Magnetic Dirac semimetal state of (Mn,Ge)Bi$_2$Te$_4

• Main Authors: Frolov, Alexander S, Usachov, Dmitry Yu, Tarasov, Artem V, Fedorov, Alexander V, Bokai, Kirill A, Klimovskikh, Ilya, Stolyarov, Vasily S, Sergeev, Anton I, Lavrov, Alexander N, Golyashov, Vladimir A, Tereshchenko, Oleg E, Di Santo, Giovanni, Petaccia, Luca, Clark, Oliver J, Sanchez-Barriga, Jaime, Yashina, Lada V
• Format: Journal Article
• Language: English
• Published: 22.06.2023
• Subjects: Physics - Materials Science
• DOI: 10.48550/arxiv.2306.13024

For quantum electronics, the possibility to finely tune the properties of magnetic topological insulators (TIs) is a key issue. We studied solid solutions between two isostructural Z$_2$ TIs, magnetic MnBi$_2$Te$_4$ and nonmagnetic GeBi$_2$Te$_4$, with Z$_2$ invariants of 1;000 and 1;001, respectively. For high-quality, large mixed crystals of Ge$_x$Mn$_{1-x}$Bi$_2$Te$_4$, we observed linear x-dependent magnetic properties, composition-independent pairwise exchange interactions along with an easy magnetization axis. The bulk band gap gradually decreases to zero for $x$ from 0 to 0.4, before reopening for $x>0.6$, evidencing topological phase transitions (TPTs) between topologically nontrivial phases and the semimetal state. The TPTs are driven purely by the variation of orbital contributions. By tracing the x-dependent $6p$ contribution to the states near the fundamental gap, the effective spin-orbit coupling variation is extracted. As $x$ varies, the maximum of this contribution switches from the valence to the conduction band, thereby driving two TPTs. The gapless state observed at $x=0.42$ closely resembles a Dirac semimetal above the Neel temperature and shows a magnetic gap below, which is clearly visible in raw photoemission data. The observed behavior of the Ge$_x$Mn$_{1-x}$Bi$_2$Te$_4$ system thereby demonstrates an ability to precisely control topological and magnetic properties of TIs.