So far, so-called topological insulators are known - crystals that are electrically insulating inside but conduct electricity outside. The conductive surface states are topologically protected and are difficult to convert into an insulating state. A group of theoretical physicists led by the University of Zurich (UZH) have now investigated a new class of topological insulators that have conductive states not on the surfaces but on the crystal edges. The team of scientists from UZH, Princeton University, Donostia International Physics Center and the Max Planck Institute for Microstructure Physics in Halle has named this new material class "topological insulators of higher order". These are particularly interesting because the electrically conductive edges are extremely robust: Impurities or disorder in the crystal do not stop the flow of topological electrons, the current simply flows around the obstacle.
In addition, the edges do not have to be specially prepared to be conductive. If the crystal breaks, the new edges are also conductive again. "But the most exciting thing is that electricity can be conducted in this way, at least theoretically without resistance," says Titus Neupert, professor at the Institute of Physics at the UZH. "You can imagine the crystal edges as a highway for electrons. You can't just turn back". This property of resistance-free conductivity, which is particularly known of superconductors at low temperatures, cannot be found in the topological insulators with conductive surfaces known to date.
The theoretical aspects still predominate in the study of physicists. They suggested tin telluride as the first compound to exhibit these novel properties. "We have to identify further possible materials of this new class and investigate them in experiments," said Neupert. The researchers hope that nanowires consisting of the higher-order topological insulators they are investigating can in future be used as conductor paths in electrical circuits. They could be combined with magnetic and supra-conductive materials and used in quantum computers.