An international team of scientists, led by a Princeton
University group, has observed an exciting and strange behavior in electrons'
spin within a new material that could be harnessed to transform computing and
electronics.
"We believe this discovery is not only an advancement in the fundamental
physics of quantum systems but also could lead to significant advances in electronics,
computing and information science," said Zahid Hasan, an assistant professor
of physics at Princeton, who led the international collaboration that included
scientists from the United States, Switzerland and Germany.
Theorists have long predicted that atoms placed in certain configurations would
trigger electrons to behave in odd "quantum" ways. The Princeton-led
team has been searching for a material that would produce these conditions.
In the Feb. 13 issue of Science, the team has reported it witnessed the exotic
behavior in a carefully constructed crystal made of an antimony alloy laced
with bismuth.
Surveying the structure on an atomic level with new techniques, the scientists
have recorded swarms of electrons spinning in a synchronized quantum dance.
The coordinated behavior observed involves a strange form of rotation. Unlike
most objects, which return to their original "face" when revolved
full circle or 360 degrees, the harmonized electrons need to be twisted two
full turns or 720 degrees in order to go back to the same face at the surface
of the material.
"As a technical achievement, or a series of physics achievements alone,
it is pretty spectacular," said Philip Anderson, the Joseph Henry Professor
Emeritus of Physics at Princeton and a winner of the 1977 Nobel Prize in physics.
He was not involved with the research. For theoreticians, Anderson added, the
work is both interesting and significant.
Others agreed.
"This discovery has the potential to transform electronics, data storage
and computing," said Thomas Rieker, program director for the National Science
Foundation's Materials Research Science and Engineering Centers. "The spin-sensitive
measurement techniques developed here may shed light on other important fundamental
questions in condensed matter physics such as the origin of high-temperature
superconductivity."
Quantum physics is the set of physical laws governing the realm of the ultra-small.
There, objects appear to follow rules that are radically different from the
world seen by the naked eye. In the quantum dominion, for example, an object
can seem to be in two places at the same time, and there is no distinction between
particles and waves. Quantum computers will be designed to take advantage of
these properties to enrich their capacities in many ways.
In addition to electrical charge, electrons possess rotational properties.
In the quantum world objects can turn in ways that are at odds with common experience.
The British physicist Paul Dirac, who won the Nobel Prize in Physics in 1933,
proposed that an electron's rotation makes it behave like a tiny bar magnet
with both north and south poles, a property he coined "quantum spin."
The 720-degree rotational property in a soup of electrons seen in the current
experiment was expected in theory by theoretical physicists at Princeton, the
University of Pennsylvania and the University of California-Berkeley. Electrons
would need to be moving at extremely high speeds in order for it to happen.
But the questions became where to look and how to detect it.
The results for the research team were the culmination of years of searching
for the right materials and experimental techniques. The current experiment
was based on the researchers' hunch that electrons in bismuth-laced antimony
or selenium (the materials are technically known as "topological quantum
spin Hall insulators") would exhibit unusual effects because they move
at high velocities that mimic the presence of a magnetic field inside the material.
This would allow for the bizarre quantum motion to take place.
"This quantum weirdness -- a coordinated twist in the spin of electrons
even though there is no magnetic field around -- is what we've been searching
for by fine tuning our experiments over the last few years," Hasan said.
"It's a very fundamental piece of new physics -- it goes beyond what you
would typically learn in a quantum physics textbook. In principle, you can use
this new quantum dance of electrons to construct a very bizarre electronic circuit."
Computer designers could employ the quantum effect to construct machines with
a far more subtle range of processing options than the simple "on"
or "off" logic that now exists, the researchers said. The team also
has been studying other materials that could produce such effects and developing
sharper imaging techniques to track the finer quantum behavior.
Today's computers employ a simple on-off logic that is based on the positive
or negative charge of an electron buried in a silicon semiconductor. New designs
could take advantage of a rich set of possibilities offered by the quantum spin
of the electrons in the experimental material to enhance power, speed and memory.
"We can use this new quantum property of spinning particles to make computers
that can store much more information and can have the capacity to perform computations
much faster than present-day machines," said David Hsieh, a graduate student
in Princeton's Department of Physics and the first author on the paper.
Until recently there was no imaging technique to detect these very subtle quantum
effects. "A key breakthrough that made the discovery possible is that through
an international collaboration we developed a set of methods based on X-rays
to see the individual spin of electrons," Hasan said. "We can now
observe the spin by taking pictures of the north and south poles of these tiny
bar magnets, which revealed this new secret of the quantum world."
The researchers expect to employ the new technique to improve medical imaging
technologies in studies of the fine quantum behavior of unusual magnets, superconductors
and other materials.
Other researchers on the Princeton team included: Robert Cava, the Russell
Wellman Moore Professor of Chemistry; YuQi Xia, Lewis Wray and Arijeet Pal,
graduate students in the Department of Physics; Dong Qian, an associate research
scholar in the Department of Physics; and Yew San Hor, a postdoctoral research
associate at the Princeton Institute for the Science and Technology of Materials.
Other participating institutions were: the Advanced Light Source at the Lawrence
Berkeley National Laboratory in Berkeley, Calif.; the Stanford Synchrotron Radiation
Lightsource at the SLAC National Accelerator Laboratory in Menlo Park, Calif.;
the University of Pennsylvania in Philadelphia; the Institut fur Festkorperforschung
in Germany; and the Swiss Light Source at the Paul Scherrer Insitute and the
Universitat Zurich-Irchel, both in Switzerland.
The research was funded by the National Science Foundation's Division of Materials
Research, the U.S. Department of Energy and Princeton University.
Posted February 18th, 2009
