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New study investigates fundamental properties of topological insulators
Topological insulators, regarded as one of the most interesting materials for future electronics, conduct electricity in a special way and provide a promise of novel circuits and faster mobile communications. A research team from Germany, Spain, and Russia with participation of the Institute of Optics and Atomic Physics at TU Berlin, has now unraveled a fundamental property of this new class of materials: how exactly electrons in the material respond when "startled" by short pulses of so-called terahertz radiation. The results are not only significant for our basic understanding of this novel quantum material but could herald faster mobile data communication or high-sensitivity detector systems for exploring distant worlds in years to come. The team's work has recently been published in NPJ Quantum Materials (DOI: 10.1038/s41535-021-00384-9, https://www.nature.com/articles/s41535-021-00384-9).
Topological insulators are a very recent class of materials and possess a special quantum property. On their surface they can conduct electricity almost loss-free while their interior functions as an insulator, where no current can flow. This opens up interesting prospects for the future. Topological insulators could form the basis for high efficiency electronic components, making them an interesting research field for physicists. "At DLR, we are for instance very interested in using quantum materials of this kind in high-performance receivers for astronomy, especially in space telescopes," explains Professor Dr. Michael Gensch, head of department at the Institute of Optical Sensor Systems at the German Aerospace Center (DLR) and professor of terahertz and laser spectroscopy at TU Berlin and the Einstein Center Digital Future (ECDF).
Differentiating internally and externally
However, a number of fundamental questions remain unanswered. What happens, for example, when you give the electrons in the material a "nudge" using specific electromagnetic waves – so-called terahertz radiation – thus generating an excited state? One thing is clear: The electrons want to rid themselves of the energy boost forced upon them as quickly as possible, such as by heating up the crystal lattice surrounding them. In the case of topological insulators, though, it was previously unclear whether dissipating this energy occurred faster in the conducting surface than the insulating core. "Until now, we simply didn't have the appropriate experiments to find out," explains study leader Dr. Sergey Kovalev from the Institute of Radiation Physics at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). "It was extremely difficult to differentiate the surface reaction from that in the interior of the material at room temperature."
New test set-up reveals the difference
In order to overcome this hurdle, Kovalev and his international team developed an ingenious test set-up, investigating more than just a single material. The Russian project partners produced three different topological insulators with different, precisely determined properties. In one case, only the electrons on the surface could directly absorb the terahertz pulses. In the others, the electrons were mainly excited in the interior of the sample. "By comparing these three experiments we were able to differentiate precisely between the behavior of the surface and the interior of the material," Kovalev explains. "We determined that the electrons in the surface became excited significantly faster than those in the interior of the material." As it happens, they were apparently able to transfer their energy to the crystal lattice immediately.
Put into figures, the results showed that while the surface electrons reverted to their original energetic state in a few hundred femtoseconds, the "inner" electrons took approximately ten times as long, that is, a few picoseconds. "Topological insulators are highly-complex systems. The theory is anything but easy to understand," emphasizes Michael Gensch. "Our results can help determine which theoretical ideas hold true."
Highly effective multiplication
The experiment also augurs well for interesting developments in digital communication like WLAN and mobile communications. Today, technologies such as 5G function in the gigahertz range. If we could harness higher frequencies in the terahertz range, significantly more data could be transmitted by a single radio channel. Frequency multipliers could play an important role in this. They are able to translate relatively low radio frequencies into significantly higher ones.
Some time ago, the research team had already realized that, under certain conditions, graphene – a two-dimensional, super thin carbon – can act as an efficient frequency multiplier. It is able to convert 300 gigahertz radiation into frequencies of some terahertz. The problem is that when the applied radiation is extremely intensive, the graphene's efficiency significantly drops. Topological insulators, on the other hand, function as frequency multipliers even with the most intensive stimulation, the study discovered. This might mean it is possible to multiply frequencies from a few terahertz to several dozen terahertz. If such a development occurs, the new quantum materials could be used in a much wider frequency range than with graphene.
The experiments were conducted using the TELBE terahertz light source at the HZDR. Researchers from the Catalan Institute of Nanoscience and Nanotechnology in Barcelona, Bielefeld University, the German Aerospace Center (DLR), TU Berlin, and Lomonosov University and the Kotelnikov Institute of Radio Engineering and Electronics in Moscow were involved in the project.
Further information is available from:
Prof. Dr. Michael Gensch
Technische Universität Berlin
Institute of Optics and Atomic Physics
Tel.: +49 (30) 314-26644
Email: michael.gensch@tu-berlin.de
http://DOI: 10.1038/s41535-021-00384-9
https://www.nature.com/articles/s41535-021-00384-9
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