Rutgers researchers
have discovered novel electronic properties in two-dimensional sheets of carbon
atoms called graphene that could one day be the heart of speedy and powerful
electronic devices.
The new findings, previously considered possible by physicists but only now
being seen in the laboratory, show that electrons in graphene can interact strongly
with each other. The behavior is similar to superconductivity observed in some
metals and complex materials, marked by the flow of electric current with no
resistance and other unusual but potentially useful properties. In graphene,
this behavior results in a new liquid-like phase of matter consisting of fractionally
charged quasi-particles, in which charge is transported with no dissipation.
In a paper issued online by the prestigious science journal Nature and slated
for print publication in the coming weeks, physics professor Eva Andrei and
her Rutgers colleagues note that the strong interaction between electrons, also
called correlated behavior, had not been observed in graphene in spite of many
attempts to coax it out. This led some scientists to question whether correlated
behavior could even be possible in graphene, where the electrons are massless
(ultra-relativistic) particles like photons and neutrinos. In most materials,
electrons are particles that have mass.
"Our work demonstrated that earlier failures to observe correlated behavior
were not due to the physical nature of graphene," said Eva Andrei, physics
professor in the Rutgers School of Arts and Sciences. "Rather, it was because
of interference from the material which supported graphene samples and the type
of electrical probes used to study it."
This finding should encourage scientists to further pursue graphene and related
materials for future electronic applications, including replacements for today's
silicon-based semiconductor materials. Industry experts expect silicon technology
to reach fundamental performance limits in a little more than a decade.
The Rutgers physicists further describe how they observed the collective behavior
of the ultra-relativistic charge carriers in graphene through a phenomenon known
as the fractional quantum Hall effect (FQHE). The FQHE is seen when charge carriers
are confined to moving in a two-dimensional plane and are subject to a perpendicular
magnetic field. When interactions between these charge carriers are sufficiently
strong they form new quasi-particles with a fraction of an electron's elementary
charge. The FHQE is the quintessential signature of strongly correlated behavior
among charge-carrying particles in two dimensions.
The FHQE is known to exist in semiconductor-based, two-dimensional electron
systems, where the electrons are massive particles that obey conventional dynamics
versus the relativistic dynamics of massless particles. However, it was not
obvious until now that ultra-relativistic electrons in graphene would be capable
of exhibiting collective phenomena that give rise to the FHQE. The Rutgers physicists
were surprised that the FHQE in graphene is even more robust than in standard
semiconductors.
Scientists make graphene patches by rubbing graphite - the same material in
ordinary pencil lead - onto a silicon wafer, which is a thin slice of silicon
crystal used to make computer chips. Then they run electrical pathways to the
graphene patches using ordinary integrated circuit fabrication techniques. While
scientists were able to investigate many properties of the resulting graphene
electronic device, they were not able to induce the sought-after fractional
quantum Hall effect.
Andrei and her group proposed that impurities or irregularities in the thin
layer of silicon dioxide underlying the graphene were preventing the scientists
from achieving the exacting conditions they needed. Postdoctoral fellow Xu Du
and undergraduate student Anthony Barker were able to show that etching out
several layers of silicon dioxide below the graphene patches essentially leaves
an intact graphene strip suspended in mid-air by the electrodes. This enabled
the group to demonstrate that the carriers in suspended graphene essentially
propagate ballistically without scattering from impurities. Another crucial
step was to design and fabricate a probe geometry that did not interfere with
measurements as Andrei suspected earlier ones were doing. These proved decisive
steps to observing the correlated behavior in graphene.
In the past few months, other academic and corporate research groups have reported
streamlined graphene production techniques, which will propel further research
and potential applications.
Posted October 14th, 2009
