Ghostly Electrons: Particles flit through atom-thin islands

Confine electrons within microscopically thin layers of material and weird things happen. Experiments on semiconductors in the 1980s demonstrated that to physicists (SN: 10/17/98, p. 247).

Now, two independent research teams have found that electrons imprisoned within a carbon sheet one atom thick behave in yet other odd ways, unlike anything seen in other materials. The electrons act as if they have no mass, so they zip along much faster than electrons moving through semiconductor layers do.

To properly account for the particles’ behavior, scientists use equations that include aspects of Einstein’s theory of relativity. The electrons act like a kind of particle known as a Dirac fermion, which shows up in particle accelerators and cosmic rays. Scientists expect it to also appear in some high-temperature superconductors, gases adjacent to neutron stars, and other exotic circumstances. The new electronic entity looks promising as a benchtop model for investigating the physics of such particles, the researchers say.

A practical payoff is possible as well. The fast electrons could enable the novel carbon sheets to serve as a new type of circuit element in electronic devices capable of operating at frequencies 1,000 times as high as today’s components commonly do, says Andre K. Geim of the University of Manchester in England, who leads one of the research teams.

For decades, scientists interested in two-dimensional arrangements of electrons have built stacks of semiconductor materials—the sorts of structures ubiquitous in today’s microcircuits—to confine the particles to the zone where two layers of a stack meet (SN: 12/18&25/04, p. 394: An Electron Runs through It). In some past studies, scientists found unexpected quantum mechanical variations in electrical resistance that became known as the quantum Hall effect (SN: 2/22/03, p. 124: Knotty Calculations). Those discoveries eventually garnered Nobel prizes.

Last year, Geim and his team reported that they could remove intact, one-atom-thick flakes of carbon from pieces of graphite (SN: 10/23/04, p. 259: Graphite in Flatland: Carbon sheets may rival nanotubes). Now, that team and another led by Philip Kim of Columbia University have zeroed in on the electrons in such carbon flakes, which are known as graphene.

In both experiments, the teams coated wafers of silicon with thin layers of silicon dioxide—an electrical insulator. They placed graphene flakes on top of the insulator and connected electrodes to them. Next, the scientists measured the electrical resistance of the flakes to reveal their electronic properties. They found a variation of the quantum Hall effect that had been predicted to occur only when relativistic effects are important.

In graphene, theoretical arguments indicate, electrons interacting as quantum waves effectively cancel each other’s masses. As a result, the particles move much faster than ordinary, mass-laden electrons in semiconductors do, Geim says. The groups describe their findings in back-to-back reports in the Nov. 10 Nature.

The “real tour de force” of these new experiments, comments Charles L. Kane of the University of Pennsylvania in Philadelphia, is that “they’re able to actually isolate the graphene and to show that indeed it has this [exotic electron] behavior.”