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We investigate the nonequilibrium steady state of driven magnetic flux lines in type-II superconductors subject to strong point or columnar pinning centers and the aging dynamics of nonequilibrium relaxation process in the presence of weak point pinning centers. We employ a three-dimensional elastic line model and Metropolis Monte Carlo simulations. For the first part, we characterize the system by means of the force-velocity / current-voltage curve, static structure factor, mean vortex radius of gyration, number of double-kink and half-loop excitations, and velocity / voltage noise features. We compare the results for the above quantities for randomly distributed point and columnar defects. Most of both numerical works have been done in two-dimensional systems such as thin film in which the structure of flux lines is treated as a point-like particle. Our main point of investigation in this paper is to demonstrate that the vortex structure and its other transport properties may exhibit a remarkable variety of complex phenomena in three-dimensional or bulk superconductors. The second part devotes to the study of aging phenomena in the absence of a driving force in disordered superconductors with much weaker point disorder. By investigating the density autocorrelation function, we observe all three crucial properties of the aging phenomena; slow power-law relaxation, breaking of time-translation invariance, and the presence of the dynamical scaling. We measure the dynamical exponents b and lambda_c/z and compare to other work. We find exponent values increase for increasing pinning strength, smaller interaction range, lower temperature, and denser defect density while the exponents measured in other approach tend to decrease. / Ph. D.

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We numerically investigate the dynamical properties of driven magnetic flux vortices in disordered type-II superconductors for a variety of temperatures, types of disorder and sample thicknesses. We do so with the aid of Langevin molecular dynamics simulations of a coarsegrained elastic line model of flux vortices in the extreme London limit. Some original findings of this doctoral work include the discovery that flux vortices driven through random point disorder show simple aging following drive quenches from the moving lattice state to both the pinned glassy state (non-universal aging) and near the critical depinning region (universal aging); estimations of experimentally consistent critical scaling exponents for the continuous depinning phase transition of vortices in three dimensions; and an estimation of the boundary curve separating regions of linear and non-linear electrical transport for flux lines driven through planar defects via novel direct measurements of vortex excitations. / Ph. D. / The works contained in this dissertation were undertaken with the goal of better understanding the dynamics of driven magnetic flux lines in type-II superconductors under different conditions of temperature, material defects and sample thickness. The investigations were conducted with the aid of computer simulations of the flux lines which preserve physical aspects of the system relevant to long-time dynamics while discarding irrelevant microscopic details. As a result of this work, we found (among other things) that when driven by electric currents, flux lines display very different dynamics depending on the strength of the current. When the current is weak, the material defects strongly pin the flux lines leaving them in a disordered glassy state. Sufficiently high current overpowers the defect pinning and results in the flux lines forming into a highly ordered crystal-like structure. In the intermediate critical current regime, the competing forces become comparable resulting in very large fluctuations of the flux lines and a critical slowing down of the flux line dynamics.

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Orientador: Oscar Ferreira de Lima / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataglin / Made available in DSpace on 2018-09-24T18:20:24Z (GMT). No. of bitstreams: 1 Oliveira_TarcisoMesquitade_M.pdf: 3236428 bytes, checksum: f8799c6f4d4ac1a196ca499b0ad67efb (MD5) Previous issue date: 2005 / Resumo: Nesta tese estudamos possíveis origens do Segundo Pico da Magnetização (SPM) em supercondutores de alta e baixa temperatura crítica, usando amostras monocristalinas de Bi2Sr2CaCu2O8+d e de Nb. Para estudar o SPM fizemos medidas de curvas de magnetização M x H e suscetibilidade AC (vs. h e T). Através da aplicação de uma lei de escala, para suscetibilidade AC, obtivemos o expoente de creep na região do SMP. Em amostras de Nb verificamos os efeitos sobre as curvas M x H de não homogeneidades na amostra: deslocamentos de planos e oxigênio intersticial. Em amostras de alta pureza (bulk e monocristalina) observamos que as curvas M x H não apresentam o SPM nem o Efeito Pico, próximo a Hc2.Ao adicionar não homogeneidades nas amostras, através de dopagem com oxigênio e deformação por elongação, observamos o aparecimento do Efeito Pico, de instabilidades termomagnéticas e de anisotropia do campo Hc2. Em amostras de Bi2Sr2CaCu2O8+d observamos que o SPM está associado com a componente do campo aplicado paralela à direção c da rede cristalina, ou seja, que o SPM está relacionado com vórtices de Abrikosov e que vórtices Josephson parecem não influenciar no SPM. Ao clivarmos a amostra observamos que a intensidade do SPM se reduz e diminui a janela de temperatura onde ele é visto. Obtivemos o expoente de creep e observamos que na região do SPM o arrasto dos vórtices diminui em campos que antecedem o HSPM , mas na região de HSPM o arrasto aumenta rapidamente e depois volta a cessar. Interpretamos a variação brusca no expoente de creep como uma mudança de fase no sistema de vórtices, que passa de um estado de quase-rede para um estado emaranhado de vórtices / Abstract: In this thesis we have studied possible origins for the Second Magnetization Peak (SMP) in high and low critical temperature superconductors, using samples of Bi2Sr2CaCu2O8+d and Nb single crystals. To study the SMP we did measurements of magnetization curves (M x H) and AC susceptibility (vs. h and T). We obtained the creep exponent in the SMP region, using a scaling law for the AC susceptibility data. In the Nb samples we have verified the effects on M x H curves due to inhomogeneities like dislocations and interstitial oxygen. In high pure samples (bulk and single crystals) we have observed that the M x H curves do not present either SMP or Peak Effect, near H2c . By adding inhomogeneities in the samples, like doping them with oxygen or deforming by elongation, we have observed the appearance of Peak Effect, thermomagnetic instabilities, and H2c anisotropy. In Bi2Sr2CaCu2O8+d samples we have observed that the SMP is associated with the magnetic field component parallel to the crystal c direction of crystal net. This means that the SMP is related to Abrikosov¿s vortices and that Josephson¿s vortices seem not to influence the SMP. After cleaving the sample we observed that the SMP intensity was reduced and the temperature window were it occurs was decreased. We have obtained the creep exponent and have observed that in the SMP region the vortices creep diminish for fields below HSPM , but around HSPM the creep increases abruptly and after the SPM it decreases again. We have interpreted the abrupt creep exponent variation as a phase transition in the vortex system, that pass from a quasi-lattice state to an entangled vortex state / Mestrado / Física / Mestre em Física

Studies on ac losses in superconductors have been a subject of great interest for a long time not only as an important topic in fundamental science, but also as a basic requirement for the application of superconductors. A proper understanding of the mechanisms of ac losses and their quantitative knowledge is an essential requirement for any application. Such studies not only yield information on the material parameters crucial for applications but can also provide a test for any possible microscopic theory of superconductivity. The main focus of the current thesis is to understand the mechanisms of ac losses in superconductors and to gain more knowledge on the ac dissipative behavior of type II superconductors. In this thesis we report our investigations on the ac losses in certain type II superconductors at different ranges of frequency through different experimental techniques. We have investigated the ac losses that arise in high Tc superconducting single crystals at rf frequency (8 MHz) using a simple LC oscillator technique. The result shows a surprising ac dissipation behavior in which the loss in the superconducting state is more than the normal state loss. Even though the superconducting state is defined as the zero resistive state, this is true only for dc transport. The ac resistivity studies have been made also on high Tc polycrystalline samples using the standard four-probe technique using a lock-in amplifier (100 kHz). The result shows different ac resistive behavior for samples with different microstructures. Non-resonant microwave absorption (NRMA) studies in MgB2 thin films is reported for the first time. The experiment has been performed using a continuous wave X band EPR spectrometer. The recorded signals give information on the ac losses that occurs at microwave frequency (9.43 GHz). The effects of ac magnetic field on the superconductors have been investigated through a dc four-probe resistivity measurements in the presence of an ac field of different magnitudes applied at different frequencies. Also a simple experimental technique based on the concept of kinetic inductance designed to study the ac losses that arise due to vortex motion is reported. In the following a chapter-wise summary of the thesis is presented.m Chapter 1 surveys the related literature on experimental and theoretical reports on ac losses in superconductors. In this chapter a brief introduction to superconductors is given with an emphasis on the high Tc superconductors. The superconducting materials studied in the thesis are described in detail along with their superconducting parameters and the form of the specimen. The origins of ac losses are discussed with various models proposed so far to explain the ac losses in superconductors. Since most of the ac losses reported in this thesis arise due to the Josephson junctions and vortex motion, they are discussed in detail. The occurrence of Josephson junctions and the various models used to describe the junctions’ characteristics are discussed. The formation of vortices their various forms in layered superconductors and the mechanisms of flux flow and flux creep are discussed. Chapter 2 describes the studies on ac losses in superconducting Bi2Sr2CaCu2O8 single crystals [1,2]. Generally in the superconducting state the dissipation is expected to be less compared with that in the normal state. However, we observe that the ac losses in the superconducting state are larger than the normal state losses. In this chapter we report on the ac losses in superconducting Bi2Sr2CaCu2O8 single crystals at radio frequencies determined from direct measurement of the absorbed power using an rf oscillator [3]. The ac response of Bi2Sr2CaCu2O8 single crystals is investigated as a function of temperature from the measured shift in current and the frequency of the oscillator. The studies are carried out at different rf amplitudes by varying the supply voltage to the oscillator circuit. To understand the magnetic field dependent behavior of ac losses, studies have been performed in the presence of magnetic field of various magnitudes applied parallel to the c-axis of the crystal. In the presence of the magnetic field two peaks are observed in ac losses in the superconducting state as a function of temperature. The presence of the peaks and their behavior are studied in detail by varying the orientation of the applied field with respect to the c-axis of the crystal. The results are discussed in terms of a new model proposed recently by us [4], which explains ac losses as a consequence of cumulative effect of the energy spent in repetitive decoupling of the Josephson junctions and in terms of Lorentz force driven motion of vortices. In Chapter 3, we discuss the ac resistivity behavior of the polycrystalline superconducting samples with different microstructures. Measurement of resistivity is the basic characterization method not only for superconductors but for any material. The superconducting state is defined as the zero resistive state; but this statement is true only for dc and not for ac. The presence of ac resistance in superconductors leads to losses. In the present work we report on the behavior of ac resistance in the superconductors. The application of a magnetic field and the variation of temperature alter the AC penetration depth of the superconducting sample, which in turn changes the AC impedance associated with it. In this chapter we report the results on the complex AC conductivity that has been measured in two types of polycrystalline YBa2Cu3O7 samples at frequencies starting from 100 Hz to 100 kHz and at temperatures from 10 K to 300 K. In the first pellet which is sintered, the possibility of presence of extrinsic Josephson junctions is less, but a large number of Josephson junctions is present in the second non-sintered pellet. In general it is expected that the AC or the DC resistivity in superconductors should decrease below Tc. In the case of DC resistivity the value of resistance goes exactly to zero and in the case of AC resistivity it keeps on decreasing towards zero with decreasing temperature. But surprisingly we find that in superconducting samples with Josephson junctions, the AC resistivity drops very close to zero at the critical temperature and instead of decreasing it increases slowly with decreasing temperature below the critical temperature. This property is also strongly dependent on the applied AC frequency. Investigation of the above phenomenon gives information regarding the contribution of JJ decoupling towards the AC resistivity of superconducting samples. The observed ac resistive behavior is well fitted with the Ambegaokar-Baratoff model for temperature dependence of critical current in the Josephson junction. In Chapter 4, the possibility of the presence of weak links in the intermetallic superconductor MgB2 is reported. The role of weak links in superconductors has been studied for a long time. Understanding the behavior of weak links has great importance for the applications of superconductors. Presence of weak links in high Tc materials due to its insulating grain boundaries limits the application potential of those materials. These weak links lead to lower critical current density and lower critical field of superconductors and lead to losses. The discovery of superconductivity in the simple intermetallic compound MgB2 has created a lot of interest from both application aspects and of fundamental science. MgB2 differs from high Tc materials and is considered as a potential candidate for applications, because of its high critical current density which arises due to the absence of weak links in MgB2. Absence of weak links is reported in most of the MgB2 literature and only in a very few studies possibility of the presence of weak links is reported. Here, our NRMA studies on the MgB2 thin films show the presence of weak links [5]. NRMA is a highly sensitive, non-invasive technique, which has proven to be a valuable tool for detecting weak links in superconductors and their characterization [6]. In this technique the sample is studied using a continuous wave electron paramagnetic resonance (EPR) spectrometer, by recording the magnetic field dependence of the power absorption. The NRMA studies on the MgB2 thin film shows the presence of weak links and hysteresis in the signal. The origin of weak links is discussed as being due to the presence of oxygen in the grain boundaries. The hysteresis appears because of remnant magnetization and due to the pinning of flux lines when there is a change in the sweeping field direction. The NRMA studies are carried out as a function of temperature, modulation field, microwave power and the scan range and the results are reported in this Chapter. In chapter 5 we report on the resistive behavior of superconducting MgB2 film in the presence of an ac field using a novel technique. In this simple technique the resistive measurements are done using the general four-probe method, but a coil is wound over the sample and connected to an ac source to generate the ac field. The resistivity measurements are carried out as a function of temperature, amplitude and the frequency of ac field. The ac field shifts the Tc towards lower temperature and increases the broadening in transition from normal to superconducting state. In the absence of Lorentz force due to the parallel orientation of ac field with the transport current, we find that Josephson junction decoupling is one of the main origins of resistivity. The results are compared with the resistive behavior of YBCO film. The epitaxial YBCO film which is free from weak links shows a different frequency dependent resistive behavior, which is explained in terms of flux-creep. In the MgB2 film the studies are carried out in the presence of a dc field that is applied perpendicular to direction of transport current in the film along with the presence and the absence of the ac field. The studies show that in superconductors the presence of ac field leads to more loss than that of dc field. Chapter 6 describes a simple experimental technique using the property of kinetic inductance to measure the vortex resistivity arising from the ac current. Since the discovery of the superconductors much attention has been given to the dynamics of the vortices because of their importance from both scientific and application point of view. When a magnetic field of amplitude more than Hc1 is applied the type II superconductors enter in to the ‘mixed state’ due to the presence of vortices. In the presence of a current, the vortices experience Lorentz force of magnitude F = J x B normal to the current and the field. The vortices move under the influence of the Lorentz force along its direction which leads to resistivity. The electric field generated by the vortex movement has two components, one acting along the current direction and the other normal to the current direction. But most of the vortex resistivity measurements are carried out either in the presence of high magnetic field or at temperatures closer to Tc due to the limitation of experimental techniques. In this chapter we are reporting a simple experimental technique to measure vortex resistivity with very high resolution even at low temperatures and fields based on the concept of kinetic inductance. Kinetic inductance is the property which arises mostly in superconductors due to the inertial mass of the charge carriers. In our measurement kinetic inductance is measured through a simple four-probe ac impedance technique, which is more commonly used for measuring resistivity. The penetration depth due to vortices is related to their resistivity and from the relation between the measured kinetic inductance and penetration depth vortex resistivity is calculated. In this report we discuss the experimental setup, principle of the method and present the results of our measurements carried out on YBa2Cu3O7 thin films.