Physics Team Observes Fast Electronic Changes in Special Class of Materials

July 24, 2020 by Luke James

Topological materials, which are the subject of intensive research due to the potential they hold for applications in electronics, are characterized by special electronic properties and are very robust against external influences and include the likes of tungsten ditelluride.

In this material, such a topologically protected state can be broken up by using a laser to change its properties which, according to research, could aid the development of ultra-fast optoelectronic switches.

Now, for the first time ever and as described in the journal Nature Communications, physicists at Kiel University (CAU) claim to have observed real-time changes to the electronic properties of tungsten ditelluride in laboratory experiments. Using laser pulses, the team put the atoms in a sample of the material into a state of controlled excitation and were able to use high-precision measurements to observe the changing electronic properties


Unusual Electronic Properties

According to the researchers, if these laser-induced changes can be reversed, they essentially have a switch that can be activated optically and can change between different electronic states.

This switching process has already been predicted by another study in which U.S. researchers were able to directly observe atomic movements within tungsten ditelluride. In their own study, the CAU research team focused on the electrons’ behavior and how the electronic properties in the same material can be altered with ultrafast laser pulses. 

“Some of the electrons in tungsten ditelluride are highly mobile, so they are excellent information carriers for electronic applications. This is due to the fact that they behave like so-called Weyl fermions,” said doctoral researcher Petra Hein.

Weyl fermions are massless particles with special properties. Until now, they have only been observed indirectly in solids like tungsten ditelluride. “For the first time, we were now able to make the changes in the areas of the electronic structure visible, in which these Weyl properties are exhibited,” Hein added. 


CAU equipment.

To record changes in the material’s electronic properties, a highly sensitive piece of equipment is required (pictured), which the CAU team has been developing for the past few years. Image credited to AG Bauer.

Time-Resolved Photoelectron Spectroscopy

To measure the almost invisible changes in the electronic properties of the material, highly precise measurements, and an extensive analysis of the obtained data was required. Over the past few years, the CAU team has been working on developing such an experiment with the necessary long-term stability. 

Using the generated laser pulses, they put the atoms inside a sample of tungsten ditelluride into a state of vibrational excitation. As different excitations overlapped, this changed the material’s electronic properties. To observe the specific process, the team irrigated the material with a second laser pulse which released electrons from the sample.

This enabled the team to draw conclusions about the material’s electronic structure—the process is known as “time-resolved photoelectron spectroscopy.” Over time, the team evaluated a large number of datasets using a new analytical approach, thus enabling them to see the electronic changes in the tungsten ditelluride material. 

“Our results demonstrate the sensitive and highly-selective interplay between the vibrations of the atoms of the solid and the unusual electronic properties of tungsten ditelluride,” says Michael Bauer. The researchers hope to continue their work by investigating whether such electronic switching processes can be triggered even faster.