Nonlinear Optical Responses and Large Rashba Splitting in Janus Monolayer MA₂Z₄ Synthetic Two-Dimensional Materials

The recently synthesized MA₂Z₄ family of two-dimensional (2D) materials opens a promising platform for a variety of functional properties in synthetic 2D crystals beyond conventional van der Waals layered materials. Janus structuring enables further symmetry breaking and corresponding novel fundamental effects. These advancements prompt our exploration of monolayer MA₂Z₄ (M = Mo, W; A = Si, Ge; Z = N, P) as well as Janus MSiGeN₄ and MSiGeP₄ (M = Mo, W) materials. Using first-principles density functional theory, we show that Janus structuring leads to significant out-of-plane nonlinear responses in both second harmonic generation (over 5 × 10⁵ pm²/V) and shift photocurrent (~30 nm µA/V²), comparable to and beyond that of 2D transition metal dichalcogenides (TMDs). These materials also express strong Rashba spin splitting, as shown by spin texture and electronic structure analysis, beyond that of 2D TMDs. In summary, our theoretical results suggest that 2D MA₂Z₄ materials offer further research opportunities for design and manipulation of optoelectronic materials, such as shift current photovoltaics, with Janus structuring.