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'Science:' Novel quantum effect directly observed and explained

Press Release Number: 
PR-HELMHOLTZ-09-1-31A
Date: 
Friday, February 13, 2009

Deutsche Fassung

Research team succeeds in observing electrons in the quantum spin Hall state; Material suitable for spin sources and quantum computing in information processing

Jülich, 13 February 2009 – An international research team has succeeded in gaining an in-depth insight into an unusual phenomenon, as reported in the current edition of the high-impact journal "Science". The researchers succeeded for the first time in directly measuring the spin of electrons in a material that exhibits the quantum spin Hall effect, which was theoretically predicted in 2004 and first observed in 2007. Astonishingly, the spin currents flow without any external stimulus as a result of the internal structure of the material. The flow of information is loss-free, even for slight irregularities. This paves the way towards fault-tolerant quantum computers and towards a source of spin currents.

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The spin is a quantum-mechanical property of elementary particles and as a rule it occurs in two variations. This is what makes it suitable for use as a binary information carrier. In hard disk drives, for example, spins are already being used to store digital information.

In 2007, physicists from Germany and the USA observed a new phenomenon that could make it possible to transport and electrically manipulate information in future storage media almost loss-free – the quantum spin Hall effect. The discovery was hailed by the high-impact journal "Science" as one of the ten most important scientific breakthroughs of 2007.


Image 1. Energy, momentum and spin distribution of the electronic states on the surface of antimony. An indication of the occurrence of the quantum spin Hall effect is when states with this spin structure (red/blue) link the valency and conduction band. (Courtesy: Forschungszentrum Jülich)

The first study that succeeded in directly observing the spin of flowing particles was published this week in "Science" by an international research team, which included Dr. Gustav Bihlmayer from Forschungszentrum Jülich, member of the Helmholtz Association. Until now, the quantum spin Hall effect could only be indirectly proven.

"We were able to show for the first time that two spin currents flow in opposite directions in the edge region of an alloy of bismuth and antimony. An external energy supply is not required; losses cannot occur," explained Dr. Gustav Bihlmayer from the Jülich Institute of Solid State Research. The causes of this astonishing phenomenon are interactions within the material. Of particular interest to materials scientists is the fact that imperfections in the material do not impair the spin currents. "This means that materials known as topological materials have spin currents that can be manipulated electrically and are therefore suitable for use as spin sources. They could even pave the way towards fault-tolerant quantum computers," said Bihlmayer. "Our process will make it possible to test the suitability of materials for this purpose in the future."

The current study makes use of theoretical calculations and photoelectron spectroscopy. The photons in a synchrotron beam cause electrons to be emitted from the material surface. The energy and momentum distribution, as well as the spin of the particles, can be used to derive concrete information on the occurrence of the quantum spin Hall effect. Previous methods were based on measurements of the conductivity in the materials at variable voltages.

Spins for data processing

Spins are a hot topic in research. Physicists and nanoelectricians have high hopes for what is known as spin electronics. Spin electronics does not just exploit the electric charge of electrons and nuclei but also their spin, and should therefore lead to the development of new approaches for the processing and coding of information in information processing. Faster, smaller and more energy-efficient computers could thus become a reality, as could completely new components capable of performing a number of different functions such as storage, logic and communication. One of the most prominent ideas is that of the quantum computer. For spin-electronic concepts, scientists conducting basic research are desperately searching for new materials and phenomena that will make it possible to control both spin orientation and spin flow.

The original news release can be found at: http://www.fz-juelich.de/portal/index.php?cmd=show&index=163&mid=671

Also: Quantum dance: Discovery led by Princeton researchers could revolutionize computing

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.

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