Spectral atom rings in

Stare at atoms long enough and you may start seeing ghosts. That’s what physicists at an IBM laboratory discovered while peering at an ellipse of atoms they had built on a copper surface.

Phantom atom (purple smudge at left) springs up when cobalt atom (purple point) is placed inside an ellipse of other cobalt atoms. IBM Research Div.

Having used the probe of a scanning tunneling microscope (SN: 10/24/98, p. 268: http://www.sciencenews.org/sn_arc98/10_24_98/Bob2.htm) to maneuver 36 individual cobalt atoms into a corral, the researchers started to put additional cobalt atoms inside the ellipse as part of a magnetism experiment. After adding just one, however, they noticed something peculiar. The microscope signal intensified elsewhere in the ellipse as if a second atom were present, but none was.

“It really came as a bit of a surprise,” says Donald M. Eigler of the IBM Almaden Research Center in San Jose, Calif. “We started with one atom and then, ‘Hey, what’s this?'”

What they had found was an echo of the response of electrons on the copper surface to the final cobalt atom’s magnetism. The electrons cancel that magnetism by redirecting their intrinsic magnetic orientation, Eigler explains. The reorientation produced a peak cancellation at the atom itself but also a cancellation signal about a third as strong some distance away.

The geometry of corrals (SN: 6/18/94, p. 389) partly explains such ghosts, report Eigler and IBM Almaden colleagues Hari C. Manoharan and Christopher P. Lutz in the Feb. 3 Nature. The scientists had put the first atom at one of two foci of the ellipse. Because of the ellipse’s shape, electron waves emanating from one focus reflect to the other. That’s where the phantom atom appeared.

Such an apparition suggests that information about the atom moves across the ellipse without wires or heat dissipation—a possible channel for future electronic devices, Eigler says.

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