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Topic Name: A new spectroscopy method
Category: Organic electronics
Research persons: Professor Ray Ashoori, and post-doc Oliver Dial
Location: Massachusetts Institute of Technology,Department of Physics
77 Massachusetts Avenue,Cambridge, MA 02139-4307tel: 617.253.4800,fax: 617.253.8554, United States
Details
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 phonesThe 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."
About The Researchers;
RAYMOND ASHOORI, Professor of PhysicsEmail: ashoori@mit.edu
Phone: (617) 253-5585Address: Room 13-2053
Oliver Dial email: dial@MIT.EDU
phone: (617) 253-8497
address: 13-2045
department: Materials Processing Center
title: Postdoctoral Associate
Funded:
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.
In The Images-
1.Professor Ray Ashoori, left, and post-doc Oliver Dial have developed a new spectroscopy method that has about 1,000 times finer resolution than other methods. They are shown above in the lab with a dilution refrigerator that they are customizing to continue the work - this refrigerator will allow them to go colder with their experiments, which would allow even better resolution.
2."High-resolution spectroscopy of two-dimensional electron systems
3. & 4.Tricks-
This experiment is technically demanding; large bandwidth signals (from around 1 kHz to 1 Ghz) need to be generated with excqusite precision, transmitted deep inside of a helium dilution refrigerator (without too much heating!), and applied to our sample. However, that's only half the trip! Once we've balanced two large signals against each-other, we're left with a tiny trickle of charge which we need to amplify and transmit back out of the cryostat to room temperature.
Here are a few specific examples of the kinds of considerations that go into making this experiment work.
Although this looks like a 3D map of a highway interchange, it's actually the interior of our "pulse shaper". This home-built device takes electrical pulses generated by commercial pulse generators, which typically "ring," or wiggle around, and turns them into the nearly perfectly flat pulses we need for this experiment.
The thick, silver colored wires that you see arcing around the inside of the box (which is normally sealed!) carry the high-frequency pulses to the circuit boards that do the actual magic. These high frequency pulses don't like turn around sharp corners, so we use these gentle curves instead.
Here, you can see the place where some of our wiring enters the cryostat. Notice the smooth curves once again in the thick, silver colored wires (they're called semi-rigid coaxial cables) as the enter. The white-banded cylinders they're attached to at the top of the stick are called attenuators; they make signals smaller. Not only do they help us by making the noise generated by our signal generators less important, but they help with reflections. We cannot afford to absorb the radio-frequency pulses we send down into our cryostat; the energy involved would heat up our fridge. Instead, we let most of the power reflect back out of the cryostat. Normally this would be a problem, because some of that power would reflect back off our signal generator (it isn't perfect), and we'd see it hit our sample again in about twice the time it takes light to travel down the length of our cables. That's a long time in our experiment; we need to have completed our entire measurement in about 100 nanoseconds, around the amount of time it takes light to travel to first base from home plate on a baseball field. However, each time the signal travels through the attenuator, it's made smaller; this reflection has passed through these attenuators two extra times on the way back to the sample, so it's small enough we can ignore it.
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