Graphene gives up more of its secrets

2015-10-19 16:38:56

Graphene, a sheet of carbon only a single atom thick, was an object of theoretical speculation long before it was actually made. Theory predicts extraordinary properties for graphene, but testing the predictions against experimental results is often challenging.


Now researchers using the Advanced Light Source (ALS) at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have taken an important step toward confirming that graphene is every bit as unusual as expected -- perhaps even more so.
"Graphene is not a semiconductor, not an insulator, and not a metal," says David Siegel, the lead author of a paper in the Proceedings of the National Academy of Sciences(PNAS) reporting the research team's results. "It's a special kind of semimetal, with electronic properties that are even more interesting than one might suspect at first glance."

Siegel is a graduate student in Berkeley Lab's Materials Sciences Division (MSD) and a member of Alessandra Lanzara's group in the Department of Physics at the University of California at Berkeley. He and his colleagues used ALS beamline 12.0.1 to probe a specially prepared sample of graphene with ARPES (angle-resolved photoemission spectroscopy) in order to observe how undoped graphene -- the intrinsic material with no extra charge carriers -- behaves near the so-called "Dirac point."
The Dirac point is a unique feature of graphene's band structure. Unlike the band structure of semiconductors, for example, graphene has no band gap -- no gap in energy between the electron-filled valence band and the unoccupied conduction band. In graphene these bands are represented by two cones ("Dirac cones") whose points touch, crossing linearly at the Dirac point. When the valence band of graphene is completely filled and the conduction band is completely empty, the graphene can be considered "undoped" or "charge neutral," and it is here that some of the interesting properties of graphene may be observed.

An ARPES experiment neatly measures a slice through the cones by directly plotting the kinetic energy and angle of electrons that fly out of the graphene sample when they are excited by an x‑ray beam from the ALS. A spectrum develops as these emitted electrons hit the detector screen, gradually building up a picture of the cone.

The way the electrons interact in undoped graphene is markedly different from that of a metal: the sides of the cone (or legs of the X, in an ARPES spectrum) develop a distinct inward curvature, indicating that electronic interactions are occurring at increasingly longer range -- distances of up to 790 angstroms apart -- and lead to greater electron velocities. These are unusual manifestations, never seen before, of a widespread phenomenon called "renormalization."