Spin orientation and sign of the Rashba splitting in Bi/Cu(111)

Whereas the magnitude of the Rashba spin splitting at surfaces has been studied in detail, less is known about its sign which determines the precise spin orientation of the electronic states. We investigate the microscopic origin of this sign by spin-resolved photoemission experiments and first-principles calculations on the strongly spin-orbit coupled surface states in Bi/Cu(111). We conclude that the sign of the Rashba splitting is determined by the particular charge asymmetry near the atomic cores. The precise spin orientation on heavy-element surfaces thus can comprise information on wave-function localizations and related aspects otherwise hardly accessible by other experimental methods.