Imaging the Structure and Dynamics of Flux Vortices in High Tc Superconductors

超导体有两个主要特性：电阻为零，并能排出磁场。然而，并不是所有的超导体都能完全排除磁场。II型超导体允许磁场沿着称为“磁通涡流”的通道穿透。这些通道中的每一个都包含量子力学定律允许的最小数量的磁场，它们可以被视为量子粒子，就像电子或光子一样。为了强调这一点，它们有时被称为“通量子”。磁通涡旋的行为是决定超导体性质的关键。当电流通过II型超导体时，会产生磁场，磁场产生磁通涡流。当这些涡流移动时，能量就会耗散，就好像超导体具有非零电阻一样。这会导致加热，这对需要高电流才能运行的超导磁体等设备是有害的。然而，如果超导体晶体结构中的缺陷能够阻止涡流的移动，那么更高的电流就可以以更低的功率损耗传输。在这项研究中，我们将使用透射电子显微镜对单个通量漩涡进行成像。该技术于1992年首次用于铌中的漩涡成像。世界上只有一个实验室成功地应用了它，直到最近我们用它来成像Bi-Sr-Ca-Cu-O的涡流，这是一种高温超导体。它具有较好的分辨率和可定量测量磁场等优点，优于其他磁成像技术。我们打算利用这种技术来研究通量子的详细结构、它们彼此之间的相互作用以及它们与不同类型的钉钉位点的相互作用，以及它们对不同几何形状的纳米级超导样品的响应。传统的II型超导体具有圆柱形通道的漩涡，但在其他材料中，如高温超导体，漩涡结构可能非常不同。我们将通过对比实验图像和理论模拟来研究不同超导体中产生的涡流结构。电子显微镜非常适合这项研究，因为它是测量样品内部磁场而不仅仅是表面磁场的唯一技术。我们还将通过录像速率记录图像和研究晶体缺陷对涡流的固定，来研究通量子如何随着磁场或温度的变化而移动。这些缺陷可以在电子显微镜下同时表现出来。这将使我们能够确定最有效地钉住涡流的缺陷类型。除了自然发生的缺陷外，我们还将使用我们的聚焦离子束显微镜来研究由离子束辐照人工产生的缺陷的影响。我们还将研究利用光刻技术在样品顶部绘制磁性纳米结构的钉钉效果，其中钉钉力来自磁相互作用而不是晶体缺陷。在非常小的超导样品中，通量涡的排列和性质与在大块样品中观察到的不同。我们计划研究这种几何约束所产生的新效应，如多重量化涡和对称诱导反涡。最近，人们对“棘轮”机制很感兴趣，在这种机制中，特殊形状的标本会导致通量子优先向特定方向移动。有人提出，这种效应可以用来降低超导器件中的电噪声。我们将通过对不同类型的棘轮装置进行图案化来扩展这项研究，并研究是否可以通过在样品表面上对磁性纳米结构进行图案化来实现类似的棘轮效应。

Superconductors have two main properties: their electrical resistance is zero and they expel magnetic field. Not all superconductors expel field completely, however. Type II superconductors allow magnetic field to penetrate along channels called 'flux vortices'. Each of these channels contains the smallest amount of magnetic field allowed by the laws of quantum mechanics and they can be treated as quantum particles just like electrons or photons. To emphasise this, they are sometimes called 'fluxons'.The behaviour of flux vortices is crucial to determining the properties of a superconductor. When an electrical current is passed through a type II superconductor, it generates a magnetic field and this field produces flux vortices. When these vortices move, energy is dissipated as though the superconductor had a non-zero resistance. This leads to heating which is detrimental for equipment such as superconducting magnets which require high electrical currents in order to operate. If, however, the vortices can be prevented from moving by being pinned by defects within the crystal structure of the superconductor, higher currents can be carried with a lower power loss.In this investigation we shall use transmission electron microscopy to image individual flux vortices. This technique was first employed to image vortices in niobium in 1992. It has only been successfully applied by one laboratory in the World until very recently when we used it to image vortices in Bi-Sr-Ca-Cu-O, a high temperature superconductor. It is superior to other magnetic imaging techniques as it has a better resolution and magnetic fields can be measured quantitatively. We intend to use this technique to study the detailed structure of fluxons and their interactions with one another and with different types of pinning site as well as their response to being confined in nanoscale superconducting samples of different geometries.Conventional type II superconductors have vortices which are cylindrical channels but in other materials, like high temperature superconductors, the vortex structure can be very different. We shall study the vortex structures produced in different superconductors by comparing experimental images of vortices with theoretical simulations. Electron microscopy is uniquely suited to this study as it is the only technique where the magnetic field within the specimen is measured rather than just the surface field.We shall also investigate how fluxons move in response to changes in magnetic field or temperature by recording images at video rate and studying the pinning of vortices by crystal defects. These defects can be simultaneously characterised in the electron microscope. This will enable us to determine the sort of defect that pin vortices most effectively. As well as naturally occurring defects, we shall investigate the effect of defects which are artificially created by ion beam irradiation using our focussed ion beam microscope. We shall also study the effect of pinning by magnetic nanostructures patterned on top of the sample using lithography where the pinning force comes from magnetic interactions rather than crystal defects.In very small superconducting samples, the arrangement and nature of flux vortices is different to that observed in bulk samples. We plan to study the novel effects that result from this geometrical confinement such as multiply quantised vortices and symmetry induced antivortices. There has been recent interest in the 'ratchet' mechanism where specially shaped specimens cause fluxons to move preferentially in a particular direction. It has been suggested that this effect could be used to reduce the electrical noise in superconducting devices. We shall extend this research by patterning different types of ratchet device and investigating whether a similar ratchet effect can be achieved by patterning magnetic nanostructures on the specimen surface.