Topological insulators are a still young class of materials with a special quantum property: on their surface they can conduct electricity almost without loss, whereas their interior acts as an insulator. This promises interesting perspectives for component development: topological insulators could serve as the basis for highly efficient electronic components. This makes them an interesting field of research in physics.
First, however, some fundamental questions remain open: For example, what happens when the electrons in the material are energetically excited with terahertz radiation? One thing is clear: the electrons want to get rid of the forcibly missed energy boost as quickly as possible, for example by heating up the crystal lattice around them. But in the case of topological insulators, it was previously questionable whether this release of energy happens faster in the conducting surface than in the insulating core. A research team from Germany, Spain and Russia led by the Helmholtz Centre Dresden-Rossendorf (HZDR) has investigated this question. "To determine this, there was previously a lack of suitable experiments," explains study leader Dr Sergey Kovalev from the Institute of Radiation Physics at the HZDR. "Until now, it has been 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 team developed a sophisticated experimental setup: Intense terahertz pulses hit the sample and excite the electrons. Immediately afterwards, laser flashes illuminate the material and record how the sample reacts to the terahertz stimulus. In a second series of experiments, special detectors measure the extent to which the sample shows an unusual non-linear effect and multiplies the frequency of the incoming terahertz pulses. Kovalev conducted these experiments at the TELBE terahertz light source in the HZDR's Centre for High Power Radiation Sources. Researchers from the Catalan Institute for Nanosciences and Nanotechnology in Barcelona, the University of Bielefeld, the German Aerospace Center