Ultrafast Imaging of Electron Waves in Graphene

NOVEMBER 9, 2010

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A frame from one of the electron-motion movies (see links at the end of this article to watch the entire sequences).

The fastest “movies” ever made of electron motion have been captured by researchers using the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne and the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign (UIUC). The movies, which were created by scattering x-rays off of graphene, show that the interaction among graphene’s electrons is surprisingly weak.

Using inelastic x-ray scattering experiments at the X-ray Science Division 9-ID x-ray beamline at the APS, physicists from UIUC imaged the motion of electrons in graphene with resolutions of 0.533 Å and 10.3 attoseconds. Their results were published in the November 5 issue of Science.

Exactly how small and how fast are these measurements? An angstrom is 1/10,000,000,000 of a meter, about the width of a hydrogen atom. And an attosecond is to a second as a second is to the age of the Universe.

The 2010 Nobel Prize for Physics was awarded to Andre Geim and Konstantin Novoselov for their work on graphene, a single-atom-thick sheet of carbon atoms arrayed in a honeycomb pattern that exhibits many intriguing properties, including great strength, flexibility, excellent electrical conductivity, and heat resistance. As a result, graphene is a candidate material for a wide range of applications, including a new generation of low-cost, flexible electronics. A major outstanding question about this material is whether the electrons in graphene move independently, or if their motion is correlated by Coulomb repulsion.

The researchers in this study found that graphene screens Coulomb interactions surprisingly effectively, causing it to act like a simple, independent-electron semimetal. Their work explains several mysteries, including why freestanding graphene fails to become an insulator as predicted. The study also demonstrates a new approach to studying ultrafast dynamics, creating a new window on the most fundamental properties of materials.

Movies from the experiment (requires a QuickTime viewer):

Movie s1 (.mov format) Animation of n(x,y,t) displayed in Fig. 2 at two different vertical scales.

See: James P. Reed,1 Bruno Uchoa,1 Young Il Joe,1 Yu Gan,1 Diego Casa,2 Eduardo Fradkin,1 Peter Abbamonte1*, “The Effective Fine-Structure Constant of Freestanding Graphene Measured in Graphite,” Science 330(6005), 805 (5 November 2010). DOI:10.1126/science.1190920

Author affiliations: 1University of Illinois at Urbana-Champaign, 2Argonne National Laboratory

Correspondence: *abbamonte@mrl.uiuc.edu

See also: This Week in Science, “Weakly Interacting Graphene.” “Many unusual properties of graphene are a consequence of the Dirac dispersion of its electrons—a linear relationship between an electron's momentum and energy.” More…

This work was supported by the U.S. Department of Energy under grants DE-FG02-07ER46459 and DE-FG02-07ER46453 through the Frederick Seitz Materials Research Laboratory.

Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

The original UIUC press release can be read here.

The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy’s Office of Science, Office of Basic Energy Sciences (DOE-BES). The APS is the source of the Western Hemisphere’s brightest high-energy x-ray beams for research in virtually every scientific discipline. More than 3,500 scientists representing universities, industry, and academic institutions from every U.S. state and several foreign nations visit the APS each year to carry out applied and basic research in support of the BES mission to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels in order to provide the foundations for new energy technologies and to support DOE missions in energy, environment, and national security. To learn more about the Office of Basic Energy Sciences and its x-ray user facilities, visit http://www.sc.doe.gov/bes/BES.html.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.