New research paves way for spintronics and quantum computing

Teams from the Johannes Gutenberg University Mainz (JGU) in Germany and Stanford University have uncovered a new quantum state of matter in Heusler compounds which they claim opens up 'previously unimagined usage possibilities'. Prof Claudia Felser (pictured) from Mainz University, said: "Heusler materials are real all rounders and a veritable goldmine for future technologies." Together with Prof Shou Cheng Zhang of Stanford University, the scientist from Mainz has shown that many Heusler compounds can behave like topological insulators (TI). TIs were discovered just five years ago. In 2006, Prof Zhang predicted that a new quantum state of matter would be identified in nanostructures of the familiar semiconductor mercury cadmium telluride (HgTe). One year later, this was confirmed in experiments carried out by the Würzburg team led by Prof Laurens Molenkamp. Completely new mathematical concepts are required to understand the physical aspects of what has been discovered. Since then, TIs have been studied in the field of solid state and material physics. Characteristic of topological insulators is the fact that the materials are actually insulators or semiconductors, although their surfaces or interfaces are made from metal - but not ordinary metal. Like superconductors, the electrons on the surfaces or interfaces do not interact with their environment - they are in a new quantum state. In contrast with superconductors, topological insulators have two non-interacting currents, one for each spin direction. These two spin currents, which are not affected by defects or impurities in the material, can be employed in the futuristic electronics field of 'spintronics' and for processing information in quantum computers. It is now supposed that Heusler materials may have the same capabilities. Heusler compounds are made up of three elements, which often have semiconductor or magnetic properties. One special feature of these compounds is that they exhibit characteristics other than those that might be expected in view of the elements of which they are composed. The first Heusler compound, for example, was made from the non-magnetic elements copper, manganese, and aluminium. Yet, Cu₂MnAl acts as a ferromagnet, even at room temperature. On the other hand, a semiconductor can result when three metals are combined. New semiconductors can be designed in the class of Heusler materials with regard to the field of renewable energies; they can be used in solar cells or in thermoelectric applications, for converting heat into electricity. Important discoveries with regard to Heusler compounds, their properties and uses in a range of potential applications have been made in Mainz. Prof Felser believes the news that Heusler materials are now being considered as possible topological insulators will create excitement all over the world. "There are two reasons for this," Felser explained. "On the one hand, this large material class with over 1000 known representatives contains more than 50 compounds that bear the hallmark of TIs. And on the other hand, it is now possible to design completely new physical effects. As the materials are made up of three elements, they can offer a range of other interesting features in addition to the topological quantum state." It is now possible to combine two quantum states such as superconductivity and topological surface effects. This paves the way for completely new and as yet undiscovered characteristics, some of which have already been predicted. "It was previously not considered possible to combine all these possibilities in just one material," concluded Prof Felser.