Research Fellows Directory
Dr James Loudon
University of Cambridge
Superconductors have two remarkable properties: they have no electrical resistance and expel magnetic field. If you apply a magnetic field to an ideal (type I) superconductor and gradually make it stronger, no field enters until a critical field strength at which the material suddenly ceases to be superconducting. However, in type II superconductors, the superconducting state is not destroyed at once but magnetic flux penetrates by flowing along narrow channels of non-superconducting material called flux vortices. Each vortex contains the smallest amount of magnetic flux allowed by the laws of quantum mechanics.
The behaviour of vortices is crucial to the performance of almost every superconducting device. Electrical current induces a force on the vortices and they move in response which uses energy so they appear to have electrical resistance. To retain the zero-resistance state the vortices must be prevented from moving which is usually done by introducing defects into the crystal structure to pin the vortices. However, determining how the vortices respond to these pinning sites is no easy matter. What is needed is a way to observe vortices in action to determine their response to constraints and stimuli.
We use transmission electron microscopy to observe vortices as it has a higher resolution than the alternatives, works in real time and measures magnetic fields quantitatively. Prior to my work, this technique had only been successfully employed by two collaborating research groups using specially built microscopes. I have demonstrated that the technique can be employed using a modern commercial electron microscope, opening up a world of possibilities for other researchers.
My research is an investigation of the structure of individual vortices and how they respond to naturally occurring defects as well as artificially patterned pinning sites. The results should be of great interest not only to academia but also to the superconductor industry.