função do campo aplicado apresentando uma leve tendência decrescente, o que está de acordo com o que foi observado em filmes infinitos. Este comportamento está associado ao gradiente de defeitos e a simetria hexagonal que a rede conforme preserva, tornando o aprisionamento mais efetivo. Em um dos arranjos conforme foi obtido um aumento significativo nas forças de depinning em baixos campos, quando comparado aos outros arranjos. Com relação aos regimes dinâmicos, em fitas os vórtices tendem a formar canais devido a influência da corrente superfícial, que reduz o movimento transversal dos vórtices, diferentemente do observado em filmes infinitos. / The behavior of vortices in type II superconductors in the presence of artificial pinning centers has attracted much attention in the last two decades, due to the possibility of enhancing the critical currents, as well as the variety of behaviors that the system can exhibit. Much has been done in recent years to understand the pinning mechanisms, including studies of infinite films under the influence of periodic and quasi-periodic arrays focusing on the commensurability effects, critical currents and vortex dynamic phases. However, when it comes to superconducting strips a lot of work is still required, especially when the system is under the influence of pinning centers. In this work, we study the dynamic behavior of vortices inside a type II superconducting strip in the presence of pinning centers with random distribution, hexagonal and two types of conformal arrays. The simulation was performed considering strips using Molecular Dynamics technique, each with different arrays of defects, immersed in the xy plane, with finite dimension in x and infinite in y, under a magnetic field applied perpendicular to the plane of the strips and zero temperature. As a result, it was found that the strip with random pinning distribution showed a gentle decay of the depinning forces as a function of the applied field. In addition, the presence of weakly pinned interstitial vortices was observed, resulting in low values of depinning force. The hexagonal array showed significant commensurability effects only at the first matching field, where there is a great increase of the depinning force, unlike that observed in infinite superconducting films, where several commensurability peaks in the critical current is observed. For the conformal arrays, we observed a depinning force stability as a function of the applied field, which agrees with what has been observed in infinite films. This stability is associated to the gradient distribution of defects in the central region of the sample associated with sixfold symmetry that the conformal transformation preserves, resulting in a more effective pinning. In one of the conformal lattices, a significant increase in the depinning forces in low fields was obtained when compared to the other arrays. Concerning to the dynamic regimes, vortices in strips tend to form well defined channels due to the influence of the surface current, which reduces the transverse motion of the vortices, different to that observed in infinite films.

Submitted by EVERLIN CAROLINA FERREIRA DA SILVA null (everlinferreira@hotmail.com) on 2016-04-19T16:13:05Z No. of bitstreams: 1 DissertaçãoFinal.pdf: 3642722 bytes, checksum: f19af99a16e56b2846a689e39c66c05b (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-04-20T19:11:13Z (GMT) No. of bitstreams: 1 silva_ecf_me_rcla.pdf: 3642722 bytes, checksum: f19af99a16e56b2846a689e39c66c05b (MD5) / Made available in DSpace on 2016-04-20T19:11:13Z (GMT). No. of bitstreams: 1 silva_ecf_me_rcla.pdf: 3642722 bytes, checksum: f19af99a16e56b2846a689e39c66c05b (MD5) Previous issue date: 2016-02-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A técnica de espectroscopia de impedância foi aplicada para caracterizar um monocristal da ludwigita homometálica Fe3O2BO3, em que uma transição de ordenamento de carga (CO) ocorre em temperatura ambiente (~ 300 K) conforme reportada na literatura. Utilizando diferentes valores de amplitude do sinal AC aplicado, pudemos pelo estudo do comportamento da condutividade AC identificar nessas medidas o fenômeno de soltura de ondas de densidade de carga (CDW) e usamos a teoria de Bardeen, na faixa de temperatura em que foi possível aplica-la, para obter a razão entre a massa do condensado MF e a massa m do elétron, da ordem de 109 - 107. As características deste material foram comparadas a uma amostra de um sistema sem ordenamento de carga reportada, uma pastilha prensada de α-MnO2 dopado com cobre. Para ambos os materiais informação sobre o comportamento da constante dielétrica também foi obtido. / The impedance spectroscopy technique was applied to the characterization of a single crystal of the homometallic ludwigite Fe3O2BO3, in which a charge ordering transition (CO) occurs around room temperature (~ 300K), as reported in the literature. By using different values of the AC amplitude signal we could study the AC conductivity behavior and identify the depinning of the charge-density wave phenomenon (CDW). Applying the Bardeen theory, in a proper temperature range, to obtain the rate of the mass of the condensate MF in relation to that of the electron m, we found a ratio the order of 109 - 107 . The characteristics of this material were compared with a system without reported charge ordering transition, Cu doped α-MnO2 pressed pellets. For both materials, information about the behavior of the dielectric constant was also obtained.

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.

No description available.

De nombreux systèmes complexes soumis à un ajout continu d'énergie réagissent à cet ajout par une accumulation de tension au cours du temps, interrompue par de soudaines libérations d'énergie appelées avalanches. Récemment, il a été remarqué que plusieurs propriétés élémentaires de la dynamique d'avalanche sont issues de processus de relaxation ayant lieu à une échelle microscopique, processus qui sont négligés dans la plupart des modèles. Lors de ma thèse, j'ai étudié deux modèles classiques d'avalanches, modifiés par l'ajout d'une forme de relaxation la plus simple possible. Le premier système est une interface viscoélastique tirée à travers un milieu désordonné. En champ moyen, nous prouvons que l'interface a un comportement périodique caractérisé par une nouvelle échelle temporelle (émergente), avec des avalanches qui touchent l'ensemble du système. Le calcul semi-analytique de la force de friction agissant sur la surface donne des résultats compatibles avec les expériences de friction classique. En dimension finie (2D), les événements touchant l'ensemble du système (trouvés en champ moyen) deviennent localisés, et les simulations numériques donnent des résultats en bon accord avec plusieurs caractéristiques importantes des tremblements de terre, tant qualitativement que quantitativement. Le second système incluant également une forme très simple de relaxation est un modèle jouet d'avalanche : c'est la percolation dirigée. Dans notre étude d'une variante non-markovienne de la percolation dirigée, nous avons observé que la classe d'universalité était modifiée mais seulement partiellement. En particulier, un exposant change de valeur tandis que plusieurs relations d'échelle sont préservées. Cette idée d'une classe d'universalité étendue, obtenue par l'ajout d'une perturbation non-markovienne offre des perspectives prometteuses pour notre premier système. / Many complex systems respond to a continuous input of energy by an accumulation of stress over time, interrupted by sudden energy releases called avalanches. Recently, it has been pointed out that several basic features of avalanche dynamics are induced at the microscopic level by relaxation processes, which are neglected by most models. During my thesis, I studied two well-known models of avalanche dynamics, modified minimally by the inclusion of some forms of relaxation. The first system is that of a viscoelastic interface driven in a disordered medium. In mean-field, we prove that the interface has a periodic behaviour (with a new, emerging time scale), with avalanche events that span the whole system. We compute semi-analytically the friction force acting on this surface, and find that it is compatible with classical friction experiments. In finite dimensions (2D), the mean-field system-sized events become local, and numerical simulations give qualitative and quantitative results in good agreement with several important features of real earthquakes. The second system including a minimal form of relaxation consists in a toy model of avalanches: the Directed Percolation process. In our study of a non-Markovian variant of Directed Percolation, we observed that the universality class was modified but not completely. In particular, in the non-Markov case an exponent changes of value while several scaling relations still hold. This picture of an extended universality class obtained by the addition of a non-Markovian perturbation to the dynamics provides promising prospects for our first system.

Cette thèse se consacre à l'étude du modèle de dénaturation de l'ADN introduit par Poland et Scheraga dans les années soixante. Les modèles de dépiégeage en milieu aléatoire, avec lesquels la correspondance a été établie, sont également traités. Dans le cas où les interactions entre le système et l’environnement sont homogènes, le problème a été résolu : selon la valeur d'un paramètre géométrique, une transition de phase d'ordre un ou deux se produit. En revanche, lorsque les interactions sont prises aléatoires (on parle d'un système en présence de désordre), nous ne connaissons ni le point critique, ni l'ordre de la transition en régime defort désordre. Pour simplifier le problème, de nombreux auteurs font usage d'une représentation hiérarchique grâce à laquelle une renormalisation exacte de la fonction de partition peut être écrite. Mais à nouveau, la question du point critique et de l'ordre de la transition n'a pas été résolue. Nous avons introduit un nouveau système (Toymodel) plus simple que la version hiérarchique en changeant la forme de la renormalisation. Le problème, ainsi posé, permet de mettre en évidence une famille de distributions qui ne varient presque pas lors d'une renormalisation, avec lesquelles nous avons pu dériver des équations du type Berezinskii-Kosterlitz- Thouless. Aussi, en présence de désordre, la transition de phase n'admet pas de point fixe critique. Ces deux éléments, en accord avec nos résultats numériques, nous poussent à croire que nous sommes en présence d'une transition de phase d'ordre infini. La seconde partie de la thèse rapporte un travail sur le processus simple d'exclusion symétrique, qui est l'un des modèles les plus simples de physique statistique hors d'équilibre pour lequel un état stationnaire est connu. La fonction de grandes déviations a été calculée dans le passé par les approches microscopiques et macroscopiques et ici, nous en avons calculé la première correction de taille finie. Le résultat a ensuite été comparé aux corrections similaires pour des systèmes à l'équilibre. / This thesis is a study of a DNA denaturation model, introduced by Poland and Scheraga during the 1960s. The depinning models with random environment, with which the similarity has been made, are also concerned. If the interactions between the system and the environment are homogeneous, the problem has been solved: depending on the value of a geometrical parameter, a first or a second order phase transition happens. On the other hand, when the interactions are random, we know neither the critical point nor the phase transition order in the case of strong disorder. In order to simplify the problem, some authors have used a hierarchical representation through which an exact renormalization can be written. Despite this simplification, the critical point and the transition order have not been found. By changing the renormalization relation, we introduced a Toy-model which is simpler than the hierarchical version. The new problem leaded us to a family of distributions, which stay almost the same under renormalization, and allow us to derive the Berezinskii-Kosterlitz- Thouless equations. Also, with strong disorder, the phase transition does not have a critical fixed point. These two elements, according to our numerical results, predict that the order transition is infinite. The second part of this thesis reports on a work about the simple symmetric exclusion process, which is one of the simplest out of equilibrium models for which a stationary state is known. The large deviation function has been calculated in the past through microscopic and macroscopic approaches. Here, we calculated the leading finite-size correction. Then the result has been compared to similar corrections for equilibrium systems.

The main focus of this thesis work has been the study of domain wall (DW) dynamics in disordered cylindrical nanomagnets. The study attempts to accurately quantify the stochasticity associated with driven (temperature/magnetic field/spin-torque) DW kinetics. Our results as summarized below, are particularly relevant with regard to the technological advancement of DW based magnetoelectronic devices. 1. Temperature dependent noise measurements showed an exponential increase in noise mag-nitude, which was explained in terms of thermally activated DW depinning within the Neel-Brown framework. The frequency-dependence of noise also indicated a crossover from nondiffusive kinetics to long-range diffusion of DWs at higher temperatures. We also observed strong collective depinning, which must be considered when implementing these nanowires in magnetoelectronic devices. 2. Our noise measurements were sensitive enough to detect not only the stochasticity in DW propagation (diffusive random walk) but also their nucleation in the presence of magnetic field down to a single DW unit inside an isolated single Ni nanowire. Controlled injection and detection of individual DWs is critical in designing DW based memory devices. 3. The spectral slope of noise was observed to be sensitive to DWkinetics that reveals a creep-like behavior of the DWs at the depinning threshold, and diffusive DW motion at higher spin torque drive. Different regimes of DW kinetics were characterized by universal kinetic exponents. Noise measurements also revealed that the critical current density and DW pinning energy can be significantly reduced in a magnetically coupled vertical ensemble of nanowires. This was attributed to strong dipolar interaction between the nanowires. Our results are particularly important in view of recent proposals for low power consumption magnetic storage devices that rely on DW motion. In all our experiments, the critical magnetic field/current density, required to set the DWs in duffusive kinetics, were found to be much smaller than the reported values for nanostrips. This could be attributed to the circular cross section of nanowires, where massless DWs results in the absence of Walker breakdown and hence in zero critical current density. At present the contribution from the non-adiabaticity, which acts as an effective field and can reduce the crit- ical current density, can not be denied. The main di±culty in quantifying the non-adiabatic spin-torque is that not only does it contain contributions due to non-adiabatic transport but also due to spin-relaxation provided by magnetic impurities or the sources for spin-orbit scattering. Fortunately, in cylindrical nanomagnet, non-adiabaticity does not affect the DW motion. There- fore, cylindrical NWs may be promising candidate for future magnetic storage devices. However, a systematic experimental study of DW dynamics in cylindrical nanomagnets is lacking. In chapter 7, silver nanowires (AgNWs) are shown to be stabilized in fcc or hcp crystal structure, depending on the electrochemical growth conditions. The AgNWs stabilized in hcp crystal structure are shown to exhibit exotic structural properties i.e. ultra low noise level, thermally driven unconventional structural phase transformation, and time dependent structural relaxation. Ultra noise level makes hcp AgNWs suitable for application in nanoelectronics and the structural transformation may be exploited for use in smart materials. Though time resolved transmission electron microscopy and noise measurements provide some understanding of the hcp AgNWs formation, the precise growth mechanism is still not clear. Future scope of the work The results in this thesis provide the groundwork for a good understanding of stochastic DW kinetics in isolated as well as ensemble of magnetic nanocylinders. Some extensions to this work that would help expand and strengthen the results, are listed below; 1. In all the nanocylinders used for our experiments the source of stochasticity in DWkinetics were randomly distributed structural defects. For a controlled injection and detection of DWs between the voltage probes, it would be of great importance to fabricate artificial notches (pinning centers) in the NW. These notches can be fabricated either by using nano-indentation or by a focussed ion beam. 2. To investigate whether DWs in different parts of the nanowire exhibit spatio-temporal correlation, a simultaneous detection of DWkinetics (through noise measurement) between different volage probes needs to be done. If the propagation time of DWs scales with the distance between the voltage probes, we can be confident of our velocity measurement. Then, by recording the DWvelocity as function of eld/current for nanowire (or nanostrip) absence (or presence) of the Walker breakdown can be probed. This would be a significant result for future spintronic devices. With an accurate determination of velocity even non- adiabaticity parameter may be calculated and one can see its effect on DW dynamics. 3. A complete understanding of sustained avalanches at finite magnetic fields, characterized by a high spectral exponent (a>¸ 2:5) in an ensemble of nanowires is still lacking. Per- forming a controlled experiment on a single nanowire, by varying the number of nanowires in the alumina matrix, one can study the chaotic dynamics of DWs in the ensemble in very accurate manner. All the experiments on AgNWs were performed on ensembles. The large change in a as well as noise magnitude in hcp AgNWs could arise from stress relaxation due to the presence of an insulating matrix or structural relaxation, determined by the nanowire growth kinetics. To resolve this issue, time and temperature dependent noise measurements should be performed on single nanowire stabilized in both hcp and fcc crystal structure.