The Fermi energy is the energy of the highest occupied electronic state in a graphene flake, and is the reference energy for this kind of measurement. "Almost no electrons tunneling from the STM tip could enter the graphene at low energies within this gap region, but at slightly higher energies there was an abrupt, giant enhancement in tunneling, like a floodgate opening up for electrons." And this was not the only odd feature in the graphene spectrum.
"There was another feature in the spectrum, a local minimum of states, which moved in a very regular way as we changed the gate voltage and thus the density of charge carriers in the material," Crommie says. The research team was able to unambiguously identify this feature as the mark of electrons tunneling from the STM tip to the Dirac point itself, the minimum in graphene density of states.
And what of the mystery gap itself? "We realized that this is not a true energy gap; it is not a feature of the electronic band structure of graphene," Crommie says. "Rather it marks the interaction of the tunneling electrons with phonons, the quantized vibrations of the graphene lattice."
Naturally occurring vibrations in the graphene sample are minimal for the Crommie group's STM setup, since it is kept very cold (just four degrees above absolute zero). However, when the bias voltage between the tip and graphene sample increased above a special threshold of 63 millivolts, "then each tunneling electron is able to create a phonon vibration in the graphene sheet, which allows the electron to get into the graphene much easier," Crommie says.
Indeed, this "phonon-assistance" causes the electron tunneling conductance to suddenly increase by more than 10 times, as phonons essentially open a new channel for electrons to flow through. Says Crommie, "We call it a phonon floodgate."
An underlying cause for this new channel arises from the carbon sigma orbitals, which no
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory