Two-dimensional Rashba semiconductors and inversion-asymmetric topological insulators in monolayer Janus MAA'ZxZ'(4−x) family

The Rashba effect in Janus structures, accompanied by nontrivial topology, plays an important role in spintronics and even photovoltaic applications. Herein, through first-principles calculations, we systematically investigate the geometric stability and electronic structures of 135 kinds of Janus MAA'ZxZ'(4−x) family derived from two-dimensional MA2Z4 (M = Mg, Ga, Sr; A = Al, Ga; Z = S, Se, Te) monolayers and design numerous Rashba semiconductors and inversion-asymmetric topological insulators. Specifically, there are a total of 26 Rashba semiconductors with isolated spin-splitting bands contributed by Se/Te-pz orbitals at conduction band minimum, and the magnitude of the Rashba constant correlates strongly with both the intrinsic electric field and the strength of spin–orbit coupling (SOC). As the atomic number increases, the bandgap of Janus MAA'ZxZ'(4−x) continually decreases until it shrinks to a point where, when SOC is considered, band inversion occurs, leading to a reopening of the bandgap with nontrivial topological phases. In conjunction with band inversion, pz orbitals near the Fermi level can introduce double Rashba splitting featuring a distinctive hybrid spin texture, which can be further effectively adjusted through small biaxial strains and show a continuous evolution from topological to non-topological accompanied by different spin textures. This work provides significant insights into Rashba and topology physics and further presents indispensable inversion-asymmetry materials for the development of nonlinear optoelectronics.