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

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 p_x + p_y-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 α^K-Γ= α^Γ-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.