Origin of giant Rashba spin splitting in Bi/Ag surface alloys

One-third of a monolayer of Bi alloyed into the Ag(111) surface yields a pair of Rashba spin-split free-electron-like surface states. The splitting in momentum space is the largest of all surface alloys investigated. Using first-principles calculations, we have determined the spin splitting in this system as a function of atomic corrugation of the top atomic layer and the strengths of the atomic spin-orbit coupling in Bi and Ag. The calculated splitting is proportional to the strengths of the atomic spin-orbit coupling, but it peaks at a certain surface corrugation parameter. These findings indicate that the observed giant spin splitting is caused by a near-optimal surface corrugation and a large atomic spin-orbit coupling in Bi. The results offer a useful guide for searching for two-dimensional systems with large surface spin effects.