Spotting a sparse crystal of trapped ions.

The neatly arranged atoms or ions that make up an ordinary crystalline solid are typically separated from their neighbors by less than 1 nanometer. Now, researchers have produced an array of regularly spaced ions that are at least 100,000 times farther apart yet still have the orderly arrangement expected of a crystal.

Wayne M. Itano, and his coworkers at the National Institute of Standards and Technology in Boulder, Colo., obtained evidence of such a pattern in a rapidly rotating cloud of nearly 1 million ions confined in a special trap. They report their findings in the Jan. 30 Science.

The NIST team is the first to observe this simple crystalline state in the laboratory, says John P. Schiffer of Argonne (Ill.) National Laboratory.

The researchers used a combination of magnetic and electric fields to trap the million beryllium ions at a temperature of less than 10 millikelvins. Settling into a roughly spherical cloud about 1 millimeter in diameter, the ions constitute what is called a one-component plasma. The magnetic field also causes the entire cloud to spin at about 100,000 rotations per second.

Laser light directed at the cloud is scattered into a distinctive diffraction pattern of concentric rings, suggesting that the ions form an equally spaced geometrical array--in other words, a single crystal.

The NIST scientists developed a method for precisely controlling the cloud's spin so as to synchronize it with pulses of laser light and thus obtain a diffraction pattern of individual spots, "You see nice square lattices," Itano says.

In much earlier experiments, researchers had confined many fewer particles in a trap and found that the ions settled into a set of concentric spherical shells (SN: 7/30/88, p. 69). The new NIST results show that with a sufficiently large number of ions, at least those in the interior fall into a cubic pattern.

"We're studying various properties of these crystallized ion plasmas, now that we can image them and control the rotational frequency," Itano says, Plasma crystals may prove useful as a model of a neutron star's crust, for example, aiding efforts to determine the ages of these stars.