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Graphite Found to Exhibit Surprising Quantum Effects



Albert Einstein, Paul Dirac and other founding physicists may have used pencils to work out the details of relativity and quantum mechanics. Now their modern successors are employing pencil lead in a new way to prove those theories--and potentially point the way toward a whole new form of electronics.

Pencil lead is actually graphite--a carbon mineral that, when dragged across paper, leaves writing behind because its atomic layers separate easily. This also means that it is an excellent conductor of electricity. Last year, Andre Geim of the University of Manchester in the U.K. used adhesive tape to strip graphite down to a layer just one atom thick; they called this superthin layer of graphite "graphene."

Experiments on graphene have revealed some strange phenomena, as detailed in two papers in today's Nature. The two-dimensional material remains capable of conducting electricity thanks to the free-floating electron in the honeycomb structure of carbon atoms. But these electrons display some unusual properties.

Geim's team found that they do not slow down, even at very low temperatures. In essence, the electrons act as if they have no mass, or no "rest mass," to use the more precise phrase from special relativity. It also means that graphite--at least the two-dimensional variety--never stops conducting. Dubbing these pseudo-relativistic particles "massless Dirac fermions," the researchers also proved that they travel far faster than electrons in other semiconductors. As such, they conform to that famous equation E=mc2 (with their actual speed, some 400 times slower than the speed of light, standing in for c).

Physicists at Columbia University, led by Philip Kim, independently confirmed these findings and also found that the massless electrons fulfilled the predictions of the Hall effect. (Edwin Hall proved in 1879 that applying a magnetic field at a right angle to a conducting material would create a voltage that is perpendicular to the regular flow of current through that material.) This effect is also applicable at the quantum level, with one caveat: instead of the voltage smoothly increasing as the magnetic field intensifies, the voltage jumps up in steps. And that is exactly how the massless electrons in the graphene behaved, according to both groups.

Ultimately, Kim writes, the findings may "lead to new applications in carbon-based electronic and magneto-electronic devices," though further research is needed. It also means that the graphite left behind by a pencil--stripped down to a layer one atom thick--can be used to prove the theories scrawled in pencil by physicists of old.

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