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 The term Artificial atom is commonly used to describe objects that have bound, discrete electronic states, as is the case with naturally occurring atoms. Semiconductor quantum dots are the most common example of 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 Massachusetts Institute of Technology, at Cambridge; coeducational; chartered 1861, opened 1865 in Boston, moved 1916. It has long been recognized as an outstanding technological institute and its Sloan School of Management has notable programs in business, . Ashoori described the fabrication fabrication (fab´rikā´sh  n),n the construction or making of a restoration. 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 American Association for the Advancement of Science (AAAS), private organization devoted to furthering the work of scientists and improving the effectiveness of science in the promotion of human welfare. . 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 Murray Hill may refer to one of the following places:
"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 capacitance, in electricity, capability of a body, system, circuit, or device for storing electric charge. Capacitance is expressed as the ratio of stored charge in coulombs to the impressed potential difference in volts. 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 repulsion /re·pul·sion/ (re-pul´shun) 1. the act of driving apart or away; a force that tends to drive two bodies apart. 2. 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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