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Prediction of Nontrivial Topological Phases and Rashba Spin-Splitting in BaABTe4 Janus Monolayers (A, B = Al, Ga, In, or Tl)

Joel D'Souza,R. Villaos,A. B. Iii,I. M. R. Verzola,Sreeparvathy Puthiya Covilakam,Zhi-Quan Huang,F. Chuang

Published 2025 in ACS Omega

ABSTRACT

Two-dimensional (2D) materials have emerged as a significant focus in materials research due to their tunable properties on thermoelectricity, spin-splitting, and nontrivial topology. Specifically, Janus-type 2D materials are interesting due to their additional breaking of inversion or mirror symmetry in the atomic structure. Based on the recently synthesized monolayer MoSi2N4 and previously studied BaIn2Te4 with the chemical formula of MA2Z4, we derive a family of 2D Janus compounds, namely BaABTe4. Using first-principles calculations, a total of six Janus BaABTe4 monolayers (BaAlGaTe4, BaAlInTe4, BaAlTlTe4, BaGaInTe4, BaGaTlTe4, and BaInTlTe4) were investigated for their dynamical stability, electronic, and topological properties. Notably, the Z2 topological invariant calculated using HSE06 hybrid functional reveals that three out of the six monolayers (BaAlGaTe4, BaAlTlTe4, and BaInTlTe4) have nontrivial topological phases, with BaInTlTe4 exhibiting the largest positive system band gap of 17 meV. These three topological monolayers were further confirmed to be dynamically stable based on phonon dispersion and formation energy calculations. Subsequent orbital analysis of BaInTlTe4 showed that the spin–orbit coupling effect drives the topological phase transition, resulting in the band inversion between the s-orbital of In + Tl and px + py-orbitals of Te around Γ. Also, the presence of the gapless edge states confirmed the nontrivial topological property. The Janus monolayers were found to exhibit significant Rashba spin-splitting except BaAlInTe4. The topologically nontrivial BaAlTlTe4 has the strongest Rashba strength of αK-Γ= αΓ-M = 1.03 eVÅ. Our results show that the coexisting nature of the nontrivial phase and Rashba-type splitting within the BaABTe4 Janus monolayers might apply to spintronics.

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