Nanotechnology Now







Heifer International

Wikipedia Affiliate Button


DHgate

Home > Press > Graphene Gives up More of its Secrets: With the Advanced Light Source Berkeley Lab scientists explore the electronic structure of graphene in regions never before tested by experiment

Undoped graphene isn’t a metal, semiconductor, or insulator but a semimetal, whose unusual properties include electron-electron interactions between particles widely separated on graphene’s honeycomb lattice - here suggested by an artist’s impression of Feynman diagrams of such interactions. Long-range interactions, unlike those that occur only over very short distances in ordinary metals, alter the fundamental character of charge carriers in graphene. (Image by Caitlin Youngquist, Berkeley Lab Public Affairs)
Undoped graphene isn’t a metal, semiconductor, or insulator but a semimetal, whose unusual properties include electron-electron interactions between particles widely separated on graphene’s honeycomb lattice - here suggested by an artist’s impression of Feynman diagrams of such interactions. Long-range interactions, unlike those that occur only over very short distances in ordinary metals, alter the fundamental character of charge carriers in graphene.
(Image by Caitlin Youngquist, Berkeley Lab Public Affairs)

Abstract:
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.

Graphene Gives up More of its Secrets: With the Advanced Light Source Berkeley Lab scientists explore the electronic structure of graphene in regions never before tested by experiment

Berkeley, CA | Posted on July 14th, 2011

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."

Experiment versus theory

To understand the significance of the team's findings, it helps to start with their experimental set-up. Ideally, measurements of undoped graphene would be done with a suspended sheet of freestanding graphene. But many experiments can't be done unless the target is resting on a solid substrate, which can influence the electronic properties of the layer on the surface and interfere with the experiment.

So Siegel and his colleagues decided to investigate a special kind of "quasi-freestanding" graphene, starting with a substrate of silicon carbide. When heated, the silicon is driven out of the silicon carbide and carbon gathers on the surface as a relatively thick layer of graphite (the kind of carbon in pencil lead). But adjacent layers of graphene in the thick graphite sample are rotated with respect to one another, so that each layer in the stack behaves like a single isolated layer.

"In solid-state physics one of the most fundamental questions one can ask about a material is the nature of its charge carriers," Siegel says. "For ordinary metals, the answer can be described by the most powerful theory of solids, known as Landau's Fermi-liquid theory," after the Soviet physicist Lev Landau and the Italian and naturalized-American physicist Enrico Fermi.

While individual electrons carry charge - the electric current in a copper wire, for example - even in a metal they can't fully be understood as simple, independent particles. Because they are constantly interacting with other particles, the effects of the interactions have to be included; electrons and interactions together can be thought of as "quasiparticles," which behave much like free electrons but with different masses and velocities. These differences are derived through the mathematical process called renormalization.

Landau's Fermi liquid is made up of quasiparticles. Besides describing features of electrons plus interactions, Fermi liquids have a number of other characteristic properties, and in most materials the theory takes generally the same form. It holds that charge carriers are "dressed" by many-body interactions, which also serve to screen electrons and prevent or reduce their longer-distance interactions.

"Since the properties of so many materials are pretty much the same in a generalized way, physicists are always interested in finding systems that differ from a normal Fermi liquid," says Siegel. "This is what makes our results so exciting. Undoped graphene really does differ from what we expect for a normal Fermi liquid, and our results are in good agreement with theoretical calculations."

Perhaps the most vivid example of the difference is the long-range interaction among electrons in semimetallic graphene, interactions which would be screened in a normal metal. Siegel grants that there may be continuing controversy about how exactly graphene should be expected to behave, "but our main result is that we have confirmed the presence of these unscreened, long-range interactions, which change the behavior of quasiparticles in graphene in a fundamental way."

"Many-body interactions in quasi-freestanding graphene," by David A. Siegel, Cheol-Hwan Park, Choongyu Hwang, Jack Deslippe, Alexei V. Fedorov, Steven G. Louie, and Alessandra Lanzara, appears in Proceedings of the National Academy of Sciences, online at www.pnas.org/content/early/2011/06/20/1100242108.abstract. Siegel, Deslippe, Louie, and Lanzara are members of Berkeley Lab's Materials Sciences Division and the UCB Department of Physics. Park is with UCB's Department of Physics, Hwang is with the Materials Sciences Division, and Fedorov is with the Advanced Light Source. This work was supported by the National Science Foundation and the U.S. Department of Energy's Office of Science.

####

About Berkeley Lab
Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

For more information, please click here

Contacts:
Paul Preuss
510-486-6249

Copyright © Berkeley Lab

If you have a comment, please Contact us.

Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

Bookmark:
Delicious Digg Newsvine Google Yahoo Reddit Magnoliacom Furl Facebook

Related News Press

News and information

Rutgers, NIST physicists report technology with potential for sub-micron optical switches March 31st, 2015

Prototype 'nanoneedles' generate new blood vessels in mice: Scientists have developed tiny 'nanoneedles' that have successfully prompted parts of the body to generate new blood vessels, in a trial in mice March 31st, 2015

Super sensitive measurement of magnetic fields March 31st, 2015

Nanomedicine pioneer Mauro Ferrari at ETH Zurich March 31st, 2015

Laboratories

Rutgers, NIST physicists report technology with potential for sub-micron optical switches March 31st, 2015

Using magnetic fields to understand high-temperature superconductivity: Los Alamos explores experimental path to potential 'next theory of superconductivity' March 27th, 2015

Graphene

'Atomic chicken-wire' is key to faster DNA sequencing March 30th, 2015

Graphene reduces wear of alumina ceramic March 26th, 2015

Square ice filling for a graphene sandwich March 26th, 2015

Govt.-Legislation/Regulation/Funding/Policy

Rutgers, NIST physicists report technology with potential for sub-micron optical switches March 31st, 2015

SUNY Poly CNSE and Title Sponsor SEFCU Name Capital Region Teams Advancing to the Final Round of the 2015 New York Business Plan Competition March 30th, 2015

Princess Margaret scientists convert microbubbles to nanoparticles: Harnessing light to advance tumor imaging, provide platform for targeted treatment March 30th, 2015

Nanoscale worms provide new route to nano-necklace structures March 29th, 2015

Chip Technology

Rutgers, NIST physicists report technology with potential for sub-micron optical switches March 31st, 2015

Next important step toward quantum computer: Scientists at the University of Bonn have succeeded in linking 2 different quantum systems March 30th, 2015

State-of-the-art online system unveiled to pinpoint metrology software accuracy March 27th, 2015

SUNY POLY CNSE to Host First Ever Northeast Semi Supply Conference (NESCO) Conference Will Connect New and Emerging Innovators in the Northeastern US and Canada with Industry Leaders and Strategic Investors to Discuss Future Growth Opportunities in NYS March 25th, 2015

Discoveries

Rutgers, NIST physicists report technology with potential for sub-micron optical switches March 31st, 2015

Prototype 'nanoneedles' generate new blood vessels in mice: Scientists have developed tiny 'nanoneedles' that have successfully prompted parts of the body to generate new blood vessels, in a trial in mice March 31st, 2015

Super sensitive measurement of magnetic fields March 31st, 2015

From tobacco to cyberwood March 31st, 2015

Announcements

Rutgers, NIST physicists report technology with potential for sub-micron optical switches March 31st, 2015

Prototype 'nanoneedles' generate new blood vessels in mice: Scientists have developed tiny 'nanoneedles' that have successfully prompted parts of the body to generate new blood vessels, in a trial in mice March 31st, 2015

Super sensitive measurement of magnetic fields March 31st, 2015

Nanomedicine pioneer Mauro Ferrari at ETH Zurich March 31st, 2015

NanoNews-Digest
The latest news from around the world, FREE




  Premium Products
NanoNews-Custom
Only the news you want to read!
 Learn More
NanoTech-Transfer
University Technology Transfer & Patents
 Learn More
NanoStrategies
Full-service, expert consulting
 Learn More










ASP
Nanotechnology Now Featured Books




NNN

The Hunger Project







© Copyright 1999-2015 7th Wave, Inc. All Rights Reserved PRIVACY POLICY :: CONTACT US :: STATS :: SITE MAP :: ADVERTISE