[Editors] MIT gets ultra-sharp glimpse of electrons

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
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MIT physicists get ultra-sharp glimpse of electrons
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For Immediate Release
FRIDAY, JULY 20, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO AVAILABLE

STORY ONLINE AT: http://web.mit.edu/newsoffice/2007/spectroscopy-0720.html

CAMBRIDGE, Mass. - MIT physicists have developed a spectroscopy 
technique that allows researchers to inspect the world of electrons 
confined to a two-dimensional plane more clearly than ever before.

Two-dimensional electron systems, in which electrons are walled in 
from above and below but are free to move in a plane as if they were 
placed on a sheet of paper, are rarely observed in the natural world. 
However, they can be created in a laboratory and used, for example, 
in high-frequency amplifiers found in cell phones.

The new spectroscopy technique measures electron energy levels with 
1,000 times greater resolution than previous methods, an advance that 
has "tremendous power to tell you what the electrons are doing," said 
MIT physics professor Ray Ashoori, author of a paper on the work 
published in the July 12 issue of Nature. This technique has already 
revealed some surprising behavior, and the researchers believe it 
will shed new light on many physical phenomena involving electrons.

Ashoori and postdoctoral associate Oliver Dial took advantage of a 
quantum phenomenon known as tunneling to create the most detailed 
image ever of the spectrum of electron energy levels in a 2D system.

The new spectroscopy technique relies on a phenomenon that defies the 
laws of classical mechanics. Electrons, because they exhibit wavelike 
behavior, can move between two locations separated by a barrier 
without having to pass over the barrier-a phenomenon known as 
"quantum tunneling."

"We anticipate that this technique will help us discover all kinds of 
new physics," said Ashoori. "We're looking into a realm that was just 
not visible to us before."

Electrons trapped in 2D systems exist in specific energy levels, just 
as electrons orbiting an atom's nucleus in three dimensions exist in 
distinct quantum energy levels. By measuring which energy levels are 
occupied, physicists can study how electrons behave together in large 
groups.

The researchers used short pulses of electricity to induce electrons 
to tunnel from a 2D system to a 3D system, and vice versa. By 
measuring the resulting voltage difference, they could calculate the 
energy states of the electrons in the 2D system.

The spectroscopy experiments were performed inside a semiconducting 
crystal cooled to 0.1 degrees above absolute zero.

Until now, the primary method for performing this kind of 
spectroscopy relied on photoemission. The new method has an energy 
resolution that is 1,000 times finer than the best photoemission 
measurements.

Physicists have also traditionally used "transport" techniques that 
measure electrical currents flowing in response to applied voltages 
to learn about 2D electron energy levels, but that technique only 
offers a partial look at what electrons are doing.

"Similar to creating small ripples on the surface of a sea, transport 
techniques only tell us about what is happening very close to the 
water's surface," said Dial. "Pictures made with this high-resolution 
spectroscopy provide, in essence, one of the first glimpses of the 
entire ocean in these systems and show what a beautiful and 
interesting  world exists beneath the surface."

The research was conducted in collaboration with crystal growers at 
Alcatel-Lucent Bell Laboratories in Murray Hill, N.J., and funded by 
the Office of Naval Research and the National Science Foundation.

--MIT--

Story written by Anne Trafton