Protected valley states and generation of valley- and spin-polarized current in monolayer MA2Z4

The optical selection rules obeyed by two-dimensional materials with spin-valley coupling enable the selective excitation of carriers. We show that several members of the monolayer ${MA}_{2}{Z}_{4}$ ($M=\mathrm{Mo}$ and $\phantom{\rule{4pt}{0ex}}W;\phantom{\rule{0.28em}{0ex}}A=\mathrm{C}$, Si, and Ge; $Z=\mathrm{N}$, P, and As) family are direct band-gap semiconductors with protected valley states and that circularly polarized infrared light can induce valley-selective interband transitions. Therefore, they are able to generate a close to 100% valley- and spin-polarized current under an in-plane bias and circularly polarized infrared light, which can be exploited to encode, process, and store information.