Scientists have published a new study that suggests the material found in pencils may display bizarre behavior thought to occur only around super-heavy atoms and black holes — a discovery that could have important implications for designing new kinds of transistors for future electronics.

This blogger once managed to create a black hole with a No. 2 pencil and a scholastic essay question regarding “materialistic self-assessment as the Cancer of society.” But never mind that, as a new study suggests an even closer tie with pencils and black holes: that the material in pencils may display bizarre behavior thought to occur only around super-heavy atoms and black holes.

Scientists investigating condensed matter have found that thin sheets of graphite — sheets of carbon linked in a honeycomb pattern, like the layers deposited on a piece of paper by someone writing a shopping list with a pencil — may be able to shoot electrons through other materials as if they’re invisible.

According to the report in a recent issue of Nature Physics, the discovery could have important implications for designing new kinds of transistors for future electronics.

The paper, by a team of United Kingdom researchers led by A.K. Geim of the University of Manchester, reports that reducing these sheets of graphite to one or two layers of atoms reveals strange properties in the material, known as graphene. Graphene, a single, one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice, is the two-dimensional building block for carbon materials of every other dimensionality.

In particular, the researchers found that electrons in the two-dimensional sheets behave as if they have no mass and travel along at the speed of light — something not seen in any other material. They do not slow down, even at very low temperatures. This also means that graphite — at least the two-dimensional variety — never stops conducting.

Yet the team of European physicists further deduced that the Klein paradox could be yet another of the strange quantum phenomena made accessible by the properties of graphene. Geim and colleagues argue that a simple tabletop experiment could reveal an effect hypothesized 80 years ago by physicist Oscar Klein but never before seen. They found that the graphene’s electrons may exhibit behavior that follows the Klein paradox, which until now had been thought only reproducible under exotic conditions.

The so-called Klein paradox — unimpeded penetration of relativistic particles through extreme (high and wide) potential barriers — is one of the most exotic and counterintuitive consequences of quantum electrodynamics, the researchers note in Nature Physics.

Although classical physics says putting an electron in an open-topped box made of silicon should trap it, in quantum physics the electron can “tunnel” out. The higher or thicker the box’s walls, the lower the chances of tunneling, and infinitely high walls completely block it. According to the Klein paradox, however, if particles are moving fast enough, even infinitely high walls look invisible and the particles pass through.

Physicists thought producing the effect required extreme conditions, such as the gravitational tide at the edges of black holes.

The researchers write:

The phenomenon is discussed in many contexts in particle, nuclear and astrophysics, but direct tests of the Klein paradox using elementary particles have so far proved impossible. Here we show that the effect can be tested in a conceptually simple condensed-matter experiment using electrostatic barriers in single- and bi-layer graphene. Owing to the chiral nature of their quasiparticles, quantum tunneling in these materials becomes highly anisotropic, qualitatively different from the case of normal, non-relativistic electrons. Massless Dirac fermions in graphene allow a close realization of Klein’s gedanken experiment, whereas massive chiral fermions in bilayer graphene offer an interesting complementary system that elucidates the basic physics involved.

Graphene’s strange properties make it possible to demonstrate the Klein paradox in the lab, Geim and colleagues argue. They hypothesize that they can create a circuit with a layer of graphene broken by a semiconductor barrier and manipulate its voltage to make electrons flow right through the barrier, a feat that runs counter to conventional wisdom. If the flow can be regulated and controlled, it could lead to some interesting developments in transistor physics.

It is “surprising” that a low-energy experiment can produce an effect thought to occur only with particles going close to the speed of light, particle physicist Norman Dombey at the University of Sussex, U.K., recently told ScienceNOW Daily News. He adds that researchers have been able to create short-lived, super-heavy atomic nuclei in particle colliders that are, in theory, big enough to show the Klein paradox. But no one could actually test for the Klein paradox this way, Dombey says.

Geim and colleagues were the first to isolate graphene in 2004. They stripped graphite down to a layer merely one atom thick; they called this super-thin layer of graphite “graphene.” Their method was surprisingly (some say astonishingly) simple: Use adhesive tape to peel off weakly bound layers from a graphite crystal and then gently rub those fresh layers against an oxidized silicon surface.

Researchers are still learning to work with it.

Resources

Paradox in a pencil
by Alex Calogeracos
Nature Physics, September 2006

Black Hole in a Pencil
by Mason Inman
ScienceNOW Daily News, Aug. 23, 2006

Electrons in Atomically Thin Carbon Sheets Behave Like Massless Particles
by Mark Wilson
Physics Today, January 2006