Electrical superball pulls itself together

A strong electric field can drive tiny particles of a superconductor to bind themselves together into a remarkably sturdy ball about 0.25 millimeter across.

Scanning electron micrograph of a ball, 0.25 mm in diameter, made up of closely packed particles of the superconductor bismuth strontium calcium copper oxide. Tao et al./Phys. Rev. Lett.

Rongjia Tao of Southern Illinois University at Carbondale and his collaborators report this surprising phenomenon in the Dec. 27, 1999 Physical Review Letters. Their finding “reveals a new property of high-temperature superconductivity,” the researchers remark.

Tao and his coworkers were initially interested in observing the motion of superconducting particles, a few micrometers in diameter, in an electric field. The researchers examined particles composed of a copper oxide compound that loses its resistance to the flow of electric current below a certain temperature. They suspended these grains in liquid nitrogen at 77 kelvins, which is below the threshold.

The researchers expected the widely dispersed particles to behave in one of two ways. The grains might bounce between the two electrodes that created the guiding electric field like bits of ordinary metals, such as copper, iron, and aluminum. Or they might act as ceramic material and form strings aligned with the field. Instead, several million particles rapidly packed themselves into a roughly spherical ball.

The sphere was robust enough to move as a single, electrically charged entity and survive repeated collisions with the electrodes. Sometimes, two or more balls would form. “There were no previous indications that aggregation of superconducting particles would occur,” says physicist N. Phuan Ong of Princeton University.

The force binding the ball together seems to be an artifact of superconductivity. The superconducting ball’s spherical shape indicates that a new type of surface tension related to superconductivity comes into play, says Princeton theorist Philip W. Anderson.

Researchers may be able to take advantage of this surface tension to lay down thin superconducting films on solid surfaces, Tao says. He and his team are now investigating possible applications of such films.

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