Topological motions: Ultra-violet probe

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  • Published: Dec 1, 2017
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Topological motions: Ultra-violet probe

European collaboration resolves paradox

The band structure of a topological insulator measured using photoemission. The dark areas indicate which energies [on the y-axis] go together with which (here inverse) wavelengths [on the x-axis] for the electron waves in the solid. After 20 seconds exposure to the UV light involved in doing a photoemission experiment (right-hand image) the band structure is very different to that after only 1 second exposure (left-hand image). The coloured circles show the position of the Dirac point.

European scientists have finally resolved a seeming paradox in the conflicting data from ultraviolet studies and magnetotransport examination of topological insulators. The explanation could open up this intriguing group of compounds to new electronics applications and quantum devices.

In 2016, the Nobel Prize for Physics was awarded to David Thouless, Duncan Haldane, and Michael Kosterlitz for their theoretical discoveries of topological phase transitions and topological phases of matter. Topological insulators are becoming increasingly familiar because of their intriguing electronic properties. They are of both great fundamental and also technological interest. Despite this interest, scientists have been grappling with a puzzle regarding the properties of these materials for a decade. The two techniques best suited to investigating topological insulators present conflicting results on the material's electronic states. Now, scientists from Amsterdam, including two FOM-funded PhD candidates, their collaborators in France, Germany, and Switzerland have resolved the seeming paradox.

Crystalline emission

In the crystalline bulk topological insulators are insulating and cannot carry an electrical current. By contrast, at their surface the same crystal is electrically conducting. This topological difference gives them their potential in a new form of electronics and perhaps even in quantum computation.

There are two powerful experimental methods for examining electrons in topological insulators. The first involves passing a current through the system in the presence of a very large magnetic field, and observing the magnetotransport effect. The second involves ultraviolet examination of the material's surface in a photoemission experiment.

Conflicting data explained

The conflicting data arises, the team suggests, because the very first ultraviolet light flash required of the photoemission experiment alters the electronic band structure at the surface of the crystal.

Usually, recording band structure in a material can takes a minute or more. In the current research, the team focused on cutting that timescale down to just one second. So that they could see the band structure prior to structural change. It was previously demonstrated that molecules adsorbed on to the surface of a topological insulator can cause a downward shift in the material's Dirac point. In the new experiments, the team could disentangle that effect at the surface from the effects of UV light and thus prove that it is the UV that leads to a different result when compared to the magnetotransport experiments.

Now that the effects of UV light in photoemission studies is more clearly defined , experimental protocols might now be developed for a whole range of experiments to ensure consistency with other techniques. Given that UV can reveal information inaccessible to other techniques it is important that such a workaround for photoemission be developed.

Related Links

Phys Rev X 2017, 7, 0410414: "Trigger of the ubiquitous surface band bending in 3D topological insulators"

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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