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Capturing the motion of magnetic vortices.

Applying a new imaging technique, researchers in Japan have for the first time directly observed the movement of tiny whirlpools of magnetism trapped in a superconducting material. Their observations help settle a long-standing debate concerning the behavior of these vortices as the temperature of an oxide superconductor -- placed in a magetic field -- is varied.

Akira Tonomura and his co-workers at the Advanced Research Laboratory of Hitachi, Ltd. in Saitama, Japan, report their findings in the Nov. 15 PHYSICAL REVIEW LETTERS.

"To those in the superconductivity business, these images are nothing less than specatcular," comments David J. Bishop of AT&T Bell Laboratories in Murray Hill, N.J., in the Nov. 18 NATURE. "It is a truly outstanding piece of experimental physics."

A superconductor normally shields itself from the effects of magnetic fields in which it is placed by preventing such fields from penetrating its interior. For oxide superconductors, however, once this external magnetic field exceeds a certain value, it begins to enter the material. This penetrating magnetic field exists within the superconductor in the form of separate vortices -- whirlpools of electric current. At low temperatures, the vortices arrange themselves into a distinctive pattern, of lattice.

The behavior of these vortices has a great influence on how well a superconductor carries electricity, particularly at high currents, when the vortices get in the way. But researchers have been unsure whether a vortex lattice remains intact below a certain temperature and "melts" into disarray above this temperature (SN: 4/189, p.197).

The Hitachi team confirmed that at low temperatures, the vortices form a lattice. As the researchers raised the temperature, the vortices moved a little with each temperature step but eventually settled again into a stationary pattern. However, once a certain temperature -- a critical point -- was reached, the lattice melted. A previous result indicated vortices start moving well below this temperature.

"Many of us have been consumed for the past six years by the issue of whether magnetic flux lattices could indeed melt," Bishop sasy. Now, "seeing is believing."
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Title Annotation:magnetic vortice lattices in superconducting material directly observed
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Nov 27, 1993
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