Topological insulators are new materials with special electronic properties that are of great interest to scientists and have many potential applications. Nevertheless, scientists have also been wrestling with a ten-year-old puzzle caused by the fact that the two best methods for probing the electronic states of topological insulators produce different results. Researchers from the University of Amsterdam (UVA) in the Netherlands, together with collaborators in France, Switzerland and Germany, have now found out exactly why, and they report their findings in a paper in Physical Review X.

Topological insulators are strange materials: the bulk is insulating and cannot carry an electrical current, yet the surface is conducting. These new materials are of great fundamental interest but are also very promising for a number of future applications in special types of electronics and in quantum computing, and so are the subject of a substantial research effort. The significance of topological materials was underlined last year with the award of the Nobel Prize for the development of fundamental theories setting out the existence and behaviour of topological matter.

There are two powerful experimental methods for examining the behavior of the electrons at the surface of a topological insulator. The first, known as magnetotransport, involves sending a current through the system in the presence of a very large magnetic field. The second involves using an ultraviolet (UV) light beam to examine the surface of the crystal. In this case, the energy of a light particle can be absorbed by an electron at the surface, giving it sufficient energy to escape from the crystal and be analyzed. Scientists can harness this photo-electric effect, known as photoemission, to gather valuable information on the electronic properties at the surface of a topological insulator.

For more than 10 years, scientists have been baffled as to why these two experiments completely disagree when applied to topological insulators. The reason, according to this new study, is that the very first UV light flash, required to record the photoemission data, alters the electronic structure at the surface.

The quantity that describes and explains how electrons in a solid do their stuff is called the band structure. It can be seen as a sort of road network that maps out the allowed combinations of energy and wavelength the electron-waves can have in the crystal. A slice through such a band structure can be easily displayed as a two-dimensional (2D) image.

This kind of snapshot contains valuable information about the electronic structure of a topological insulator, and in particular the energy location of the crossing-point of the two branches visible in the band structure. This special feature is called the Dirac point, named after theoretical physicist Paul Dirac, who first developed the theory that describes electrons.

Normally, recording a band structure image takes a minute or more, but the UVA-led research team worked hard to bring this down to just a single second. Their studies showed that, initially, the Dirac point appears at an energy matching that from magnetotransport data. After only 20 seconds of UV exposure, however, they found that the Dirac point, and the rest of the band structure with it, slid way down in energy, far from the value found in the transport experiments.

It was already known that molecules that stick to the surface of a topological insulator can cause a downward shift of the Dirac point. These new experiments were able to disentangle the effect of the molecules at the surface and that of the UV light, so the scientists could demonstrate that the very first flash of light plays the role of the starter’s pistol, triggering a rapid downward slide of the Dirac point.

Does this finding imply that photoemission needs to be abandoned as a way for studying topological insulators? On the contrary, now that the effect of the UV light is properly understood, protocols can be developed for ensuring photoemission is used in the right way in future studies.

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

This shows 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 a topological insulator. After 20 seconds of exposure to the UV light required for photoemission experiments (right-hand image), the band structure is very different to that after just one second of exposure (left-hand image). The coloured circles show the position of the Dirac point.
This shows 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 a topological insulator. After 20 seconds of exposure to the UV light required for photoemission experiments (right-hand image), the band structure is very different to that after just one second of exposure (left-hand image). The coloured circles show the position of the Dirac point.