One-dimensional confined Rashba states in a two-dimensional Si$_{2}$Bi$_{2}$ induced by vacancy line defects
Advanced defect engineering techniques have enabled the creation of unique quantum phases from pristine materials. One-dimensional (1D) atomic defects in low-dimensional systems are particularly intriguing due to their distinct quantum properties, such as 1D Rashba states that allow for the generation of nondissipative spin currents, making them ideal for spintronic devices. Using density-functional calculations and model-based symmetry analysis, we report the emergence of 1D Rashba states in a two-dimensional Si$_{2}$Bi$_{2}$ monolayer (ML) with vacancy line defects (VLDs). We show that introducing VLDs in the Si$_{2}$Bi$_{2}$ ML induces 1D confined defect states near the Fermi level, which are strongly localized along the extended defect line. Notably, we observed 1D Rashba spin-split bands in these defect states with significant spin splitting originating mainly from the strong $p-p$ coupling orbitals between Si and Bi atoms near the defect sites. These spin-split defect states exhibit perfectly collinear spin polarization in momentum $\vec{k}$-space, which is oriented perpendicularly to the VLD orientation. Moreover, using $\vec{k}\cdot\vec{p}$ perturbation theory supplemented with symmetry analysis, we show that the 1D Rashba states with collinear spin polarization are enforced by the lowering of symmetry of the VLDs into the $C_{s}$ point group, which retains the $M_{xz}$ mirror symmetry along with the 1D nature of the VLDs. The observed 1D Rashba states in this system protect carriers against spin decoherence and support an exceptionally long spin lifetime, which could be promising for developing highly efficient spintronic devices.