Explaining How "Oil And Water" Mix In Superconductors May Lead To Practical Applications, Lucent Scientists Report

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 can stop the current cold. "Nailing" down these tubes, or vortices, so that they cannot move and disrupt the flow of current has been a worldwide effort for many scientists, including the Lucent researchers.

"We're all trying to find the best way to keep the snakes from wandering," said physicist David Bishop of Lucent's Bell Labs. "In this case, we first needed to find out where the snakes were."

For the last three years, Bishop and Peter Gammel, also of Bell Labs, have been leading an international team studying an unusual class of superconductors. Known as nickel borocarbides, they are comprised of both magnetic and superconducting materials, which appear to peacefully coexist as their atoms happily intermingle.

"Just like water and oil, you wouldn't expect magnetic material to mix with superconducting material," Bishop said, referring to the nickel borocarbides, which were discovered five years ago at Bell Labs by his former colleague Bob Cava.

In a series of experiments, Bishop, Gammel and their colleagues bombarded the nickel borocarbide with subatomic particles known as neutrons, which are not charged but have a specific magnetic spin. This approach is much like throwing a little magnet at a large magnet and then being able to deduce the latter's magnetic properties by seeing how the small one scatters. They hoped that the interaction of the neutrons with both the magnetic material and the whirlpool-like vortices (generated by an external magnetic field) would reveal the inner workings of nickel borocarbides.

The experiments showed that as the magnetic properties of the vortices changed, the shape of the lattice they were in also changed suddenly from a square to a hexagon. And as the vortices changed, the molecular structure of the magnetic material also changed. Based on these observations, the researchers concluded that the structure of the magnetic material directly influenced the structure of the vortex lattice.

Scientists had never before observed these sudden transitions in vortex patterns in a superconducting material. So this phenomenon provides evidence that researchers can alter the vortex patterns by merely changing a material's underlying magnetic structure.

"Magnetic vortices have always been elusive and difficult to hold onto," Bishop said, "but the observations in this experiment give us a strong new handle - the magnetism produced by the magnetic material itself -- to grab them." Previous ways to pin down vortices in other superconductors have included introducing defects or impurities into the material.

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