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'Artifical atoms' work leads to Packard Award

Assistant Professor Raymond Ashoori of physics, who won a $500,000 David and Lucile Packard Fellowship this fall for his work in developing a technique that is aiding scientists' research with "artificial atoms," clearly remembers how it all began.

"That first experiment [in 1991] was pretty wild," Professor Ashoori, 29, said. "At the time, I was told it wouldn't work-it would be too difficult."

But late one night about a year later Dr. Ashoori, then a postdoc at AT&T Bell Labs, found the first evidence that his experiment was indeed working: a series of "bumps" across a graph. Each bump represented individual electrons going one by one into an ultra-microscopic "box," a creation otherwise known as an artificial atom. That first successful experiment has since led to a technique known as single-electron capacitance spectroscopy, which "allows single electrons to be manipulated and rapidly detected with extraordinary sensitivity [as they enter or leave an artificial atom]," Professor Ashoori said.

Scientists have been creating artificial atoms, which are key to basic research in condensed-matter physics and could have a variety of applications in the semiconductor industry, for the last 10 years. Like real atoms, artificial atoms contain electrons, and scientists would like to understand how these electrons interact. But until Dr. Ashoori's work (in collaboration with colleagues at AT&T), they could only study masses of electrons-more than about 100-which precluded observations of individual interactions. In other words, the resolution of previous techniques was limited.

Single-electron capacitance spectroscopy, however, has solved that problem. It allows scientists to study interactions between individual electrons by measuring the change in electrical charge associated with the addition of each electron to an artificial atom.

The technique works by placing an artificial atom between two electrically conducting plates, or capacitors. (The "atom" itself is created by sandwiching a layer of the semiconducting material gallium arsenide between two layers of insulating material. The "atom" corresponds to a tiny area in the center of the gallium arsenide.)

When voltage is applied, electrons "tunnel" from one conducting plate-"it's like an electron reservoir," Dr. Ashoori said-to the artificial atom. The electrons are attracted to the specially configured "atom" much like electrons in a real atom are attracted to the nucleus.

The movement of single electrons to the artificial atom can be detected, Professor Ashoori said, "because when a single electron tunnels from the reservoir plate to the atom, it induces a tiny amount of charge on the other plate." And that charge can be detected "to determine what voltage is required to bring one electron into the artificial atom," he said.

Already Professor Ashoori has used the technique to study phenomena that scientists have long been interested in but heretofore could not observe. For example, he has been able to detect a single electron's "spin flip." This phenomenon, which occurs when a magnetic field is applied to an artificial atom containing two electrons, happens because the magnetic field forces one of the electrons to change the direction in which it is spinning, which in turn forces it into a higher energy state.

Professor Ashoori notes that spin flipping in a real two-electron atom-otherwise known as helium-requires a magnetic field stronger than that on the surface of the sun. Spin flipping can be coaxed from electrons in an artificial atom because such atoms are much, much bigger than the real thing. "The large size of the artificial atom tends to magnify interactions between two electrons," he said.

Professor Ashoori is excited about future work with single-electron capacitance spectroscopy, and with the help of the Packard Fellowship he plans to continue studying the effects of magnetic fields on the electrons in artificial atoms. (He will receive $100,000 in unrestricted funds per year for five years to support his research.)

In addition, he would like to apply single-electron capacitance spectroscopy to the study of other small things. "It has broad applications in terms of what you can do," he said. For example, "you could measure the [electrical] properties of single defects or single impurities in a material." One specific example: in silicon there are defects that trap electrons and reduce the efficiency of transistors, "and these things are poorly understood," Dr. Ashoori said. By studying single traps, he said, "we can learn more about them, and perhaps take them out."

Professor Ashoori is among 20 young university faculty from around the United States to receive Packard Fellowships this year. The Fellowships were established to further the work of promising young scientists and engineers, and to encourage a steady flow of talented graduate students to undertake university research in this country, according to a press release from the Packard Foundation.

Explained David Packard, chairman of the Foundation and co-founder of Hewlett-Packard Company, "Many of the most important technological contributions of this century have come from university faculty members who began their research early in their careers and have worked in their areas of interest over a long period of time. We want to nurture this kind of research."

Raymond Ashoori earned his BA from the University of California, San Diego in 1984, and his PhD from Cornell University in 1990.

A total of six MIT professors have received Packard Fellowships. In addition to Professor Ashoori, they are: Moungi Bawendi, chemistry, 1992; Scott Virgil, chemistry, 1991; Jacqueline Hewitt, physics, 1990; Ruth Lehmann, biology, 1989, and Arthur Lander, brain and cognitive sciences, 1988.

A version of this article appeared in the November 17, 1993 issue of MIT Tech Talk (Volume 38, Number 14).

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