Research: It's all about the surface

The extraordinary quantum material manganese bismuth telluride offers an opportunity for novel electronic devices that encode and transport information magnetically. Called "spintronics," such devices promise to make information technologies more sustainable and energy-efficient in the future.

In 2019, the team led by Anna Isaeva, at that time a junior professor at the Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter, succeeded in producing this new type of quantum material for the first time. The crystal manganese bismuth telluride, or MnBi₂Te₄ for short, which was tailored in Dresden, does not need an external magnetic field to exhibit certain topological effects such as the quantized anomalous Hall effect. This extraordinary material is a so-called "magnetic topological insulator" - it brings its magnetic field along. This makes it more suitable for practical applications than its predecessors. Scientists worldwide have since been analyzing different facets of this novel material.

Researchers from the Würzburg-Dresden Cluster of Excellence ct.qmat have now identified two new materials - MnBi₄Te₇ and MnBi₆Te₁₀ - as magnetic topological insulators and found out which ultrathin atomic layer needs to be on the surface for these particular topological effects to become visible. The results of their work were published in the journal Physical Review Letters. In addition to the cluster members from Julius-Maximilians-Universität Würzburg (JMU), Technische Universität Dresden (TUD) and Leibnitz Institute for Solid State and Materials Research Dresden (IFW), groups from the Research Center Jülich and Hiroshima University (Japan) are also involved in the publication.

First, crystals with distinct layer structures were produced in a laboratory furnace in Dresden. Subsequently, scientists in Würzburg examined the material samples using photoelectron spectroscopy. The theoretical computations used to analyze certain layer arrangements of MnBi₄Te₇ and MnBi₆Te₁₀ came from the ct.qmat partner institute IFW.

"We were able to experimentally demonstrate in detail how the topological surface states behave in both material structures. In doing so, we found profound differences in the electronic properties. Dissipationless current conduction is only possible for certain atomic layers on the surface," explains Dr. Hendrik Bentmann, an early career scientist from Würzburg and the head of the study. "Now we are working to better understand the interplay of two fundamental phenomena of solid-state physics - magnetism and topology - in these materials and to control them in more complex structures."

Currently, scientists are in the process of producing this material atom by atom in such a way that the correct atomic layer sits immediately on the surface.

R. C. Vidal, H. Bentmann, J. I. Facio, T. Heider, P. Kagerer, C. I. Fornari, T. R. F. Peixoto, T. Figgemeier, S. Jung, C. Cacho, B. Büchner, J. van den Brink, C. M. Schneider, L. Plucinski, E. F. Schwier, K. Shimada, M. Richter, A. Isaeva, F. Reinert,
Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi₄Te₇ and MnBi₆Te₁₀,
Phys. Rev. Lett. 126, 176403 (2021) (arXiv)