An international team of scientists, including physicists from St Petersburg University, has discovered a new class of materials that are both antiferromagnets and topological insulators. The MnBi2Te4 single crystal synthesised by the researchers can be used in developing superfast memory cells, spintronics devices, quantum computers, and even a dark matter detector. The research results have been published in Nature.

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The Laboratory of the Electronic and Spin Structure of Nanosystems of St Petersburg University is headed by Eugene Chulkov, professor at the University of the Basque Country. Researchers from the laboratory note that they have been working to achieve this result for several years. First, the existence of single crystals with unusual properties was predicted in theory. Then they were synthesised in laboratory  at Technische Universität Dresden and Azerbaijan State Oil and Industry University. The new material turned out to have simultaneously the properties of an antiferromagnet and a topological insulator.

Ferromagnets are materials in which the magnetic moments of all atoms are aligned. They create a macroscopic magnetic field in the material. For example, computer hard drives are made of ferromagnets. However, everything is different in antiferromagnets: the magnetic moments of the atoms are oppositely directed. They therefore do not create a stray magnetic field, which, in fact, negatively affects the elements of electronics. It is antiferromagnets that might be used to produce storage devices in the future. Unlike ferromagnets, such memory devices can be put close to each other as many times as you wish. And this will make your computer more powerful. Additionally, the resonant frequency of antiferromagnets is not gigahertz, but terahertz. This means that devices based on them will work 1,000 times faster than classical ones. By the way, a prototype of an element of antiferromagnetic memory based on the new material MnBi2Te4 has been recently proposed in one research paper.

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A discovered single crystal is also a topological insulator. It is a special material on the surface of which electrons behave in a fundamentally different way to how they do inside a single crystal. On the surface it is an extra fine conductive layer, and inside it is a semiconductor. It is these unique surface electrons, which form the so-called Dirac cone, that have been measured in the laboratory of St Petersburg University. What is important, even if the material surface is destroyed, it does not lose its properties and remains topologically protected. This property can be useful in the development of quantum computers. At present, one of the main problems in developing such computers is related to the fact that a qubit - a unit of information storage - is subject to decoherence. It means that, according to quantum laws, it collapses over time. However, if we make a qubit based on a topological insulator, hypothetically this problem can be avoided.

‘This single crystal is also of interest because of the fact that it provides researchers with a whole class of new materials,’ said 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.’

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By the way, the physicists have already managed to observe the quantum anomalous Hall effect in these single crystals. In solid state physics, the ordinary Hall effect is that if an external voltage is applied to a material placed in a magnetic field, there appears a current perpendicular to this voltage. It is used, for example, in magnetometers in smartphones and in electronic ignition systems of internal combustion engines. There is also a quantum Hall effect. However, it is the quantum anomalous Hall effect that has never been observed before in systems where the magnetic layer is precisely ordered, as in a MnBi2Te4 single crystal. Since in this case the effect is possible without applying an external magnetic field, the new material becomes very promising for developing a wide variety of electronic devices. For example, another paper has already proposed a model of a topological spin field-effect transistor based on MnBi2Te4 material.

Additionally, as the researchers note, the single crystal that is obtained can give an impetus to the development of elementary particle physics. There is a hope that topological insulators will help experimentally detect Majorana fermions – specific particles that are their own antiparticles at the same time. They were hypothesised by the Italian physicist Ettore Majorana in the 1930s, but have not yet been discovered. According to theoretical studies, the Majorana fermion can exist as a quasiparticle in topological insulators. As a matter of fact, it is this particle that due to its topological protectability is an excellent candidate for a qubit in a quantum computer.

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‘Another interesting example is the theoretical work which states that it is possible to develop a dark matter detector based on our material,’ said Ilya 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.’

At St Petersburg University, the researchers measured the magnetic characteristics and photoelectron spectra of the new single crystal. It was done using the equipment of the resource centres 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. Interestingly, the preliminary version of the scientific article (preprint), which appeared in the public domain before publication, has been cited more than 60 times. In total, the scientific collaboration supervised by St Petersburg University Professor Evgeny Chulkov includes 22 research institutions from all over the world.

‘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,’ said Eugene Chulkov.

This research is  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.

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