fuzziness of a particular atom with that of a neighbor. This overlap means that the two atoms share electrons to a slight degree, and the force that results resembles that of a chemical bond.

Such forces come into play both in attaching a xenon atom to the underlying surface and in attracting it to a pointed tip. Eigler notes that the tip can operate in two modes: Imaging and manipulation. In the imaging mode the tip rides above the surface at a distance of several angstroms, mapping the location of xenon atoms without disturbing them. When manipulating, the tip simply goes lower; more properly, it uses the feedback system to position itself so as to receive a stronger tunneling current, which is the measure of this closeness. "That increases the magnitude of the attractive force between the atom and the tip," Eigler adds. The consequence is that the atom remains attached to the surface but can also follow the tip as it moves about. The atom does not rise from the surface but acts somewhat like a heavy steel ball that can roll, with the tip attracting and guiding it as if the tip were a magnet.

"We find that the ability to slide a xenon atom over a nickel surface is independent of both the sign and the magnitude of the electric field, the voltage, and the current," Eigler writes. "It does, however, critically depend upon the separation between the tip and the atom." This leads to a simple procedure for repositioning such atoms, one by one: Locate them using imaging; lower the tip to attract an atom of interest; move the tip as desired; and then raise the tip. The atom will then stay behind, and the low temperatures of the surface will keep it there (see Figure 2.4).

FIGURE 2.4 Dragging an atom across a surface, in five steps: (1) locate atom and place tip over it; (2) lower tip to increase its force on the atoms; (3) move tip sideways, pulling atom with it; (4) atom is now in desired position; (5) raise tip, leaving atom in place on surface. (Courtesy of Donald Eigler, IBM.)

In this fashion Eigler and his colleagues have placed such atoms on the surface as if they were lightbulbs on a theater marquee, spelling out the letters IBM. More in the nature of basic research, these researchers have studied the fundamental behavior of such atoms in small groups, carrying out observations that in no way could have previously been performed. In one of them they built a row of seven xenon atoms (see Figure 2.5). In Eigler's words,

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