Study of the threshold line between macroscopic and bulk behaviors for homogeneous type II superconductors

In this work we solved the time dependent Ginzburg-Landau equations to simulate homogeneous superconducting samples with square geometry for several lateral sizes. As a result of such simulations we notice that in the Meissner state, when the vortices do not penetrate the superconductor, the response of small samples are not coincident with that expected for the bulk ones, i.e., $4\pi M=-H$. Thus, we focused our analyzes on the way which the $M(H)$ curves approximate from the characteristic curve of bulk superconductors. With such study, we built a diagram of the size of the sample as a function of the temperature which indicates a threshold line between macroscopic and bulk behaviors.Comment: 6 pages, 5 figure

Vortex-antivortex annihilation in mesoscopic superconductors with a central pinning center

In this work we solved the time-dependent Ginzburg-Landau equations, TDGL, to simulate two superconducting systems with different lateral sizes and with an antidot inserted in the center. Then, by cycling the external magnetic field, the creation and annihilation dynamics of a vortex-antivortex pair was studied as well as the range of temperatures for which such processes could occur. We verified that in the annihilation process both vortex and antivortex acquire an elongated format while an accelerated motion takes place.Comment: 4 pages, 5 figures, work presented in Vortex VII

Thermal treatment of superconductor thin film of the BSCCO system using domestic microwave oven

In this work, we report the preparation of a superconductor thin film of the BSCCO system using a good quality powder with nominal composition Bi_{1.8}Pb_{0.4}Sr_2CaCu_2O_x which was thermally treated using a domestic microwave oven (2.45 GHz, 800 W). This film was grew on a single crystal of LaAlO_3(100) substrate and exhibited a crystalline structure with the c-axis perpendicular to the plane of the substrate. An onset superconducting transition temperature was measured at 80 K.Comment: 4 pages, 5 figure

A DIMENSÃO ONTOLÓGICA NA GEOGRAFIA: Um paralelo entre o horizonte da crítica-radical e o pensamento de Martin Heidegger

A presente pesquisa está estruturada através da articulação de três elementos básicos. O primeiro elemento refere-se ao âmbito temático e corresponde à ontologia do espaço enquanto ―objeto formal‖ da pesquisa na ciência geográfica. Propomos sua problematização através de duas orientações de método distintos, que correspondem aos dois elementos referidos ao âmbito metodológico: a via do horizonte da crítica-radical, filiada ao materialismo histórico e dialético marxista e a via do pensamento de Martin Heidegger, ou seja, através da fenomenologia hermenêutica. Os três elementos da problemática se inscrevem na historiografia da Geografia, desde o contexto do movimento de renovação iniciado na década de 1970. Naquele momento se efetivaria tanto a assimilação da reflexão ontológica do espaço junto à constituição do horizonte da crítica-radical, como também se daria a assimilação da fenomenologia e do pensamento de Heidegger no bojo da constituição do horizonte humanista da disciplina. A pesquisa aborda a temática da fundamentação ontológica do espaço e sua articulação com o problema da relação sociedade-natureza, através do paralelo entre os dois métodos de investigação destacados que, por suas características particulares, contribuem para o debate teórico da Geografia, sob perspectivas, escopos e finalidades distintos. De um lado, apresenta um perfil de reflexão ontológica marcado pela influência do pensamento marxista e lukácsiano e determinado essencialmente pela ontologia do ser social. Trata-se de uma problematização ontológica produzida a partir da centralidade da noção do trabalho (ou de produção social) como princípio ontológico, que repercutiu amplamente na análise da realidade e junto à formulação de uma teoria espacial na disciplina. De outro lado, apresenta os elementos básicos indispensáveis para desenvolver uma investigação ontológica com bases fenomenológicas. Os termos básicos do projeto heideggeriano são expostos através dos seguintes elementos: a diferença ontológica entre ser e ente; a analítica do ser-aí como fonte da elaboração da questão do ser e; a análise da estrutura ser-no-mundo e da mundanidade do mundo e, no bojo da analítica existencial, a interpelação fenomenológica do espaço como fenômeno originário do ser-aí. Ambas proposições são fundamentalmente divergentes no que diz respeito ao encaminhamento que a questão sobre a relação sociedade-natureza adquire diante de seus pressupostos mais fundamentais

Microstructure and paramagnetic Meissner effect of YBa2Cu3Oy nanowire networks

The microstructure and magnetic characterizations of non-woven, fabric-like YBa2Cu3Oy (YBCO) nanofiber mats are reported. The samples were produced by solution blow spinning (SBS), starting from a sol-gel solution of the precursor materials in polyvinylpyrrolidone. In the present work, the nanowire network samples were morphologically characterized by scanning electron microscopy, and the superconducting properties were measured by magnetometry. An interesting feature is the appearance of a paramagnetic Meissner effect (PME) when field-cooling, firstly verified in that sort of sample. The PME appears only in very small applied magnetic fields, which is similar to previous observations of the PME on an artificially granular YBCO thin film, but distinctly different from bulk samples investigated in the literature. Thus, we explain the PME by flux trapping within the voids of the nanoporous structure of the nanofiber mats

Profile and crowding of currents in mesoscopic superconductors with an array of antidots

Studies with mesoscopic superconducting materials have made significant advances in the last decades. One of the applications of such systems is in devices for single-photon and single-electron detectors. However, depending on the geometry of these systems, crowding current effects take place, and as a consequence, the total critical current could decrease, which facilitates the penetration of vortices. This effect could be also responsible for a variety of penetration morphologies of flux avalanches in macroscopic samples. Thus, in this paper, we used the time-dependent Ginzburg Landau theory to study the crowding current effects in mesoscopic superconducting systems with an array of antidots. It is demonstrated that the profile of the currents is influenced by the antidots, i.e., in the vertices of the antidots, the intensity of the currents increases and distinguishably presents profiles, which depends on the size of the systems. Thus, we demonstrate that the distance between the antidots influences the current crowding effect, and the fabrication of future devices should be thought in order to minimize such effect.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Influence of thermal gradient in vortex states of mesoscopic superconductors

In general, the studies of finite size effects in mesoscopic superconductors have been carried out in such a way that the temperature parameter is constant in the entire system. However, we could have situations where a real sample is near a heater source, as an example. In such situations, gradients of temperature are present. On the other hand, mesoscopic superconductors are interesting systems due to the fact that they present confinement effects which influence all the vortex dynamics. Thus, in this work we studied the influence of thermal gradients on the vortex dynamics in mesoscopic superconductors. For this purposes, we used the time dependent Ginzburg-Landau equations. The thermal gradients produce an asymmetric distribution of the currents around the system which, in turn, yield interesting vortex configurations and difficult the formation of giant vortices