Single-electron control on the atomic level.

A microelectronic circuit typically requires a massive electrical surge involving billions of electrons to register or transfer one unit, or bit, of information. However, novel circuitry in which a single electron controls the motion of other electrons potentially provides a faster, more efficient, and more economical means of storing and processing information.

Intrigued by this possibility, researchers have over the last few years explored the feasibility of designing and fabricating such "single-electron" devices (SN: 3/21/92, p. 180). Now, a group of physicists has used calculations and computer simulations to demonstrate theoretically that a single-electron device can operate reasonably reliably, even when it consists of nothing more than a string of atoms.

"We proved that if we can assemble, atom by atom, the necessary structure and have electrons hop from one atom to the next, we have the physical basis for logic operations at the atomic level:' says Konstantin K. Likharev of the State University of New York at Stony Brook. But at this time, "we do not feel that either we or anyone else has the experimental techniques to [fabricate] such atomic structures."

Likharev, Stony Brook colleague Dmitri V. Averin, and Karl Hess and Leonard E Register of the Beckman Institute at the University of Illinois at Urbana-Champaign report their findings in the Jan. 3 APPLIED PHYSICS LETTERS.

The researchers based their work on the notion of creating a line of atoms that incorporates a "well" capable of trapping an electron. Once an electron occupies the well, it blocks the motion of other electrons traveling along this atomic "wire." In effect, the trapped electron repels the mobile electrons. When no trapped electron is present, electrons travel unimpeded along the atomic wire.

However, quantum mechanical effects on an atomic scale, which make an electron behave as if it were both a particle and a wave, muddle this behavior. An empty well sometimes reflects electrons, and an occupied well sometimes allows the passage of electrons. To ensure that single-electron devices based on such a design are sufficiently reliable for practical purposes, the probabilities of these unwanted outcomes must be small.

The researchers demonstrated that quantum effects can significantly decrease the reliability of the control that one electron has over the motion of another. But for the atomic structure considered, a sufficiently high reliability can be attained by making the wire at least 50 atoms long.

"In this example, quantum mechanics is in principle incompatible with creating a zero or a one:' Hess says. "If you require such precision... your device needs to be a certain size." This, in turn, sets limits on the number of such single-electron devices that can be packed into a given area.

Likharev looks at the results a little differently "We have a proof of principle," he concludes. "We can do logic operations on the atomic level in a natural way, using only [electrostatic] repulsion [between electrons]."

The trouble is that no one has a clear idea of how to build the necessary atomic structures in practice. For example, no technique that involves molecules and atoms assembling themselves into units can yet achieve the appropriate geometry,

Meanwhile, Likharev and his colleagues are concentrating on elucidating the electronic characteristics of larger-scale structures that can be made using present-day technology "We try to design these devices and understand what exactly can be done," he says.