Scientists have observed yet another strange phenomenon caused by superconductivity, a state in which materials have lost all electrical resistance. When they applied an electrical field to a superconducting powder, the material spontaneously assembled itself into solid little balls that started racing back and forth between the two electrodes. The quirky behavior, described in the 27 December issue of Physics Review Letters, if tamed could be useful for creating thin films of superconductors.
The team, led by physicist Xuesong Zhang at Southern Illinois University in Carbondale, had been studying substances, such as powdered copper in oil, that flow or otherwise change their properties in response to an electric field. One day, Zhang started dabbling with a souped-up powder: cuprates, superconducting compounds made of copper, oxygen, and a variety of metal atoms.
When zapped with an electric field, Zhang's powders usually formed either long chains that run the distance from one electrode to the other, or individual grains that shuttled back and forth from cathode to anode. But when the researchers mixed the cuprates with liquid nitrogen and placed the slurry between two electrodes, the cuprate grains clung to each other in a single ball. A few tenths of a millimeter thick, the ball would catapult violently between the electrodes, yet would not shatter. "I didn't believe what I saw," says Zhang, "so I tried it with other [cuprate] powders. They did the same thing." "It's an amusing effect," adds Princeton physicist Philip Anderson.
Anderson, Zhang, and their colleagues propose that the weird behavior may stem from the cuprates' peculiar structure; the crystals are layered, with planes of copper and oxygen atoms sandwiched between planes of rare earth metals. "When something goes superconducting, all the electrons pair up in the planes; they march like an army," explains Mike Norman, a materials scientist at the Argonne National Laboratory in Illinois. Unlike an ordinary superconductor, which is like one large corps, the cuprate layers form a number of smaller regiments that are weakly "coupled"--they feel each other's influences--so the "regiment" of electrons in one plane marches in the same direction as the regiment in the neighboring plane. Because the regiments are in lockstep, an electron pair can move from one plane to another with minimal disruption.
However, at the outermost layer of a granule, an electron pair can only move inward into the neat array of electron pairs in a nearby layer, not outward into the chaos of the vacuum, says Anderson. This creates a "surface energy" that the material tries to reduce as much as possible, by minimizing its top layer and maximizing the amount of material on the inside--which is done most effectively by forming a sphere. "That would explain it," Norman agrees.
Zhang says the balls may help refine theories about superconductivity by giving scientists an indirect way of measuring the forces between layers in superconducting crystals; the size and stability of the balls would reveal the strength of the "surface tension" that holds them together. He also notes that exploiting this surface tension may lead to a new process for making superconducting films by "wetting" a surface with the powder, just as you can make a thin film of liquid on a table by pouring water on one corner. Such films may help produce new superconducting microcircuits.