Mason Inman - science journalist

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Black Hole in a Pencil

23 August 2006, for ScienceNOW

The stuff of pencil lead may display bizarre behavior thought to occur only around superheavy atoms and black holes. That's the implication of a new study, which suggests that the material can shoot electrons through other materials as if they're invisible. The effect could be useful for designing new kinds of transistors for electronics.
 

Rubbing a pencil across paper sloughs off layers of graphite--sheets of carbon linked in a honeycomb pattern. Reduce these sheets to one or two layers of atoms, and this material, known as graphene, reveals strange properties. In particular, electrons in the 2-dimensional sheets behave as if they have no mass and zip along at the speed of light--something not seen in any other material.

Now, physicists suggest that graphene has even more exotic behaviors. In the current issue of Nature Physics, Andre Geim at the University of Manchester in the United Kingdom and colleagues argue that a simple tabletop experiment could reveal an effect hypothesized 80 years ago by physicist Oscar Klein but never before seen.

Putting an electron in an open-topped box made of silicon should trap it, according to classical physics. But 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. But according to the so-called Klein paradox, 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.

But graphene's strange properties make it possible to demonstrate the Klein paradox in the lab, Geim and colleagues argue. They envision a simple circuit with a ribbon of graphene broken by a barrier of semiconductor. By tweaking the voltage across the semiconductor, they can adjust how high the barrier appears to electrons. If the barrier is low, the electrons will be blocked. But make the barrier high enough, and counterintuitively, electrons should sail through. As the effect can be turned on and off, it could be exploited to make new kinds of transistors, says Geim. Geim and colleagues were the first to isolate graphene in 2004, and researchers are still learning to work with it. But he hopes to perform the experiment within a couple of years.

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, says particle physicist Norman Dombey at the University of Sussex, U.K. He adds that researchers have been able to create short-lived, superheavy 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, he says.