Saturday, April 19, 2008

Carbon mesh pins down universal constant

Carbon mesh pins down universal constant
Graphene, a mesh of carbon one atom thick, has been used to finally pin down a mysterious universal constant.
Image: Thomas Szkopek

SYDNEY: The world's thinnest material can shed light on the exact measurement of one of the universe's fundamental physical constants, a new study reveals.

Researchers led by physicist Andre Geim from the University of Manchester in the U.K., used graphene – a sheet of carbon just one atom thick – to gauge the exact measurement of the fine structure constant, a fundamental physical constant defining the interaction between fast moving electric charges and light.

Their results were published online in the current edition of the journal Science Express, ahead of publication in the U.S. journal Science.

The fine structure constant was first introduced by physicists in attempts to understand atomic structure and has long mystified scientists because there seemed to be no natural mathematical relationship that described the constant, like a circle's circumference divided by its diameter describes the universal constant pi.

Foundations of life

In this new study, the U.K. and Portuguese researchers shone light through sheets of graphene and found that it absorbs a surprising amount of light considering its extreme thinness. The material's opacity is due to its molecular structure: a mesh of carbon atoms and bonds that looks something like chicken wire (when rolled up, graphene forms carbon nanotubes and when piled in layers it forms graphite).

They found that the exact value of light absorbed by graphene – 2.3 per cent of visible light – divided by pi gives the value of the fine structure constant (approximately 1/137). As the researchers point out, few other universal constants can be described so simply.

"We were absolutely flabbergasted when we realised that such a fundamental effect could be measured in such a simple way. One can have a glimpse of the very foundations of our universe just looking through graphene," said Geim, who was part of the team that discovered graphene in 2004.

"Change this fine-tuned number by only a few per cent and life would not be here because nuclear reactions in which carbon is generated from lighter elements in burning stars would be forbidden. No carbon means no life," he added.

Acting like light

Theoretical physicist Ross McKenzie from the School of Physical Sciences and the Centre for Organic Photonics and Electronics (COPE) at the University of Queensland, Australia, describes the research as "very beautiful".

"It's rare in condensed matter physics to get something so clean and elegant, particularly in the way the theory agrees with the experiment," he said.

Graphene can be used to calculate the fine structure constant because its crystal structure is unique among solids, according to McKenzie. As electron waves travel through the crystal, the symmetry of the carbon atoms forces the relationship between the electron wavelength and energy to be the same as the relationship for photons in light. As a result, the electrons effectively act as photons, but move at a much slower velocity. This property in turn leads to other unique properties that rely on the fine structure constant.

Chemical physicist Paul Meredith, also from COPE, said the research represents a "great leap forward" in terms of manipulating graphene. "The first step towards making a device, especially a nanoscopic device, is the ability to manipulate this material and they've cracked it," he said.

Graphene has very high conductivity so could be used in a variety of structured electronic materials, Meredith said. Possible uses include flexible transparent electronics or transparent electrodes for solar cells, as well as innovative uses in medicine.

Original here

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