Emergence of Rashba spin valley state in two-dimensional strained bismuth oxychalcogenides Bi${}_2$O${}_2$Se

Abstract

The experimental evidence of the ultra-high electron mobility and strong spin-orbit coupling in the two-dimensional (2D) layered bismuth-based oxyselenide, Bi${}_2$O${}_2$Se, makes it a potential material for spintronic devices. However, its spin-related properties have not been extensively studied due to the centrosymmetric nature of its crystal structure. By using first-principles density-functional theory calculation, this study reports the emergence of Rashba-spin-valley states in Bi${}_2$O${}_2$Se monolayer (ML). Breaking the crystal inversion symmetry of Bi${}_2$O${}_2$Se ML using an external electric field enables the Rashba-spin-valley formation, causing the appearance of the Rashba-type splitting around the $\Gamma$ valley and spin-valley coupling at the $D$ valleys located near the middle of $\Gamma -M$ line. In addition to the typical Rashba-type spin textures around the $\Gamma$ valley, the study also observed in-plane unidirectional spin textures around the $D$ valleys, which is a rare phenomenon in 2D materials. The observed Rashba-spin-valley states are driven by the lowering point group symmetry of the crystal from $D_{4h}$ to $C_{4v}$ enforced by the electric field, as clarified through $\vec{k}\cdot \vec{p}$ model derived from symmetry analysis. More importantly, tuning the Rashba and spin-valley states by using biaxial strain offers a promising route to regulate the spin textures and spin splitting preventing the electron from back-scattering in spin transport. Finally, we proposed a more realistic system, namely, Bi${}_2$O${}_2$Se ML/SrTiO${}_3$ (001) heterointerface that supports the strong Rashba-spin-valley states and highlighting the potential of the Bi${}_2$O${}_2$Se ML for future spintronics and valleytronics-based devices.

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