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Packing electrons into an artificial atom.

Over the years, researchers have prodded, stretched, squeezed, illuminated, and even smashed atoms into yielding their quantum secrets. Now they can create and tailor "artificial atoms" to study the behavior of individual electrons confined to spaces much larger than atomic dimensions.

Such novel structures allow researchers to investigate certain quantum effects under conditions not possible in ordinary atoms. "There's a continuum of physics to study as you vary the size," says Raymond C. Ashoori of the Massachusetts Institute of Technology.

Ashoori described the fabrication of an artificial atom and the result of adding electrons to it one by one this week in Boston at a meeting of the American Association for the Advancement of Science.

An ordinary atom stays together because of the attraction between its nucleus and orbiting electrons. An artificial atom is more like a tiny box whose walls keep electrons confined. Nonetheless, in both types of confinement, electrons can have only certain welldefined energies.

Ashoori and his collaborators at AT&T Bell Laboratories in Murray Hill, N.J., create an artificial atom by sandwiching a thin layer of gallium arsenide between two layers of aluminum gallium arsenide. The artificial atom itself corresponds to a location - a few hundred angstroms wide - in the gallium arsenide crystal that can be completely emptied of electrons, then gradually refilled.

"By observing how much energy it takes to add each successive electron, we can directly learn how the electrons interact with one another," Ashoori says.

To detect and measure the energy needed to add successive electrons, the researchers use a new, remarkably sensitive technique known as single-electron capacitance spectroscopy (SN: 4/4/92, p.222). "We can count them [electrons] one by one as they go in:' Ashoori says.

Ashoori and his colleagues can also study how much the repulsion between electrons contributes to an electronic energy level. By applying an external magnetic field, they can squeeze an artificial atom; this squeezing makes it easier to distinguish betWeen effects caused by electron repulsion and those attributed to electron motion.

Observing changes in energy level while increasing the magnetic field allows an unprecedented measurement of how much electrons interact with each other, Ashoori says.

Indeed, attempting an equivalent measurement in a helium atom would require a magnetic field of 400,000 teslas - far larger than the 2 teslas that Ashoori and his colleagues need to see this electron-electron interaction in a two-electron artificial atom.

For three electrons, the interactions among electrons become exceedingly complicated, and the corresponding energy measurements are difficult to interpret. But when more than 10 electrons are packed into an artificial atom, their behavior begins to resemble that of electrons in a metal.
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Author:Peterson, Ivars
Publication:Science News
Date:Feb 20, 1993
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