Researchers have produced the first microscopic images of a superconducting material near the critical current, the exact point at which it loses its ability to carry current without electrical resistance. In an applied magnetic field, a superconductor is permeated by a dense array of "magnetic vortices," swirling whirlpools of electric current. At and above the critical current, these vortices can move around and form intriguing patterns. Understanding the nature of magnetic vortex patterns is very important because they control the flow of current through superconductors.
Shown below is an electron-microscope image of flowing magnetic vortices created with a technique known as "magnetic decoration." In this technique, iron particles are evaporated onto the surface of a superconductor in which vortices are present. The particles preferentially land where the vortices are and form small piles, seen in the image as the white dots against the blue background. The image has been processed in such a way as to highlight the vortex array's topological defects which are the grey, star-shaped regions in the picture. In the picture, no current is flowing in the lower half, while in the upper half a current is flowing at the critical current. Clearly, the flowing current dramatically reduces the density of topological defects in the magnetic vortices.
This research is reported by Flavio Pardo, Francisco De La Cruz, Peter L. Gammel, Ernst Bucher & David J. Bishop in the 26 November 1998 issue of Nature. Images courtesy of the authors.
Further Reading: "Resistance in High-Temperature Superconductors," by David J. Bishop, Peter L. Gammel, and David A. Huse, in Scientific American, February 1993, p. 48.
"Magnetic Flux-Line Lattices and Vortices in the Copper Oxide Superconductors" by D.J. Bishop, P.L. Gammel, D.A. Huse and C.A. Murray in Science, Vol. 255, pages 165-172; January 10, 1992
"Fingerprinting Superconducting Vortices with Magnetic Smoke," P.L. Gammel and D.J. Bishop, Science, Vol. 279, 410; January 16, 1998.