Exotic Quantum State Of Matter Discovered - known as the quantum Hall effect.
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Exotic Quantum State Of Matter Discovered
ScienceDaily (Apr. 25, 2008) - A team of scientists from Princeton
University has found that one of the most intriguing phenomena in
condensed-matter physics -- known as the quantum Hall effect -- can occur in
nature in a way that no one has ever before seen.
Writing in the April 24 issue of Nature, the scientists report that they
have recorded this exotic behavior of electrons in a bulk crystal of
bismuth-antimony without any external magnetic field being present. The
work, while significant in a fundamental way, could also lead to advances in
new kinds of fast quantum or "spintronic" computing devices, of potential
use in future electronic technologies, the authors said.
"We had the right tool and the right set of ideas," said Zahid Hasan, an
assistant professor of physics who led the research and propelled X-ray
photons at the surface of the crystal to find the effect. The team used a
high-energy, accelerator-based technique called "synchrotron photo-electron
And, Hasan added, "We had the right material."
The quantum Hall effect has only been seen previously in atomically thin
layers of semiconductors in the presence of a very high applied magnetic
field. In exploring new realms and subjecting materials to extreme
conditions, the scientists are seeking to enrich the basis for understanding
how electrons move.
Robert Cava, the Russell Wellman Moore Professor of Chemistry and a
co-author on the paper, worked with members of his team to produce the
crystal in his lab over many months of trial-and-error. "This is one of
those wonderful examples in science of an intense, extended collaboration
between scientists in different fields," said Cava, also chair of the
Department of Chemistry.
"This remarkable experiment is a major home run for the Princeton team,"
said Phuan Ong, a Princeton professor of physics who was not involved in the
research. Ong, who also serves as assistant director of the Princeton Center
for Complex Materials, added that the experiment "will spark a worldwide
scramble to understand the new states and a major program to manipulate them
for new electronic applications."
Electrons, which are electrically charged particles, behave in a magnetic
field, as some scientists have put it, like a cloud of mosquitoes in a
crosswind. In a material that conducts electricity, like copper, the
magnetic "wind" pushes the electrons to the edges. An electrical voltage
rises in the direction of this wind -- at right angles to the direction of
the current flow. Edwin Hall discovered this unexpected phenomenon, which
came to be known as the Hall effect, in 1879. The Hall effect has become a
standard tool for assessing charge in electrical materials in physics labs
In 1980, the German physicist Klaus von Klitzing studied the Hall effect
with new tools. He enclosed the electrons in an atom-thin layer, and cooled
them to near absolute zero in very powerful magnetic fields. With the
electrons forced to move in a plane, the Hall effect, he found, changed in
discrete steps, meaning that the voltage increased in chunks, rather than
increasing bit by bit as it was expected to. Electrons, he found, act
unpredictably when grouped together. His work won him the Nobel Prize in
physics in 1985.
Daniel Tsui (now at Princeton) and Horst Stormer of Bell Laboratories did
similar experiments, shortly after von Klitzing's. They used extremely pure
semiconductor layers cooled to near absolute zero and subjected the material
to the world's strongest magnet. In 1982, they suddenly saw something new.
The electrons in the atom-thin layer seemed to "cooperate" and work together
to form what scientists call a "quantum fluid," an extremely rare situation
where electrons act identically, in lock-step, more like soup than as
individually spinning units.
After a year of thinking, Robert Laughlin, now at Stanford University,
devised a model that resembled a storm at sea in which the force of the
magnetic wind and the electrons of this "quantum fluid" created new
phenomena -- eddies and waves -- without being changed themselves. Simply
put, he showed that the electrons in a powerful magnetic field condensed to
form this quantum fluid related to the quantum fluids that occur in
superconductivity and in liquid helium.
For their efforts, Tsui, Stormer and Laughlin won the Nobel Prize in physics
Recently, theorist Charles Kane and his team at the University of
Pennsylvania, building upon a model proposed by Duncan Haldane of Princeton,
predicted that electrons should be able to form a Hall-like quantum fluid
even in the absence of an externally applied magnetic field, in special
materials where certain conditions of the electron orbit and the spinning
direction are met. The electrons in these special materials are expected to
generate their own internal magnetic field when they are traveling near the
speed of light and are subject to the laws of relativity.
In search of that exotic electron behavior, Hasan's team decided to go
beyond the conventional tools for measuring quantum Hall effects. They took
the bulk three-dimensional crystal of bismuth-antimony, zapped it with
ultra-fast X-ray photons and watched as the electrons jumped out. By
fine-tuning the X-rays, they could directly take pictures of the dancing
patterns of the electrons on the edges of the sample. The nature of the
quantum Hall behavior in the bulk of the material was then identified by
analyzing the unique dancing patterns observed on the surface of the
material in their experiments.
Kane, the Penn theorist, views the Princeton work as extremely significant.
"This experiment opens the door to a wide range of further studies," he
The images observed by the Princeton group provide the first direct evidence
for quantum Hall-like behavior without external magnetic fields.
"What is exciting about this new method of looking at the quantum Hall-like
behavior is that one can directly image the electrons on the edges of the
sample, which was never done before," said Hasan. "This very direct look
opens up a wide range of future possibilities for fundamental research
opportunities into the quantum Hall behavior of matter."
Other researchers on the paper include graduate students David Hsieh, Andrew
Lewis Wray, YuQi Xia and postdoctoral fellows Dong Qian and Yew San Hor. The
team members are in the departments of physics and chemistry, and are
members of the Princeton Center for Complex Materials. They used facilities
at the Lawrence Berkeley Laboratory in Berkeley, Calif., and the University
of Wisconsin's Synchrotron Radiation Center in Stoughton, Wis.
This work was supported by U.S. Department of Energy and the National
Adapted from materials provided by Princeton University.