Tese (doutorado)—Universidade de Brasília, Instituto de Física, 2016. / Submitted by Fernanda Percia França (fernandafranca@bce.unb.br) on 2017-02-13T14:20:53Z No. of bitstreams: 1 2016_DanielDouradodeAragãoSantos.pdf: 11726662 bytes, checksum: 466b96f2b7d83ab56c89e06f864925ea (MD5) / Approved for entry into archive by Raquel Viana(raquelviana@bce.unb.br) on 2017-03-22T16:49:32Z (GMT) No. of bitstreams: 1 2016_DanielDouradodeAragãoSantos.pdf: 11726662 bytes, checksum: 466b96f2b7d83ab56c89e06f864925ea (MD5) / Made available in DSpace on 2017-03-22T16:49:32Z (GMT). No. of bitstreams: 1 2016_DanielDouradodeAragãoSantos.pdf: 11726662 bytes, checksum: 466b96f2b7d83ab56c89e06f864925ea (MD5) / Nesse trabalho investigamos os efeitos da interação feixe-plasma por meio de um modelo hamiltoniano para um sistema composto de N partículas eletrostaticamente acopladas a uma onda. A discretização do sistema em feixes monocinéticos permite fazer uma abordagem perturbativa do problema e destacar importantes conexões entre as formulações hamiltoniana e vlasoviana. Por meio de resultados numéricos, mostramos que tanto o amortecimento como a amplificação de Landau surgem como consequência de um processo de phasemixing dos vários modos normais do sistema linearizado. Observamos que esse mecanismo possui características distintas dependendo se o sistema é estável ou instável. Os resultados obtidos para sistemas de muitos graus de liberdade foram possíveis graças ao desenvolvimento de uma nova técnica para obtenção de raízes complexas. Implementamos simulações de Vlasov via método semi-lagrangiano e descrevemos o processo de relaxação não colisional da distribuição de velocidades das partículas ressonantes e a evolução da intensidade da onda. Analisamos o espectro dos modos não amortecidos em plasmas livres quando se assume uma distribuição de velocidades estacionária com um pequeno (e suave) plateau nas vizinhanças da velocidade de fase da onda. Verificamos que essas oscilações são bastante sensíveis à forma do plateau principalmente quando possuem velocidades de fase próximas à velocidade térmica das partículas. Investigamos também, por meio do cálculo da função dielétrica, as modificações na relação de dispersão dos modos eletrostáticos em plasmas livres e plasmas magnetizados sujeitos a campo externo de radiação. Os cálculos são feitos com base em um modelo semiclássico, em que elétrons são descritos quanticamente e os campos classicamente. / In this work we discuss the effects of the beam-plasma interaction by means of a hamiltonian model for a system composed of N particles electrostatically coupled to a single wave. The discretization of the system in monokinetic beams allows us to use a perturbative approach of the problem and highlight important connections between the hamiltonian and vlasovian formulations. By means of numerical calculations, we show that both Landau damping and growth emerge due to a phase mixing mechanism that involves many normal modes of the linearized system. We observe that this mechanism exhibits a distinct behavior depending on whether the system is stable or unstable. For systems with many degrees of freedom, the results obtained were possible due to the development of a new root-finding technique in the complex plane. Using vlasovian simulations via semi-Lagrangian scheme we describe the process of collisionless relaxation of the velocity distribution for the resonant particles and the evolution of the wave intensity. We analyse the spectrum of undamped modes in free plasmas when one assumes a stationary velocity distribution with a small (and smooth) plateau centred on the wave phase speed.We find that these oscillations are quite sensitive to the shape of the plateau, mainly for those that have velocities close to the thermal particle velocity. We also investigate, using a semiclassical approach, the modifications in the dispersion relation of the electrostatic modes in free and magnetized plasmas subjected to external radiation field.

Made available in DSpace on 2018-03-22T15:55:39Z (GMT). No. of bitstreams: 1 tese_11668_Tese de doutorado - Fernando H. S. Eleutério.pdf: 1773499 bytes, checksum: e9c2c4b8631ab20a64b11ecf5e0aa507 (MD5) Previous issue date: 2017-12-15 / O foco deste trabalho é a investigação das duas fases intrínsecas de supercondutores estruturais, utilizando como objeto de estudo a resposta magnética e a sua composição granular. Ao investigar a susceptibilidade magnética ac na cerâmica policristalina a base de mercúrio dopada com rênio (Hg0.82Re0.18Ba2Ca2Cu3O8.16) (Hg,Re)- 1223) com o precursor com teor ótimo de oxigênio, pulverizada (reduzida a pó e peneirada) foi possível determinar a temperatura crítica da amostra e observar que ela depende do tamanho de cada do grão. Para a partícula de 20 μm foi encontrada somente uma temperatura crítica de 133K, porém são observadas duas temperaturas críticas (133K e 98 K) ao ser reduzir ainda mais o tamanho da partícula, a uma escala mesoscópica de 600 nm. Este fato foi elucidado por uma extensão da teoria de Ginzburg-Landau utilizando dois parâmetros de ordem para parametrizar o campo escalar complexo e fazendo a conexão com o modelo microscópico responsável por descrever as peculiaridades destes tipos de grãos.

Made available in DSpace on 2018-08-01T21:59:52Z (GMT). No. of bitstreams: 1 tese_11668_Tese final Fernando Henrique Santos Eleutério - PPGFis20180316-142248.pdf: 1478639 bytes, checksum: 8a421c51018e8816f936d7cac9314677 (MD5) Previous issue date: 2017-12-15 / O foco deste trabalho é a investigação das duas fases intrínsecas de supercondutores estruturais, utilizando como objeto de estudo a resposta magnética e a sua composição granular. Ao investigar a susceptibilidade magnética ac na cerâmica policristalina a base de mercúrio dopada com rênio (Hg0.82Re0.18Ba2Ca2Cu3O8.16) (Hg,Re)- 1223) com o precursor com teor ótimo de oxigênio, pulverizada (reduzida a pó e peneirada) foi possível determinar a temperatura crítica da amostra e observar que ela depende do tamanho de cada do grão. Para a partícula de 20 μm foi encontrada somente uma temperatura crítica de 133K, porém são observadas duas temperaturas críticas (133K e 98 K) ao ser reduzir ainda mais o tamanho da partícula, a uma escala mesoscópica de 600 nm. Este fato foi elucidado por uma extensão da teoria de Ginzburg-Landau utilizando dois parâmetros de ordem para parametrizar o campo escalar complexo e fazendo a conexão com o modelo microscópico responsável por descrever as peculiaridades destes tipos de grãos.

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In this dissertation, I present optical emission and light scattering studies on ultraclean two-dimensional electron systems. These studies focus on emerg- ing phases in the second Landau level. I report for the excitation spectrum for fractional quantum Hall states at filling factors ν = 2+1/3, ν = 2+3/8, and ν = 2+2/5 through resonant inelastic light scattering. Resonant Rayleigh scattering is used to demonstrate that these fractional quantum Hall states are anisotropic. This work provides new insights into the nature of quasiparticle interactions of these states. It also sets the stage for the subsequent discussions about competing and coexistent phases. I present studies of emergent phases in the filling factor range 2 ≤ ν ≤ 3 using weak optical emission from the second Landau level and resonant inelas- tic light scattering by spin wave excitations. A multiplet of optical emission peaks observed that exhibit striking filling factor dependence amnifest phase competition in the second Landau level. A correlation of emission peaks in the multiplet with anomalies observed in the spin wave spectrum uncover major impact of the spin degree of freedom on the emergent phases in the second Landau level. These experiments demonstrate the promise of optical emission from excited Landau levels as a probe of emergent phases. Results from optical emission and resonant inelastic light scattering stud- ies of the second Landau level conducted at higher temperatures (T ≈ 1 K) are also presented. Evidence that many phases observed at these higher temperatures are shown to be the same as those at lower (T ≈ 40 mK) temperatures. Striking and anomalous temperature-dependence of optical emission experiments is used to gain further insight into the nature of these competing phases.

Orientador: Rafael Zadorosny / Resumo: Este trabalho está estruturado em duas partes. A primeira, envolve o estudo do estado de vórtices e comportamento magnético de um supercondutor mesoscópico submetido a um gradiente térmico. Foi veri cado que a influência de um gradiente térmico altera o estado de vórtice gigante e pode levar a magnetização a ter um comportamento anômalo, o qual depende do parâmetro de Ginzburg-Landau e das temperaturas nas regiões "quente" e "fria". Na segunda parte, abordamos o estado resistivo durante o movimento de um vórtice e durante a dinâmica de par vórtice-antivórtice (V-AV) para os tipos de Abrikosov e Cinemáticos. Estudamos quais parâmetros afetam as propriedades do não equilíbrio e os mecanismos de dissipação presentes no processo de difusão térmica. Para os processos envolvendo vórtice ou V-AV de Abrikosov, os mecanismos de dissipação devido à processos de relaxação contribuem para a maior parte da potência total dissipada e, consequentemente, exercem uma grande contribuição para a variação de temperatura. Por outro lado, para o caso de V-AV cinemáticos, os elétrons normais aparecem como principal mecanismo de dissipação, todavia os processos de relaxação tem uma contribuição considerável para esse caso. Para realizar tais estudos, utilizamos o teorema da energia livre e as equações de Ginzburg- Landau generalizadas. / Abstract: This work is structured in two parts. In the rst one, the vortices states and the magnetic behavior were studied for a mesoscopic superconductor under a thermal gradient. It was veri ed that the thermal gradient changes the giant vortex state and could lead a exotic behavior of the magnetization curve, depending on the parameter and the temperature of the hot and the cold . In the second part, the focus was the resistive state during one vortex motion and during the annihilation process of a vortex-antivortex (V-AV) pair both with Abrikosov and Kinematic types. We de ned which parameters a ect the nonequilibrium proprieties and the dissipative mechanisms present in the thermal di usion processes. For the cases involving a vortex or an Abrikosov's V-AV pair, the dissipative mechanisms due to the relaxation process have the major contribution for the total power dissipated power and consequently, exert great contribuition on the temperature variation. On the other hand, for the case of kinematic V-AV, the normal eletrons rised as the main dissipative mechanism, however relaxation process has a considerable contribuition for this case. To realize such studies, we used the free energy theorem and the generalized Ginzburg-Landau equations. / Doutor

In this dissertation, we employ variational methods to obtain a new existence result for solutions of a Ginzburg-Landau type equation on a Riemannian manifold. We prove that if $N$ is a compact, orientable 3-dimensional Riemannian manifold without boundary and $\gamma$ is a simple, smooth, connected, closed geodesic in $N$ satisfying a natural nondegeneracy condition, then for every $\ep>0$ sufficiently small, $\exists$ a critical point $u^\ep\in H^1(N;\mathbb{C})$ of the Ginzburg-Landau functional \bd\ds E^\ep(u):=\frac{1}{2\pi |\ln\ep|}\int_N |\nabla u|^2+\frac{(|u|^2-1)^2}{2\ep^2}\ed and these critical points have the property that $E^\ep(u^\ep)\rightarrow\tx{length}(\gamma)$ as $\ep\rightarrow 0$. To accomplish this, we appeal to a recent general asymptotic minmax theorem which basically says that if $E^\ep$ $\Gamma$-converges to $E$ (not necessarily defined on the same Banach space as $E^\ep$), $v$ is a saddle point of $E$ and some additional mild hypotheses are met, then there exists $\ep_0>0$ such that for every $\ep\in(0,\ep_0),E^\ep$ possesses a critical point $u^\ep$ and $\lim_{\ep\rightarrow 0}E^\ep(u^\ep)=E(v)$. Typically, $E$ is only lower semicontinuous, therefore a suitable notion of saddle point is needed. Using known results on $\mathbb{R}^3$, we show the Ginzburg-Landau functional $E^\ep$ defined above $\Gamma$-converges to a functional $E$ which can be thought of as measuring the arclength of a limiting singular set. Also, we verify using regularity theory for almost-minimal currents that $\gamma$ is a saddle point of $E$ in an appropriate sense.

In this dissertation, we employ variational methods to obtain a new existence result for solutions of a Ginzburg-Landau type equation on a Riemannian manifold. We prove that if $N$ is a compact, orientable 3-dimensional Riemannian manifold without boundary and $\gamma$ is a simple, smooth, connected, closed geodesic in $N$ satisfying a natural nondegeneracy condition, then for every $\ep>0$ sufficiently small, $\exists$ a critical point $u^\ep\in H^1(N;\mathbb{C})$ of the Ginzburg-Landau functional \bd\ds E^\ep(u):=\frac{1}{2\pi |\ln\ep|}\int_N |\nabla u|^2+\frac{(|u|^2-1)^2}{2\ep^2}\ed and these critical points have the property that $E^\ep(u^\ep)\rightarrow\tx{length}(\gamma)$ as $\ep\rightarrow 0$. To accomplish this, we appeal to a recent general asymptotic minmax theorem which basically says that if $E^\ep$ $\Gamma$-converges to $E$ (not necessarily defined on the same Banach space as $E^\ep$), $v$ is a saddle point of $E$ and some additional mild hypotheses are met, then there exists $\ep_0>0$ such that for every $\ep\in(0,\ep_0),E^\ep$ possesses a critical point $u^\ep$ and $\lim_{\ep\rightarrow 0}E^\ep(u^\ep)=E(v)$. Typically, $E$ is only lower semicontinuous, therefore a suitable notion of saddle point is needed. Using known results on $\mathbb{R}^3$, we show the Ginzburg-Landau functional $E^\ep$ defined above $\Gamma$-converges to a functional $E$ which can be thought of as measuring the arclength of a limiting singular set. Also, we verify using regularity theory for almost-minimal currents that $\gamma$ is a saddle point of $E$ in an appropriate sense.

In part I, a numerical study of the mixed states in a mesoscopic type-II superconducting cylinder is described. Steady-state configurations and transient behavior of the magnetic vortices for various values of the applied magnetic field H are presented. Transitions between different multi-vortex states as H is changed is demonstrated by abrupt changes in vortex configurations and jumps in the B vs H plot. An efficient scheme to determine the equilibrium vortex configuration in a mesoscopic system at any given applied field, not limited to the symmetry of the system, is devised and demonstrated. In part II, a superconducting thin film is subject to a non-uniform magnetic field from a vertical magnetic dipole, consisting of two magnetic monopoles of opposite charges. For a film with constant thickness and with no pins, it has been found that the film carries two pairs of vortex-antivortex in the steady state in the imposed flux range of 2.15 < (Phi)+ < 2.90 (in units of flux quantum) and no vortex at all for (Phi)+ <= 2.15. Transitions from a superconducting state with 3 pairs of vortex-antivortex to one with 2 pairs, where a pair of vortex-antivortex annihilates, have been observed in the pseudo-time sequence. With a perturbation with antidots (holes), vortexantivortex pair has been created for lower magnetic fluxes down to (Phi)+ = 1.3. In the sample of size 16(Xi) x 16(Xi), the attraction force between the vortex and antivortex always dominates over the pinning force, so that they eventually come out of pins, move toward each other, and annihilate each other. The annihilation rate, measured with time taken for the annihilation, is reduced noticeably by the increase of the distance between pins, or the increase in the pin size. A simulation of the magnetic vortex pinning in the sample of size 32(Xi) x 32(Xi) suggests we are likely to achieve pinning of the vortex-antivortex pair with the sample size around this and vortex-antivortex separation of 22(Xi). Using this sample as a template, the maximum density of pinned vortices achievable is calculates to be about 7.6 x 10^14 vortices/m2 for (Xi) =~ 1.6A°.