SUPERCONDUCTING BALLS, a new phenomenon, have been observed by physicists at Southern Illinois University. Rongjia Tao (618-536-2117, rtao@physics.siu.edu) and his colleagues began by wanting to observe the motion of micron-sized copper oxide (e.g., Br-Sr-Ca-Cu-O) superconducting particles (suspended in liquid nitrogen) in an electric field running between two electrodes. Metal particles in this situation would bounce between the two electrodes or tend to line up; after all, an electric field helps to define a preferred direction in space. The superconducting particles ignored this hint and, to the researchers' great surprise, formed themselves into a ball. The ball, about .25 mm across and containing over a million particles, formed quickly and was quite sturdy, surviving constant collisions with the electrodes (see figure at Physics News Graphics).

What binds the ball together against the dictates of the rectilinear field? Tao and his collaborator, Princeton theorist Philip Anderson, have concluded that the effect is an artifact of superconductivity (the same particles, above their superconducting transition temperature, do not ball up but instead queue into lines), perhaps something to do with the way in which the surface energy of the particle ensemble is reduced by self-assembly into a ball. This unprecedented new surface energy is related to the acquired surface charges on the particles and the reactions among the layers of the balls.

Granular properties of the particles might also play a role in the process and in the ball's internal structure, but this is difficult to gauge since the inter-particle interactions (frictional dissipation being the hallmark of granular materials) are mitigated by the liquid nitrogen needed in the experiment to neutralize gravity. A way around this is to do the experiment in the microgravity of space. The basic scientific novelty of this new phenomenon is paramount, but Tao is also turning his attention to possible applications in the area of superconducting thin films and unusual forms of wetting. (Tao et al., Physical Review Letters, 27 December 1999; for the text see Physics News Select Articles.)