MnBi₂Te₄: The name of the first antiferromagnetic topological insulator

Overview

Quantum materials are worldwide in the focus of research activities within diverse scientific disciplines. This material class appears to be increasingly complex and rich in physical phenomena such as magnetism, superconductivity or topology, and is therefore promising for technological advances in the fields of information processing, sensors, computing and others. Also in Würzburg and Dresden quantum-materials research is under the spotlight and has gained further importance through the establishment of the Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter between TU Dresden and JMU Würzburg.

The extraordinary properties in quantum materials often require special, hardly achievable conditions such as low temperatures, strong magnetic fields or high pressure. Scientists are therefore looking for materials that exhibit their exotic properties even at ambient conditions. One class of quantum materials that has received a broad interest over the recent years are so-called magnetic topological insulators (MTI), which combine topological electronic properties with long-range magnetic order. They are considered a source of novel quasi-particles and unconventional quantum phenomena, but their experimental implementation is very challenging.

In a large international cooperation of over 40 scientists from over 20 research institutions, several teams of experimentalists from Dresden and Würzburg are strongly involved in the discovery of a new quantum material that constitutes the first antiferromagnetic topological insulator and also the first MTI that does not require magnetic impurities as dopants. The research team at TU Dresden led by Dr. Anna Isaeva together with their colleagues from the Leibniz IFW Dresden developed the first crystal-growth technique for MnBi2Te4 and performed structural X-ray diffraction characterization, magnetization, transport and electron-spin-resonance studies of the crystals. The main contribution to synthesis was made by Dr. Alexander Zeugner. The research cooperation was able to prove both in theory, led by the Donostia International Physics Center in Spain, and in spectroscopic experiments, headed by teams from JMU Würzburg, led by Dr. Hendrik Bentmann, and from St. Petersburg University, that MnBi2Te4 is the first antiferromagnetic topological insulator (AFMTI) below its Néel temperature. In particular, the combination of magnetically and electronically sensitive techniques, namely X-ray magnetic dichroism and angle-resolved photoelectron spectroscopy, confirmed the simultaneous presence of antiferromagnetic order and topological states at the surface of MnBi2Te4.

This discovery is significant for the scientific community: An MTI crystal is expected to feature an edge state on its surface that may realize a quantized Hall conductivity even without an external magnetic field. In addition, the fabrication of an AFMTI could make an important contribution to the booming field of antiferromagnetic spintronics. The new research area of magnetic van der Waals materials could also benefit from novel two-dimensional ferromagnets.

Dr. Isaeva's team has already further optimized the synthesis protocol for the new material so that MnBi2Te4 single crystals can be produced more easily. Research teams worldwide have joined the study of the interaction of magnetism and topology in MnBi2Te4. Recent findings show that there are even more structural derivatives of MnBi2Te4 that are relevant in the context of MTI, such as MnBi4Te7. The close collaboration between TU Dresden, IFW Dresden and JMU Würzburg already resulted in a number of additional joint publications that offer a more detailed view on the complex properties of this new class of MTI.

Otrokov et al., Nature 576, 416–422 (2019).

Zeugner et al., Chem. Mater. 31, 2795-2806 (2019).

Vidal et al., Phys. Rev. B 100, 121104(R) (2019). (Editor´s suggestion)

Vidal et al., accepted for publication in Phys. Rev. X (2019).