Crystal reality check: Wonder stuff

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  • Published: Aug 1, 2017
  • Author: David Bradley
  • Channels: X-ray Spectrometry
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A decade of anticipation

(left) Surface state dispersion inside bulk band gap of a topological insulator. (right) Spin-momentum locking of the surface states (spin orientation as indicated by red arrows) | Banerjee Lab

It is well over a decade since materials scientists started to investigate so-called topological insulators. These substances have been flagged as wonder materials, breakthroughs and the answer to many problems. Unfortunately, they are yet to live up to the hype alluded to by theoretical studies. Now, researchers from the University of Groningen in the Netherlands, think they know why and their answer could open up new developments and applications.

By definition, topological insulators are insulating in the bulk but allow charge to flow across their surface because of the ordering patterns there that allow electrons to move in ways that are not possible in conventional materials. The ordering derives from topology, hence the name. One electron property, spin, which is either up or down is normally locked to an electron's movement. Electrons moving to the right have spin down, and those moving to the left have spin up, Eric de Vries, Tamalika Banerjee, and their colleagues explain in the journal Physical Review B. The research group is part of the Zernike Institute for Advanced Materials at Groningen and is focused on understanding and using topological insulators. One of the consequences of that left-right, up-down constraint is that if one injects spin up electrons into a topological insulator, they will travel to the left. And spin down electrons will move right. It is hoped that this behaviour might be exploited in spintronics devices that use the quantized spin value of electrons and not just their charge for their functionality.

Spin voltage mimic

Theory predicts such properties of topological insulators, especially in considering crystalline materials containing heavy atoms. However, experimental proof has fallen short and at best offered mixed results that don't live up to the theoretical predictions. "We wondered why, so we devised experiments to investigate the behaviour of the surface state electrons," de Vries explains. "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."

The team's earlier tests used ferromagnets to detect electron spin in a topological insulator and gave them surprising results, de Vries says. "We demonstrated that a voltage presumably originating from spin detection can originate in factors other than the locking of electron spin to its movement. Using different geometries, we showed that artefacts related to stray magnetic fields generated by the ferromagnets can mimic similar spin voltages." This observation might require that scientists take another look at other published results in this field.

Current work

In the current work, the researchers, used a somewhat different approach. "We analysed the topological insulators using strong magnet fields," de Vries explains. "This causes electrons to oscillate in transport channels." Crystal structure experiments showed that a considerable part of the charge transport occurs in the bulk phase of the material, and not only at the surface, contrary to theoretical explanations of how topological insulators actually work. De Vries suggests that this is down to imperfections in the crystal structure, such as missing atoms. This allows electrons to move freely where they otherwise might be restricted. The defects open up new transport channels, generating electric current in the bulk of the material.

The findings beggar the question as to why no other scientists have noticed this effect before. De Vries points out that interpreting transport measurements on topological insulators is 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," he says. High-field magnetic studies helped with their work but X-ray crystallography will also provide additional clues and evidence. "The key is to grow the crystals without any missing atoms," de Vries says. "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."

Related Links

Phys Rev B 2017, online: "Coexistence of bulk and surface states probed by Shubnikov-de Haas oscillations in Bi2Se3 with high charge carrier density"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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