- An international team of researchers uses experiments to show that topological insulators could serve as the basis for highly efficient electronic components.
- Scientists from the DLR Institute for Optical Sensor Systems are participating in the study.
- These quantum materials could ensure faster mobile data transmission in the future.
- In the future, this material can also play a major role in the development of detector systems such as space telescopes.
COLOGNE, Germany (DLR PR) — They are considered to be extremely interesting materials for the electronics of the future: Topological insulators conduct electricity in a special way and promise new types of circuits and faster mobile communications. An international team of researchers, with the participation of the German Aerospace Center (DLR), has now unraveled a fundamental property of the new class of materials: How do the electrons in the material react when they are “startled” with short pulses of so-called terahertz radiation? The results are not only important for a fundamental understanding of these novel quantum materials, but could also ensure faster mobile data communication in the future or be used in highly sensitive detector systems for the exploration of distant planets.
“The properties offer promising prospects in the future,” emphasizes Prof. Anke Kaysser-Pyzalla, chairman of the DLR, “The so-called topological insulators can be the basis for highly efficient electronic components and ensure faster mobile data transmission in the future.”
Topological insulators are still a young class of materials with a special quantum property. On their surface, they can conduct electricity with almost no loss. On the other hand, their interior acts as an insulator, so that no current can flow here. These properties offer promising prospects in the future: Topological insulators could serve as the basis for highly efficient electronic components, which makes them an interesting field of research in physics.
“At DLR, we are very interested in using such quantum materials in high-performance heterodyne receivers for astronomy, especially in space telescopes,” explains Michael Gensch from the DLR Institute for Optical Sensor Systems and Professor at the Institute for Optics and Atomic Physics at the TU Berlin .
Topological isolators under terahertz radiation
However, some fundamental questions remain open: What happens, for example, if the electrons in the material are “pushed” with certain electromagnetic waves – so-called terahertz radiation – and thereby energetically stimulated? The electrons want to get rid of the forcibly missed energy boost as quickly as possible, for example by heating the crystal lattice around them. In the case of topological insulators, however, it has so far been questionable whether this loss of energy happens faster in the conductive surface than in the insulating core.
“So far, there has been a lack of suitable experiments to determine this,” explains study leader Dr. Sergey Kovalev from the Institute for Radiation Physics at the Helmholtz Center Dresden-Rossendorf (HZDR). “Up until now it was extremely difficult to distinguish between the reaction of the surface and that of the interior of the material at room temperature.”
To overcome this hurdle, the international research team led by Kovalev developed a sophisticated test setup. Intense terahertz pulses hit the sample and stimulate the electrons. Immediately afterwards, laser flashes illuminate the material and record how the sample reacts to the terahertz stimulus. In a second series of tests, special detectors measure the extent to which the sample shows an unusual non-linear effect and the frequency of the incoming terahertz pulses multiplied. Kovalev carried out these experiments at the terahertz light source TELBE (High-Field High-Repetition-Rate Terahertz facility @ ELBE) in the ELBE center for high-power radiation sources of the HZDR.
Scientists from the Catalan Institute of Nanosciences and Nanotechnology in Barcelona, the Bielefeld University, the German Aerospace Center (DLR), the Technical University of Berlin as well as the Lomonossov University and the Kotelnikov Institute of Radio Technology and Electronics in Moscow.
Rapid Energy Transfer
It was crucial that the team didn’t just scrutinize a single material. The Russian project partners are producing three different topological insulators with different, precisely coordinated properties: In one, only the electrons on the surface could directly absorb the energy of the terahertz pulses, in the other, mainly electrons inside the sample were excited.
“The comparison of these three experiments made it possible to precisely differentiate between the behavior of the surface and that of the interior of the material,” explains Kovalev. “The electrons on the surface de-excited much faster than those inside the material.” Apparently, they were able to transfer their energy immediately to the material’s crystal lattice.
If the surface electrons had returned to their original energetic state after a few hundred femtoseconds, this took around ten times as long for the “inner” electrons, i.e. a few picoseconds. “Topological isolators are highly complex systems, they are anything but easy to understand theoretically,” emphasizes DLR scientist Michael Gensch. “Our results can help in deciding which of the theoretical ideas are applicable.”
Highly Effective Multiplication
But the experiment also promises interesting perspectives for digital communication, for example for WLAN and mobile communications. Technologies like 5G work in the gigahertz range today. If higher frequencies in the terahertz range could be used, significantly more data could be transmitted over a radio channel. So-called frequency multipliers could play an important role. They are able to translate relatively low radio frequencies into significantly higher ones.
Some time ago the research team realized that graphene — two-dimensional, super-thin carbon — can serve as an efficient frequency multiplier under certain conditions. It can convert 300 gigahertz radiation into frequencies of a few terahertz. But if the incoming radiation is extremely intense, graphene loses a lot of its efficiency. Topological insulators, on the other hand, still work even with the most intense stimulation, according to the result of the new study.
“This could make it possible to multiply frequencies from a few terahertz to several dozen terahertz,” believes HZDR physicist Dr. Jan-Christoph Deinert, who leads the TELBE team together with Kovalev. “So far, we don’t see the end of this with topological insulators.” This means that the new quantum materials can be used in a much broader frequency range than graphene, for example.