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

高温超导体中磁通涡旋的结构和动力学成像

• 批准号: EP/E027903/1
• 负责人: Paul Midgley
• 金额: $ 36.1万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2008
• 资助国家: 英国
• 起止时间: 2008 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FE027903%2F1
• 关键词: Imaging; Structure; Dynamics; Flux; Vortices

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.

超导体有两个主要性质：它们的电阻为零，它们排出磁场。然而，并不是所有的超导体都能完全排斥场。第二类超导体允许磁场沿着称为“磁通涡旋”的通道穿透。这些通道中的每一个都包含量子力学定律所允许的最小量的磁场，它们可以像电子或光子一样被视为量子粒子。为了强调这一点，它们有时被称为“磁通子”。磁通涡旋的行为对确定超导体的性质至关重要。当电流通过II型超导体时，它会产生磁场，而这个磁场会产生磁通涡旋。当这些涡旋运动时，能量被耗散，就好像超导体具有非零电阻一样。这导致加热，这对于诸如超导磁体的设备是有害的，超导磁体需要高电流来操作。然而，如果超导体的晶体结构中的缺陷可以阻止磁通涡旋的移动，那么就可以用较低的功率损耗来输送较高的电流。这项技术于1992年首次用于铌中的涡旋成像。直到最近我们用它对高温超导体Bi-Sr-Ca-Cu-O中的涡旋成像，世界上只有一个实验室成功地应用了它。与其他磁成像技术相比，它具有更高的分辨率和磁场的定量测量等上级优点。我们打算用这种技术来研究磁通子的详细结构，它们之间的相互作用，以及与不同类型的钉扎位置，以及它们对被限制在不同几何形状的纳米超导样品中的响应。传统的II型超导体具有圆柱形通道的涡旋，但在其他材料中，如高温超导体，涡旋结构可能非常不同。我们将通过比较涡旋的实验图像和理论模拟来研究不同超导体中产生的涡旋结构。电子显微镜是唯一适合这项研究，因为它是唯一的技术，在样品内的磁场测量，而不仅仅是表面field.We也将调查如何磁通量移动响应磁场或温度的变化，通过记录图像在视频速率和研究钉扎的漩涡晶体缺陷。这些缺陷可以在电子显微镜中同时表征。这将使我们能够确定的缺陷，针涡最有效的种类。以及自然发生的缺陷，我们将调查缺陷的影响，这是人为创建的离子束照射使用我们的聚焦离子束显微镜。我们还将研究使用光刻法在样品顶部图案化的磁性纳米结构的钉扎效应，其中钉扎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，磁通涡旋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，如多重量子化涡旋和对称诱导反涡旋。最近有兴趣在“棘轮”机制，特别是形状的标本导致磁通量优先在一个特定的方向移动。有人建议，这种效应可以用来减少超导设备中的电噪声。我们将通过图案化不同类型的棘轮装置来扩展这项研究，并研究是否可以通过图案化样本表面上的磁性纳米结构来实现类似的棘轮效应。

项目成果

Magnetic structure of individual flux vortices in superconducting MgB 2 derived using transmission electron microscopy

使用透射电子显微镜导出超导 MgB 2 中各个通量涡旋的磁结构

• DOI: 10.1103/physrevb.87.144515
• 发表时间: 2013
• 期刊: Physical Review B
• 影响因子: 3.7
• 作者: Loudon J

Publisher's Note: Magnetic structure of individual flux vortices in superconducting MgB 2 derived using transmission electron microscopy [Phys. Rev. B 87 , 144515 (2013)]

出版商注：使用透射电子显微镜导出的超导 MgB 2 中各个通量涡旋的磁结构 [Phys.

• DOI: 10.1103/physrevb.87.179903
• 发表时间: 2013
• 期刊: Physical Review B
• 影响因子: 3.7
• 作者: Loudon J

The Magnetic Structure of Individual Flux Vortices in Superconducting MgB2 Derived using Transmission Electron Microscopy

使用透射电子显微镜导出超导 MgB2 中单个通量涡旋的磁结构

• DOI: 10.48550/arxiv.1303.5642
• 发表时间: 2013
• 作者: Loudon J

Determination of the Nature of the Structural Phase Transitions in 122 Pnictide Systems

122 磷元素体系中结构相变性质的测定

• DOI: 10.1088/1742-6596/391/1/012134
• 发表时间: 2012
• 期刊: Conference Series
• 作者: Loudon J