Nontrivial topology in monolayer MA₂Z₄ (M = Ti, Zr, or Hf; A = Si or Ge; and Z = N, P, As, Sb, or Bi)

Ina Marie R. Verzola^1*, Rovi Angelo B. Villaos¹, Zhi-Quan Huang¹, Chia-Hsiu Hsu², Yoshinori Okada², Hsin Lin³, Feng-Chuan Chuang^1,4,5,6

¹Department of Physics, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan
²Quantum Materials Science Unit, Okinawa Institute of Science and Technology, Okinawa, 904-0495, Japan
³Institute of Physics, Academia Sinica, Taipei, 115201, Taiwan
⁴Physics Division, National Center for Theoretical Sciences, Taipei, 10617, Taiwan
⁵Center for Theoretical and Computational Physics, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan
⁶Department of Physics, National Tsing Hua University, Hsinchu, 30013, Taiwan

* Presenter:Ina Marie R. Verzola, email:irverzola@g-mail.nsysu.edu.tw

The search for two-dimensional (2D) materials with interesting topological properties is still attracting growing interest as they offer exotic physical phenomena. Recently, the emergent 2D MA₂Z₄ has been gaining attention because it exhibits versatile properties due to its tunable elemental components M, A, and Z. In this study, first-principles calculations were performed to investigate 30 monolayer MA₂Z₄ (M is Group IV transition metals Ti, Zr, or Hf; A is Si or Ge; and Z is pnictogens N, P, As, Sb, or Bi) under two crystal phases called T-phase and H-phase. Our calculations revealed that all the materials prefer the T-phase. Our Z₂ topological invariant calculation under hybrid functional HSE06 reveals five MA₂Z₄ (TiSi₂Bi₄, ZrGe₂P₄, ZrGe₂As₄, HfSi₂As₄, and HfGe₂As₄) to have nontrivial topology. This topological phase transition was driven by the spin-orbit coupling resulting in the splitting of the d_z² orbital of transition metal elements and p orbitals of pnictogen elements. The nontrivial properties were further confirmed by the presence of gapless edge states. Finally, phonon spectra and ab initio molecular dynamics verified that all the nontrivial materials are thermodynamically stable. This indicates that the new MA₂Z₄ family has potential for electronics and topological devices, which will stimulate interest in experimental synthesis.

Keywords: Nontrivial topology, Quantum spin Hall effect, Novel quantum materials, Two-dimensional materials, First-principles calculations