- by Kitta MacPherson -
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.
"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.
The original news release can be found at: http://www.princeton.edu/main/news/archive/S23/49/78A93/index.xml?section=topstories
Note: Published in Science, 13 February 2009: Vol. 323. no. 5916, pp. 919 - 922; DOI: 10.1126/science.1167733; D. Hsieh, Y. Xia, L. Wray, D. Qian, A. Pal, J.H. Dil, F. Meier, J. Osterwalder, G. Bihlmayer, C.L. Kane, Y.S. Hor, R.J. Cava, M.Z. Hasan; "Observation of Unconventional Quantum Spin Textures in Topological Insulators".
ARPES measurements were performed at the Stanford Synchrotron Radiation Lightsource (SLAC National Accelerator Laboratory), beamline 5-4; at the Advanced Light Source (Lawrence Berkeley National Laboratory), beamline 12.0.1; and at the Swiss Light Source, SIS beamline. SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory are funded by the U.S. Department of Energy's Office of Science.