Topological insulators prove less insulating than theory suggests

Topological insulators, a class of materials that has been investigated for just over a decade, have been heralded as a new 'wonder material'. But so far, topological insulators have not quite lived up to the expectations fueled by theoretical studies. Physicists at the University of Groningen in the Netherlands have now discovered why and report their analysis in a paper in Physical Review B.

Topological insulators are materials that are insulating in the bulk but allow charge to flow across their surface. These conducting states at the surface originate from ordering patterns in the electrons that are different from ordinary materials. This ordering is linked to the physical concept of 'topology', analogous to the concept in mathematics. This property gives rise to very robust states with some special properties.

For one, the electron spin – a magnetic property of electrons that can have the values 'up' or 'down' – is locked to their movement. “This means that electrons moving to the right have spin down, and those moving to the left have spin up,” explains first author Eric de Vries, a PhD student in the 'Spintronics of Functional Materials' research group led by Tamalika Banerjee. “But it also means that when you inject electrons with spin up into such a topological insulator, they will travel to the left!” With this property, topological insulators might help to realize spintronics: electronics based on the quantized spin value rather than the charge of electrons.

The special properties of topological insulators are predicted from theoretical analyses of their surface structures, made from crystals of heavy atoms. But experiments show mixed results, which don't quite live up to theoretical predictions. “We wondered why, so we devised experiments to investigate the behavior 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,” says de Vries.

Earlier experiments by the group, in which they used ferromagnets to detect the spins of electrons in the topological insulator, were surprising. “We demonstrated that a voltage presumably originating from spin detection can originate in factors other than the locking of electron spin to its movement,” explains de Vries. “Using different geometries, we showed that artifacts 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.

For this study, de Vries and his group took 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 in the Netherlands, where a 33-Tesla magnet is available, one of the strongest 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,” he says. De Vries's experiments showed that a considerable part of the charge transport occurred in the bulk phase of the material, and not just at the surface.

The reason for this, explains De Vries, is the imperfect crystal structure of the topological insulator. “Sometimes there are atoms missing in the crystal structure. This results in freely moving electrons. These start to conduct as new transport channels, generating electric current in the bulk of the material.”

So why has no one noticed 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.” Further experiments that might lead to clearer conclusions will require very high magnetic fields in specialized labs.

These results also suggest a way to improve topological insulators. “The key is to grow the crystals without any missing atoms,” says de Vries. “Another solution is to fill the holes, for example with calcium ions that bind the free electrons. But that might cause other disturbances to the electrons' mobility.”

For 10 years, topological insulators were all the rage and compared to the wonder material graphene. The discovery that, in practice, topological insulators have glitches serves as a reality check. “We need to study and understand the interaction between the surface states and the bulk material in much more detail,” de Vries concludes.

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.