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A phonon floodgate in monolayer carbon
Date:7/21/2008

gate voltage is to vary the density of the charge carriers in the graphene," Crommie says. "The purpose of varying the STM bias voltage is to perform spectroscopy, so we can look at the graphene's local density of states at different energies. We want to know where are the electrons? How are they behaving?"

These questions are of particular interest because of graphene's odd electronic properties. The carbon atoms in graphene are arranged at the corners of hexagons, as in chicken wire, with three of each atom's four electrons involved in molecular bonds with its neighbors; these are sigma orbitals that lie in the plane of the material. The remaining electrons are in pi orbitals extending above and below the plane. The hybridization of the pi orbitals spreads across the graphene sheet, and the unconfined electrons are free to move as high-speed "relativistic quasiparticles," so-called Dirac fermions which act as if they have no mass.

The plot of energy states for Dirac fermions in graphene looks quite different from that of a conventional 3-D semiconductor, which typically consists of two opposing parabolic curves, a lower-energy valence band and a higher-energy conduction band, with a band gap between them that no charge carriers can occupy.

Graphene's unusual electronic properties

By contrast, the Dirac fermion energy states of graphene can be represented as two cones with their vertices meeting at a point of minimum electronic density, called the Dirac point. Thus one might expect the spectrum of the density of states resulting from electrons tunneling into graphene to be linear, following the smooth edge of the touching cones.

"When we plotted the LDOS spectra of our gated graphene flakes, however, we found a gap-like feature that was centered on the Fermi energy no matter how we changed the density of charge carriers in the graphene with the gate voltage and no matter where we looked on the flake," Crom
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Contact: Paul Preuss
paul_preuss@lbl.gov
510-486-6249
DOE/Lawrence Berkeley National Laboratory
Source:Eurekalert  

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