by Staff Writers
Groningen, Netherlands (SPX) Jul 31, 2017
Topological insulators, a class of materials which has been investigated for just over a decade, have been heralded as a new 'wonder material', as has graphene. But so far, topological insulators have not quite lived up to the expectations fueled by theoretical studies. University of Groningen physicists now have an idea about why. Their analysis was published on 27 July in the journal Physical Review B.
Topological insulators are materials that are insulating in the bulk but allow charge to flow across the surface. These conducting states at the surface originate from ordering patterns in the states where electrons reside that are different from ordinary materials. This ordering is linked to the physical concept of 'topology', analogous to that used in mathematics. This property gives rise to very robust states with some special properties.
'But it also means that when you inject electrons with spin up into such a topological insulator, they will travel to the left!' Topological insulators might therefore be very useful in the realization of spintronics: electronics based on the quantized spin value rather than the charge of electrons.
The special properties of topological insulators are predicted by the theoretical analysis of the surface structures of these materials, made from crystals of heavy atoms. But experiments show mixed results, which don't quite live up to the theoretical predictions. 'We wondered why, so we devised experiments to investigate the behaviour of the surface state electrons. Specifically, we wanted to see if transport is really limited to the surface, or if it is also present in the bulk of the material.'
Using different geometries, we showed that artefacts related to stray magnetic fields generated by the ferromagnets can mimic similar spin voltages.' This observation may lead to a re-evaluation of some published results.
This time, they used a different approach. 'We analyzed the topological insulators using strong magnet fields. This causes electrons to oscillate in transport channels.' De Vries went to the national High Field Magnet Laboratory at the Radboud University Nijmegen, where a 33-Tesla magnet is available, one of the stronger magnets in the world.
'Others have done similar tests with weaker magnets, but these are not sensitive enough to reveal the additional transport channels that coexist with the surface states.' De Vries's experiments showed that a considerable part of the charge transport occurred in the bulk phase of the material, and not only at the surface.
So why has no one noted this before? De Vries stresses that interpreting transport measurements made on topological insulators can be difficult. 'We experienced this in our previous experiments. Our message is that extreme care is needed in the interpretation of experimental observations for devices based on these materials.' Also, experiments which might lead to clearer conclusions require very high magnetic fields in specialized labs.
They were compared to the wonder material graphene. The discovery that, in practice, topological insulators have glitches serves as a reality check. De Vries: 'We need to study and understand the interaction between the surface states and the bulk material in much more detail.'
Newark DE (SPX) Jul 31, 2017
A pair of engineers at the University of Delaware has developed a process to form interwoven polymer networks more easily, quickly and sustainably than traditional methods allow. Their secret ingredient? Blue light. Abhishek Shete, graduate research assistant in materials science and engineering, and Christopher Kloxin, assistant professor in materials science and engineering and chemical ... read more
University of Groningen
Space Technology News - Applications and Research
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