Ideal two-dimensional systems with a gain Rashba-type spin splitting: SrFBiS 2 and BiOBiS 2 nanosheets

The Rashba effect is increasingly seen as the key ingredient in the burgeoning field of spintronics. However, the lack of two-dimensional materials with a large Rashba effect hinders its practical applications. Here, with density functional theory, we report giant Rashba-type spin splitting in SrFBiS2 and BiOBiS2 nanosheets originating from their hidden local polar atomic configurations. Owing to the weak interactions between the two BiS2 layers, they hold two individual Rashba spin splittings belonging to two opposite BiS2 layers, which are degenerated because of their inversion symmetry. As a result, with small perturbations, such as a tiny electric field, we observe two sets of Rashba spin splittings in both materials; this is different from most of the previous studied Rashba systems, in which there is only one set of Rashba spin splitting. More interestingly, the strength of the Rashba effect in both nanosheets can be preserved while their bandgap can be effectively tuned by applying an external electric field.