Scientists discover exotic quantum state of matter
Posted on Monday, April 28, 2008 @ 19:12:10 UTC by vlad
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 These images collected by Princeton University scientists show (top)
the first direct image of the dancing pattern of electrons on the edge
of the bismuth-antimony bulk crystal, which is a quantum Hall
insulator; (center) a schematic and another image showing the electron
distribution in three dimensions; and (bottom) a schematic and an image
conveying the distribution of edge-electrons in two dimensions. Images:
Zahid Hasan
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 spectroscopy."
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 worldwide.
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 in 1998.
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 said.
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."
Source: Princeton University
Via: http://physorg.com/news128261028.html
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