Puzzling New Physics from Graphene Quartet’s Quantum Harmonies

Using a one-of-a-kind instrument designed and built at the
National Institute of Standards and Technology (NIST), an international
team of researchers have unveiled a quartet of graphene’s electron
states and discovered that electrons in graphene can split up into an
unexpected and tantalizing set of energy levels when exposed to
extremely low temperatures and extremely high magnetic fields. Published
in the Sept. 9^th, 2010, issue of Nature,* this new
research raises several intriguing questions about the fundamental
physics of this exciting material and reveals new effects that may make
graphene even more powerful than previously expected for practical
applications.

This artist's rendition illustrates the electron energy levels in graphene as revealed by a unique NIST instrument. Because of graphene's properties, an electron in any given energy level (the wide, purple band) comprises four quantum states (the four rings), called a "quartet." This quartet of levels split into different energies when immersed in a magnetic field. The two smaller bands on the outermost ring represent the further splitting of a graphene electronic state. Credit: T. Schindler and K. Talbott/NIST View hi-resolution image

Graphene is one of the simplest materials—a single-atom-thick sheet
of carbon atoms arranged in a honeycomb-like lattice—yet it has many
remarkable and surprisingly complex properties. Measuring and
understanding how electrons carry current through the sheet is important
to realizing its technological promise in wide-ranging applications,
including high-speed electronics and sensors. For example, the electrons
in graphene act as if they have no mass and are almost 100 times more
mobile than in silicon. Moreover, the speed with which electrons move
through graphene is not related to their energy, unlike materials such
as silicon where more voltage must be applied to increase their speed,
which creates heat that is detrimental to most applications.

NIST recently constructed the world’s most powerful and stable
scanning-probe microscope, with an unprecedented combination of low
temperature (as low as 10 millikelvin, or 10 thousandths of a degree
above absolute zero), ultra-high vacuum and high magnetic field. In the
first measurements made with this instrument, the team has used its
power to resolve the finest differences in the electron energies in
graphene, atom-by-atom.

Because of the geometry and electromagnetic properties of graphene’s
structure, an electron in any given energy level populates four possible
sublevels, called a “quartet.” Theorists have predicted that this
quartet of levels would split into different energies when immersed in a
magnetic field, but until recently there had not been an instrument
sensitive enough to resolve these differences. The experiment, according
to the research team, revealed unexpected complex quantum behavior of
the electrons in a high magnetic field at extremely low temperatures.
The electrons apparently interact strongly with one another in ways that
affect their energy levels.

One possible explanation for this behavior, the team says, is that
the electrons have formed a “condensate” in which they cease moving
independently of one another and act as a single coordinated unit. If
so, the work could point the way to the creation of smaller,
very-low-heat-producing, highly energy efficient electronic devices
based upon graphene.

The research team includes collaborators from NIST, the University of
Maryland, Seoul National University, the Georgia Institute of
Technology and the University of Texas at Austin. For more details, see
NIST's Sept. 8^th, 2010, news announcement, “NIST Researchers Hear Puzzling New Physics from Graphene Quartet’s Quantum Harmonies” online at www.nist.gov/cnst/graphene_quartet.cfm.

Media Contact: Mark Esser, mark.esser@nist.gov, 301-975-8735