May 20, 2010

First Observation of Plasmarons in Graphene

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At Berkeley Lab’s Advanced Light Source, scientists working with graphene have made the first observation of the energy bands of complex particles known as plasmarons. Their discovery may hasten the day when graphene can be used to build ultrafast computers and other electronic, photonic, and plasmonic devices on the nanoscale. Understanding the relationships among these three kinds of particles—charge carriers, plasmons, and plasmarons—may hasten the day when graphene can be used for “plasmonics” to build ultrafast computers—perhaps even room-temperature quantum computers—plus a wide range of other tools and applications.

“The interesting properties of graphene are all collective phenomena,” says Rotenberg, an ALS senior staff scientist responsible for the scientific program at ALS beamline 7, where the work was performed. “Graphene’s true electronic structure can’t be understood without understanding the many complex interactions of electrons with other particles.”

The electric charge carriers in graphene are negative electrons and positive holes, which in turn are affected by plasmons—density oscillations that move like sound waves through the “liquid” of all the electrons in the material. A plasmaron is a composite particle, a charge carrier coupled with a plasmon.

“Graphene has no band gap,” says Bostwick, a research scientist on beamline 7.0.1 and lead author of the study. “On the usual band-gap diagram of neutral graphene, the filled valence band and the empty conduction band are shown as two cones, which meet at their tips at a point called the Dirac crossing.”

Graphene is unique in that electrons near the Dirac crossing move as if they have no mass, traveling at a significant fraction of the speed of light. Plasmons couple directly to these elementary charges. Their frequencies may reach 100 trillion cycles per second (100 terahertz, 100 THz)—much higher than the frequency of conventional electronics in today’s computers, which typically operate at about a few billion cycles per second (a few gigahertz, GHz).

Plasmons can also be excited by photons, particles of light, from external sources. Photonics is the field that includes the control and use of light for information processing; plasmons can be directed through channels measured on the nanoscale (billionths of a meter), much smaller than in conventional photonic devices.

And since the density of graphene’s electric charge carriers can easily be influenced, it is straightforward to tune the electronic properties of graphene nanostructures. For these and other reasons, says Bostwick, “graphene is a promising candidate for much smaller, much faster devices—nanoscale plasmonic devices that merge electronics and photonics.”

The usual picture of graphene’s simple conical bands is not a complete description, however; instead it’s an idealized picture of “bare” electrons. Not only do electrons (and holes) continually interact with each other and other entities, the traditional band-gap picture fails to predict the newly discovered plasmarons revealed by Bostwick and his collaborators.

The team reports their findings and discuss the implications in “Observations of plasmarons in quasi-free-standing doped graphene,” by Aaron Bostwick, Florian Speck, Thomas Seyller, Karsten Horn, Marco Polini, Reza Asgari, Allan H. MacDonald, and Eli Rotenberg, in the 21 May 2010 issue of Science


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