An international research team has succeeded in gaining an in-depth insight into an unusual phenomenon. 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.
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 journal Science as one of the ten most important scientific breakthroughs of 2007.
The first study that succeeded in directly observing the spin of flowing particles was published February 13 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.
- 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 topologically ordered materials. Science, 13 February 2009; vol. 323, issue 5916
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