Tunable spin polarization and electronic structure of bottom-up synthesized MoSi2N4 materials

Manipulation of spin-polarized electronic states of two-dimensional (2D) materials under ambient conditions is necessary for developing new quantum devices with small physical dimensions. Here, we explore spin-dependent electronic structures of ultra-thin films of recently introduced 2D synthetic materials $M{\mathrm{Si}}_{2}{Z}_{4}$ $(M=\mathrm{Mo} \mathrm{or} \mathrm{W} \mathrm{and} Z=\mathrm{N} \mathrm{or} \mathrm{As})$ using first-principles modeling. Stacking of $M{\mathrm{Si}}_{2}{Z}_{4}$ monolayers is found to generate dynamically stable bilayer and bulk materials with thickness-dependent properties. When spin-orbit coupling (SOC) is included in the computations, $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ monolayers display indirect band gaps and large spin-split states at the $K$ and ${K}^{\ensuremath{'}}$ symmetry points at the corners of the Brillouin zone with nearly 100% spin polarization. The spins are locked in opposite directions along an out-of-the-plane direction at $K$ and ${K}^{\ensuremath{'}}$, leading to spin-valley coupling effects. As expected, spin polarization is absent in the pristine bilayers due to the presence of inversion symmetry, but it can be induced via an external out-of-plane electric field much like the case of $\mathrm{Mo}(\mathrm{W}){\mathrm{S}}_{2}$ bilayers. A transition from an indirect to a direct band gap can be driven by replacing N by As in $M{\mathrm{Si}}_{2}{(\mathrm{N},\mathrm{As})}_{4}$ monolayers. Our study indicates that the $M{\mathrm{Si}}_{2}{Z}_{4}$ materials can provide a viable alternative to the ${\mathrm{MoS}}_{2}$ class of 2D materials for valleytronics and optoelectronics applications.