Materials called topological insulators are those that do not let the electric current in its volume, but on its surface. Unlike the usual conductors, that is, metals, the current circulating on the surface of a topological insulator does not suffer any loss of energy. This property opens up great possibilities of applications in electronics, as it would facilitate the manufacture of more efficient devices; faster and with low energy consumption; an objective as desirable as necessary in the current scenario of rapid progress in global energy demand due to the consequences that this generates in our environment. For this reason, the discovery of topological insulators about a decade ago caused a worldwide 'boom' of research in the fields of nanotechnology and the physics of condensed matter.
Due to the technological applications that it could have, for example in information technologies, one of the challenges during these years of intense research has been the creation of a magnetic topological insulator. Until recently, magnetism was introduced into non-magnetic topological insulators exclusively by the so-called extrinsic pathway, which consists of adding magnetic atoms. However, thanks to the efforts of a group of researchers from the Center for Materials Physics (CFM, mixed center CSIC-UPV/EHU), Donostia International Physics Center (DIPC) and the University of the Basque Country (UPV/EHU), now it is possible to grow an intrinsic magnetic topological insulator, that is, it has magnetic properties by its own nature. The team of researchers Mikhail Otrokov (Ikerbasque researcher at CFM), Evgueni Chulkov (UPV/EHU, Euskadi Research Award 2019), María Blanco Rey (UPV/EHU) and Pedro M. Etxenike (UPV/EHU, President of the DIPC), has managed to theoretically predict the first magnetic topological insulator, whose chemical formula is MnBi2Te4. The key to the success of this prediction has been the great experience that this group of researchers has in the fields of topological insulators, magnetism and materials science in general. The Ikerbasque researcher and leader of this study, Mikhail Otrokov, states that “previous work from different approaches led us to the conclusion that the intrinsic route was the only viable one today. Then, we directed our efforts to find an intrinsic magnetic topological insulator based on previous experiences, thanks to which we knew what crystalline structure and atomic composition should have such material. ”
Donostia is not only the place where the theoretical prediction of this first magnetic topological insulator has been made, but it has been the base field from which the experimental confirmation of it has been coordinated, a work that has involved expert researchers in different areas of reference research centers in Russia, Azerbaijan, Germany, Austria, Japan, Italy and the USA. The results of this study have been published in the prestigious magazine ‘Nature’. Otrokov explained that for experimental confirmation, the first task was the synthesis of the crystals of the compound by experts in chemical synthesis. Once synthesized, the samples were subjected to many experiments of structural characterization, magnetic, electronic, transport, atomic composition, etc. until the predicted characteristics were observed and verified.
The results of the study, which had previously been disseminated through an open access server and talks given by the authors at international conferences, have been well received by the international scientific community. Today the MnBi2Te4 and other materials based on it are studied in dozens of research centers, being those of the USA and China the ones that show more intense activity.
This compound of Manganese (Mn), Bismuth (Bi) and Telurium (Te) has great potential both fundamentally and technologically. It is extraordinarily rich in exotic properties, such as several Hall effects; including the quantum Hall effect; some of which are used in the calibration of physical constants for their exceptional accuracy. The MnBi2Te4 can also be used for the creation of the so-called Majorana fermions. A type of particle that has been considered the cornerstone of quantum computing. Likewise, MnBi2Te4 is the first intrinsic material for which an electromagnetic response is predicted very similar to that of an axion, a hypothetical particle postulated within the framework of quantum chromodynamics, which is a good candidate to solve the problem of dark matter. Therefore, many experiments are designed aimed precisely at the detection of signals of an axion-like behavior in the family of this compound.
As for practical applications, several devices based on magnetic topological insulators have already been patented. For example, MnBi2Te4 could be used in the chiral interconnections of integrated circuits, which promise superior performance to ordinary copper connections currently used in commercially available circuits. Other applications include optical modulators, magnetic field sensors and memory elements.
Researchers based in Donostia, together with their network of international collaborators, hope to be able to observe in the MnBi2Te4 some of the exotic properties mentioned above and discover new intrinsic magnetic topological insulators with characteristics even superior to those of MnBi2Te4.