MURRAY HILL, N.J -- Bell Labs researchers have produced the first microscopic pictures of a superconducting material at the precise point it no longer carries current without resistance, commonly known as its critical current. The images clearly show previously unobserved magnetic-field patterns.
"If researchers were to better explain the critical current, which is one of the most widely measured and least understood characteristics of superconductors," said physicist Dave Bishop, "it might be possible to extend the uses of superconducting materials." The research findings are described in the Nov. 26 issue of the journal Nature.
In 1986, when scientists found superconducting materials that lose all resistance to electric current at world-record high temperatures, they thought these superconductors might be used to build levitating trains and very high-speed electronic circuits. They soon discovered an obstacle, however. As the current flows through the superconducting material, it generates a magnetic field, whose whirlpool-like tubes of electric charge -- known as vortices -- can stop the current cold under certain conditions. The study of these vortices has intensified ever since.
The Bell Labs researchers imaged the vortices below, at and above the critical current to better understand the phenomena of the critical current. "At first, the vortices are stationary" Bishop said. "But at the critical current, the vortices begin to move, and this movement can impede the flow of the current. So it's crucial to understand how these vortices move and arrange themselves under various temperature and magnetic-field conditions."
The latest research reveals that when the vortices begin moving, they can form distinct patterns. "It's very similar to a flock of ducks. When they're stationary on the ground, they are arranged haphazardly. However, when they fly, they're in formation, and this pattern of flight never changes," Bishop said.
In nature, this is a common phenomenon known as dynamic pattern formation, where the static structure is disordered, but motion causes a pattern to form. The same phenomenon happens with blowing snow when the wind forms pronounced patterns in the snow, such as uniform ripples.
"While this is not a new phenomena," Bishop said, "we were surprised to find such a clear example in vortices within a superconductor."
Bishop and his colleagues at Bell Labs, which is the research and development arm of Lucent Technologies, and the Atomic Center Bariloche in Argentina made their findings by essentially "decorating" the vortices within the superconductor with microscopic iron particles. The process is similar to when children place iron filings on a piece of paper and then move them by shifting a magnet beneath the paper.
To perform the magnetic decoration, the researchers applied a magnetic field -- roughly 10 to 100 times stronger than the Earth's magnetic field -- to a superconductor carrying a current. By decorating the vortices below, at and above the critical current, the researchers were able to observe many unexpected patterns in the vortices.
However, Bishop cautioned against believing a breakthrough is imminent. "Understanding these materials is one of the greatest challenges in science, and we still have a long way to go," he said. "These results, while very exciting, represent only one piece in a very complicated puzzle."
The other authors on the Nature article include Peter Gammel, Ernst Bucher and Flavio Pardo of Bell Labs and Francisco de la Cruz of the Atomic Center Bariloche.
Lucent Technologies (LU) designs, builds, and delivers a wide range of public and private networks, communications systems and software, consumer and business telephone systems and microelectronics components. Bell Labs is the research and development arm of the company. For more information about Lucent Technologies, headquartered at Murray Hill, N.J., visit our website at http://www.lucent.com.
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