Rashba Spin-Splitting Control at the Surface of the Topological Insulator Bi$_2$Se$_3$

The electronic structure of Bi$_2$Se$_3$ is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultra-high-vacuum conditions, can be overcome via {\it in situ} potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. {\it Ab initio} slab calculations reveal that these Rashba states are derived from the 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface. Phys.Rev.Lett. 107 186405 (2011)