Step edges in topological crystalline insulators can produce electrically-conducting pathways where electrons with opposite spins move in opposite directions and U-turns are prohibited. Image: Thomas Bathon/Paolo Sessi/Matthias Bode.
Step edges in topological crystalline insulators can produce electrically-conducting pathways where electrons with opposite spins move in opposite directions and U-turns are prohibited. Image: Thomas Bathon/Paolo Sessi/Matthias Bode.

Physicists at the University of Würzburg in Germany have made an astonishing discovery about the structure of a specific type of topological insulator, which they have reported in a paper in Science.

Topological insulators, which conduct electricity at their surface but not within their bulk, are currently a hot topic in physics. Only a few weeks ago, their importance was highlighted when the Royal Swedish Academy of Sciences in Stockholm awarded this year's Nobel Prize in Physics to three British scientists for their research into so-called topological phase transitions and topological phases of matter.

Topological insulators are also being studied at the departments for Experimental Physics II and Theoretical Physics I of the University of Würzburg, but the focus here is on a special type of insulator called topological crystalline insulators (TCI). In cooperation with researchers at the Polish Academy of Sciences in Warsaw and the University of Zurich in Switzerland, Würzburg physicists have now achieved a major breakthrough by detecting new electronic states of matter in these insulators.

"TCIs are relatively simple to produce and they are already different from conventional materials because of their special crystalline structure," says Paolo Sessi, a research fellow at the Department of Experimental Physics II and lead author of the paper. In topological materials, the direction in which the electrons travel is determined by their spin: simply put, the ‘spin’ can be interpreted as a magnetic dipole that can point in two directions (‘up’ and ‘down’). Up-spin electrons in TCIs move in one direction and down-spin electrons in the other.

"But previously scientists didn't know how to produce the conductive channels required to this end," says Matthias Bode, head of the Department for Experimental Physics II and co-author of the paper. By chance, Bode and his colleagues discovered that very narrow conductive channels occur naturally when splitting lead tin selenide (PbSnSe), a crystalline insulator.

This happens because small, atomically-flat terraces emerge at the split, separated from each other by step edges. The conductive channels can then form within these step edges, depending on their precise height; these channels, which can be imaged using a high-resolution scanning tunneling microscopy, are extremely narrow, at around 10 nm, and surprisingly robust against external disturbance. "Edges that bridge an even number of atomic layers are totally inconspicuous, " explains Sessi. "But if the edges span an odd number of atomic layers, a small area about 10nm in width is created that has the electronic conductive channel properties we were looking for."

Supported by their colleagues from the Department of Theoretical Physics I and the University of Zurich, the experimental physicists were able to shed light on the origin of these new electronic states. "The crystalline structure causes a layout of the atoms where the different elements alternate like the black and white squares on a chessboard," Bode says. This alternating black-and-white pattern applies to both squares that are adjacent to each other and squares that are on top of each other.

This means that if the split in the crystal runs through different atomic layers, more than one edge is created. Seen from above, white squares may abut other white squares along this edge and black squares to other black squares – or identical atoms to identical atoms. This only works, however, if an odd number of atomic layers is responsible for the difference in height between the two surfaces.

"Calculations show that this offset at the surface is actually causative of these novel electronic states," says Sessi. Furthermore, the calculations prove that the phenomenon of the spin-dependent conductive channels, which is characteristic of topological materials, occurs here as well. According to the scientists, these channels could prove of use in many applications, including ultra-fast and energy-efficient computers, because they cause low conduction loss and can be used directly to transmit and process information in the field of spintronics.

However, several questions need to be answered and challenges overcome before this will become reality. For instance, the scientists are not yet sure over what kind of distance currents can be transported in the newly-discovered conductive channels. Also, in order for the channels to be implemented in circuits, methods would have to be developed that allow step edges of a defined height to be created along specified directions.

This story is adapted from material from the University of Würzburg, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.