A scanning tunnelling microscope image of the bismuthene film. The honeycomb structure of the material (blue) is visible, analogous to graphene; a conducting edge channel (white) forms at the edge of the insulating film (on the right). Image: Felix Reis.
A scanning tunnelling microscope image of the bismuthene film. The honeycomb structure of the material (blue) is visible, analogous to graphene; a conducting edge channel (white) forms at the edge of the insulating film (on the right). Image: Felix Reis.

A promising new ultra-thin material developed by physicists at the University of Würzburg in Germany is electrically conducting at the edge and highly insulating within – and all at room temperature.

This makes the new material a form of topological insulator, which is presently the focus of much international research. These materials are electrically insulating within, because the electrons maintain strong bonds to the atoms; at their surfaces, however, they are electrically conductive, due to quantum effects.

Electrons have a built-in compass needle, the spin, whose orientation is capable of transmitting information very efficiently, and the electrons in topological insulators are protected against scattering when moving through the surface channels. With these properties, topological insulators could form the basis for spin-based data processing, also known as spintronics.

Until now, however, there has been one major obstacle to using these surface channels for technical applications. "As the temperature of a topological insulator increases, all quantum effects are washed out and with them the special properties of the electrically conducting edges," explains Jörg Schäfer, a lecturer at the Chair of Experimental Physics 4 of the University of Würzburg.

For this reason, all known topological insulators have to be cooled to very low temperatures – usually down to -270°C – to be able to study the quantum properties of the edge channels. "Of course, such conditions are not very practicable for potential applications such as ultra-fast electronics or quantum computers," Schäfer says.

A team of Würzburg physicists has now presented an entirely new concept to elegantly bypass this problem. In addition to Schäfer, members of the team included Ralph Claessen from the Chair of Experimental Physics IV, and Ronny Thomale, Werner Hanke and Gang Li from the Chair of Theoretical Physics I. The scientists have now published their results in a paper in Science.

The Würzburg breakthrough involves the development of an ultra-thin film comprising a single layer of bismuth atoms deposited on a silicon carbide substrate. "The crystalline structure of the silicon carbide substrate causes the bismuth atoms to arrange in a honeycomb geometry when depositing the bismuth film – very similar to the structure of the 'miracle material' graphene, which is made up of carbon atoms," explains Claessen. Because of this similarity, the waver-thin film is termed ‘bismuthene’.

Despite the similarity, however, bismuthene has one decisive difference. "Bismuthene forms a chemical bond to the substrate," explains Thomale, and this plays a central role in providing the material with the desired electronic properties, as highlighted by computer-based modelling. "Whereas common bismuth is an electrically conductive metal, the honeycomb monolayer remains a distinct insulator, even at room temperature and far above." This is achieved by combining the heavy bismuth atoms with the insulating silicon carbide substrate.

The electronic conduction channels come into play at the edge of a piece of bismuthene. This is where the metallic edge channels are located, which can potentially be used for data processing. This has not only been determined theoretically by the Würzburg research team, but has also been proven in experiments using microscopic techniques.

In order to harness the edge channels for electronic components, it is crucial that there is no short-circuit through the inside of the topological material or through the substrate. "Previous topological insulators required extreme cooling to assure this," Schäfer says. The new bismuthene concept removes this cooling requirement: the distinct insulating behavior of the film and the substrate eliminate any short-circuits.

The Würzburg scientists believe that this ability to work at room temperature will make the material of interest for spintronics applications under realistic conditions. "Such conduction channels are 'protected topologically'. This means they can be used to transmit information virtually without loss," Claessen says.

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.