Scientists measure what it takes to push an atom

About one-130-millionth of an ounce of force pushes a cobalt atom across a smooth, flat piece of platinum.

Pushing the same atom along a copper surface is easier, just one-1,600-millionth of an ounce of force.

The scientists report these minuscule findings in a recent issue of the journal Science.

I.B.M. scientists have been pushing atoms around for some time, since Donald M. Eigler of the company’s Almaden Research Center in San Jose, Calif., spelled “IBM” using 35 xenon atoms in 1989. Since then, researchers at the company have continued to explore how they might be able to construct structures and electronic components out of individual atoms.

Knowing the precise forces required to move atoms “helps us to understand what is possible and what is not possible,” said Andreas J. Heinrich, a physicist at Almaden and an author of the new Science paper. “It’s a stepping stone for us, but it’s by no means the end goal.”

In the experiment, Dr. Heinrich and his collaborators at Almaden and the University of Regensburg in Germany used the sharp tip of an atomic force microscope to push a single atom. To measure the force, the tip was attached to a small tuning fork, the same kind that is found in a quartz wristwatch. In fact, in the first prototype, Franz J. Giessibl, a scientist at Regensburg who was a pioneer in the use of atomic force microscopes, bought an inexpensive watch and pulled out the quartz tuning fork for use in the experiment.

The tip vibrates 20,000 times a second until it comes into contact with an atom. As the tip pushes, the tuning fork bends, like a diving board, and the vibration frequency dips.

A single atom does not roll, and even a perfectly smooth surface is not perfectly smooth. Instead, the atom rests in small indentations in the lattice, in effect like an egg in an egg carton. The resistance — what becomes friction when multiplied by millions and billions of atoms — comes from the energy needed to rearrange the bonds between the cobalt atom and surface.

When the tip pushes hard enough, the atom hops, almost instantaneously to the next indentation. “It’s not smooth,” said Markus Ternes, another Almaden scientist working on the research. “It’s faster than we can detect.”

From the changes in the frequency of the tuning fork vibrations, the scientists calculated the force that the tip applied to the cobalt atom.

Copper is less sticky than platinum, because of differences between the underlying bonds, and hence allowed the greater ease is pushing the cobalt atom along.