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Weird "Particles" Spotted in Hot New Material

14 October 2009 (All day)
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In the past 5 years, no material has excited more interest from condensed matter physicists than graphene, a sheet of carbon only one atom thick. Electrons zing through the stuff in an unusual way, and they flow so easily that graphene could someday replace silicon and other semiconductors as the material of choice for microchips. Now, a team of physicists has taken a key step in fulfilling graphene's promise as a hotbed of exotic physics by showing that the electrons within it can team up to behave like particles with a fraction of the electron's charge.

The effect is called the fractional quantum Hall effect, and it's an esoteric embellishment of an already esoteric phenomenon known as the Hall effect. Discovered in 1879, the Hall effect works like this: Suppose you take a horizontal bar of metal and apply a voltage from one end to the other. A current will run down the length of the bar. If you then apply a strong vertical magnetic field, the flowing electrons will experience a sideways shove that will cause them to crowd to the side of the bar as they go so that a voltage develops across the width of the bar too. Sideways Hall voltage increases in proportion to the strength of the magnetic field.

Things get weirder if the bar is made of semiconductor and is extremely thin top to bottom. In that case, the electrons can flow in only a few quantum channels that close one by one as the magnetic field increases. The Hall voltage climbs as the magnetic field increases in a series of even steps whose spacing is set by the electron's charge. The discovery of that quantum Hall effect won the Nobel Prize in physics in 1985. Weirder still, if the slab of semiconductor is made very pure and cold, then the electrons can gang up to act like "quasiparticles" with fractional charges--say, 1/3 of an electron's charge--adding more steps to the Hall-voltage stairway. That's the fractional quantum Hall effect, which bagged a Nobel in 1998.

The fractional effect is a sign of very strong interactions among the electrons, a condition that can lead to a variety of surprising phenomena and which marks the conceptual frontier in condensed matter physics. Many physicists had hoped to see the fractional effect in graphene as proof that it would be an especially fruitful material to study. But they couldn't be entirely sure it would appear. Because of the arrangement of the atoms in graphene, the electrons zip through less like ordinary massive particles that can stop and start and more like massless and always-moving particles of light. Nobody was sure such "relativistic" electrons would interact strongly enough to produce the effect, though several groups had looked for it. "It was the biggest disappointment in this really hot field, that it hadn't been seen," says Eva Andrei of Rutgers University in Piscataway, New Jersey.

Fret no more, physicists. Andrei and her team have finally spotted electrons in graphene getting together in the right way. To do it, the team suspended micrometer-sized bits of graphene to avoid interference from the underlying substrate. The researchers then used a special arrangement of electrodes to keep from shorting out their own measurements, they report online this week in Nature. They observed quasiparticles with 1/3 an electron's charge. In fact, Andrei says, the researchers saw the effect at higher temperatures and lower magnetic fields than are needed to see it in semiconductors, suggesting that the electrons in graphene interact especially strongly.

"It's absolutely convincing," says physicist Kostya Novoselov of the University of Manchester, U.K. "It definitely proves it's reasonable to study electron-electron interactions in graphene." Andrei says now that physicists have spotted this effect, they may see electrons in graphene joining together in completely new and even weirder ways. And if researchers can produce quasiparticles with charge 5/2, then in principle they could make a type of quantum computer that would work by braiding the particles' paths together.

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