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

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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.