New MnBi2Te4 Crystal Acts as Antiferromagnet and Topological Insulator

An international group of researchers, which includes physicists from St Petersburg University, has identified a new category of materials that behave not only as antiferromagnets but also as topological insulators.

The MnBi₂Te₄ single crystal developed by the scientists can be used to create superfast memory cells, quantum computers, spintronics devices, and even a dark matter detector. The study outcomes have been reported in Nature.

The Laboratory of the Electronic and Spin Structure of Nanosystems of St Petersburg University is led by Evgeny Chulkov, professor at the University of the Basque Country. Scientists from the lab state that they have been striving for many years to achieve this result.

Initially, the occurrence of single crystals with strange properties was theoretically predicted. Later, they were developed in the lab at Technische Universität Dresden and Azerbaijan State Oil and Industry University. The new material was found to exhibit the properties of an antiferromagnet and a topological insulator at the same time.

In ferromagnetic materials, the magnetic moments of all atoms are aligned, forming a macroscopic magnetic field within the material. For instance, computer hard drives are developed using ferromagnets. But when it comes to antiferromagnets, everything is different: the magnetic moments of the atoms are aligned oppositely.

Hence they do not form an external magnetic field, which actually has a negative impact on the elements of electronics. In the future, antiferromagnets could be used to develop storage devices. In contrast to ferromagnets, memory devices such as these can be placed close to each other any number of times. This would make a computer more robust.

Moreover, antiferromagnets have terahertz range of resonant frequency, and not gigahertz. This implies that devices developed using antiferromagnets will work 1,000 times faster compared to classical ones. Incidentally, a prototype of an antiferromagnetic memory element based on the new material MnBi₂Te₄ was recently described in a research paper.

A single crystal is also a topological insulator. It is a unique material whose surface electrons act in a typically different manner than how they behave within a single crystal. The surface is an additional fine conductive layer, and from within, it is a semiconductor.

These distinctive surface electrons, forming what is called the Dirac cone, have been quantified in the laboratory of St Petersburg University. The most significant fact is that the material does not lose its properties and stays topologically protected even if its surface is destroyed. This characteristic can be helpful in developing quantum computers.

One of the main challenges faced now in the development of such computers is due to the fact that a qubit—a unit of information storage—is affected by decoherence. It implied that, based on quantum laws, it declines over time. But if we develop a qubit based on a topological insulator, this challenge can be hypothetically overcome.

“This single crystal is also of interest because of the fact that it provides researchers with a whole class of new materials,” stated Professor Aleksandr Shikin, the deputy head of the laboratory.

If layers that are connected antiferromagnetically are separated by layers of a topological insulator, we can create unique magnetic characteristics of the material with a gradual transition from antiferromagnetism to two-dimensional ferromagnetism. This is a completely new system with new features, which, by and large, have not even been discovered yet.

Aleksandr Shikin, Deputy Head, Laboratory of the Electronic and Spin Structure of Nanosystems, St Petersburg University

Moreover, the physicists have already been able to detect the quantum anomalous Hall effect in these single crystals. The ordinary Hall effect observed in solid-state physics is that the application of an external voltage is to a material placed in a magnetic field leads to the formation of a current perpendicular to the voltage. For instance, it is used in electronic ignition systems of internal combustion engines and in magnetometers of smartphones.

In addition, there is a quantum Hall effect. But it is the quantum anomalous Hall effect that has not been detected earlier in systems in which the magnetic layer is exactly ordered, similar to a MnBi₂Te₄ single crystal.

Here, the effect is generated without an external magnetic field being applied, thus the new material turns highly promising for creating a broad array of electronic devices. For instance, another research paper has already described a topological spin field-effect transistor model that is based on MnBi₂Te₄ material.

Furthermore, as stated by the scientists, the single crystal that is synthesized can offer momentum to the advancement of elementary particle physics. Researchers hope that topological insulators will be useful for the experimental detection of Majorana fermions—unique particles that act as their own antiparticles simultaneously.

They were proposed by Ettore Majorana, an Italian physicist, in the 1930s. However, they are yet to be discovered. Theoretical studies propose that the Majorana fermion can occur as a quasiparticle in topological insulators. In fact, this particle is a brilliant candidate for a qubit in a quantum computer owing to its topological protectability.

“Another interesting example is the theoretical work which states that it is possible to develop a dark matter detector based on our material,” said Ilia Klimovskikh, laboratory assistant and Candidate of Physics and Mathematics.

Since it is a magnetic topological insulator, it is possible to realise the phase of an axion insulator in it. On its basis it is possible to develop a dark matter detector with a certain range that does not exist yet. This is very unexpected, but such papers are likely to appear because the material has completely new and unique properties.

Ilia Klimovskikh, Laboratory Assistant and Candidate of Physics and Mathematics, St Petersburg University

The researchers at St Petersburg University quantified the photoelectron spectra and magnetic properties of the new single crystal. This was achieved using the equipment at the resource centers of the University Research Park: the Centre for Physical Methods of Surface Investigation and the Centre for Diagnostics of Functional Materials for Medicine, Pharmacology and Nanoelectronics.

Fascinatingly, the initial version of the scientific paper (preprint), which appeared in the public domain prior to being published, has been cited over 60 times. The scientific collaboration managed by St Petersburg University Professor Evgeny Chulkov includes a total of 22 research institutions from across the globe.

So many institutions participating in a single publication in the field of condensed matter may seem unusual. However, to solve effectively complex problems in modern solid state science requires consolidated efforts of various highly professional teams. They include theorists, chemists, physicists and materials scientists. This trend will only grow stronger in the foreseeable future.

Evgeny Chulkov, Professor, St Petersburg University

This study was supported by grants from St Petersburg University (ID 40990069), the Russian Science Foundation (No 18-12-00062), the Russian Foundation for Basic Research (No 18-52-06009), and other scientific institutions.