[tt] QED in graphene

[Eugen Leitl]

http://www.telegraph.co.uk/digitallife/main.jhtml?xml=/connected/2005/11/15/ecfpenc15.xml

Do pencils point to the Holy Grail of physics?

Last Updated: 12:01am GMT 15/11/2005

The Manchester professor who showed how to levitate frogs and replicated the
glue that sticks lizards to the ceiling believes that graphite from the
pencil on his desk could lead to super-computers capable of probing the
greatest theory ever devised. Science Editor Roger Highfield reports

Did you hear the physicist's joke about the pencil? Never mind, it's
pointless. Over the past few days, however, physicists have discovered
something exotic lurking at the tip of even a blunt pencil that can be used
to work out many things, from how to build the next generation of superfast
computers to creating a cheap way to test the most successful theory ever
devised.
 	
Graphene

Metal detecting: electric charges in graphene's honeycomb film of carbon
atoms reveal effects of Einstein's relativity

The flat, parallel sheets of carbon atoms in the graphite of pencil lead can
be peeled apart to yield a single atomic layer with peculiar properties. This
new material is exciting physicists because it provides the wherewithal to
speed the number-crunching of computers and to probe the most precise theory
ever devised: QED.

This is the "jewel of physics" and began in earnest a century ago when Albert
Einstein unveiled special relativity, a theory that replaced Sir Isaac
Newton's notions of space and time and linked it with James Clerk Maxwell's
theory of electromagnetism.

Then scientists blended special relativity with quantum physics, the theory
that governs the subatomic world. The end result is quantum electrodynamics,
QED, which can predict with amazing accuracy the way electrons jump about in
atoms.

Until now it was possible to probe QED physics only by spending billions on
atom smashers or by studying distant galaxies. Now an easy way to put QED
through its paces has been found in that pencil tip by Prof Andre Geim of
Manchester University, a scientist best known for his earlier use of magnetic
fields to levitate frogs and his recreation of the dry adhesive that enables
a gecko to crawl along ceilings.  advertisement

Working with colleagues in Manchester, Russia and Holland, he has used the
graphite to find the real-world equivalent of a super-simple material that
for the past half-century has been known only to theoreticians: a
two-dimensional crystal - a single sheet of atoms - which enabled them to
work out how to model the three-dimensional salt crystals that we scatter
over our food, among other things, but was thought not to exist.

That was until Prof Geim gazed closely at his pencil. Pencils use graphite
because its atomic layers separate easily to leave a trace on paper (which is
also why it is an excellent lubricant).

Last year, Prof Geim used Sellotape to strip graphite down to a layer one
atom thick; the atomic fabric is called "graphene" and, as he explains, it
enables many theoretical implications of QED to now be put to the test in
efforts to develop the next generation of theory.

In the current issue of the journal Nature, his team describes how electric
charges in graphene's honeycomb film of carbon atoms reveal effects of
Einstein's relativity never seen before in materials. These effects are
thought to occur naturally only in neutron stars, superdense objects with
huge magnetic fields.

While the buzz of electrons in microchips is usually described by
old-fashioned quantum mechanics, developed in the first half of the last
century, their movements in graphene can only be described with QED.

"In essence, studies of electron transport in graphene allow access to the
rich and subtle physics of quantum electrodynamics in a bench-top
experiment," said Prof Geim. "QED comes out of a pencil trace."

QED, like old fashioned quantum theory, has many mind- boggling implications.
No wonder, then, that graphene "turns out to exhibit truly exceptional
properties," he said.

Graphene behaves as if the electrical current is not carried by normal
electrons but by charged particles with no mass at all - when at rest akin to
photons of light but which carry electric charge. Scientists call them Dirac
fermions and love to study them, said Prof Geim.

These particles do not slow down. Even when there are none at all, current
will still flow in graphene. The mass of these particles also varies with
changing energy, just as predicted in a paper published a century ago this
month by Einstein on the relationship between mass and energy, expressed by
the most famous equation of all, E = mc2.

Physicists led by Dr Philip Kim and Nobel laureate Prof Horst Stormer at
Columbia University in New York independently confirmed these findings and
also found that the massless electrons fulfilled the predictions of quantum
Hall effect.

This was glimpsed in 1879 when Edwin Hall showed that if an electric current
flows through a conductor in a magnetic field, the field exerts a force on
the moving electrons which deflects them to one side of the conductor to
create a measurable voltage. The same also goes for graphene, with one
quantum caveat: instead of the voltage smoothly rising as the magnetic field
intensifies, the voltage jumps in steps.

There is also the "half-integer quantum Hall effect", where only certain
numbers of electrons are allowed to flow through the graphene, said Dr Kostya
Novoselov, one of Prof Geim's colleagues.

Other new and peculiar quantum and relativistic effects may well lurk in the
electronic flatland of graphene. The odds are that graphene can do things for
electronics that are beyond the reach of conventional materials. "By
attaching electrical contacts and measuring the resistivity of graphene we
have found that charge carriers in it move without scattering over many
thousands of inter-atomic distances," said Prof Geim.

Graphene brings scientists close to the Holy Grail of electronics which
engineers call ballistic transistors - ultimately faster than any current
technology. "A ballistic transistor is one in which electrons can shoot
through without collisions, like a bullet," he said.

Judging by how quickly scientists have devised applications of related
molecules called carbon nanotubes - in effect rolled up graphene - the tip of
a pencil could well point towards a ballistic electronic revolution within a
decade.