Experimental detection of local orbital angular momentum and its role …

Angle resolved photoelectron spectroscopy can be utilized to observe local orbital angular momentum (OAM). Local OAM in strongly spin-orbit coupled materials is found to form a chiral structure. It is proposed that local OAM on the surfaces of high-Z materials plays a crucial role in the formation of Rashba-type surface band splitting. Local OAM state in a Bloch state produces asymmetric charge distribution (electric dipole). Surface-normal electric field aligns the electric dipole and results in chiral OAM states and the relevant Rashba- type splitting. Therefore, the band splitting originates from electric dipole interaction, not from the Zeeman splitting as proposed in the original Rashba picture. The characteristic spin chiral structure of Rashba states is formed through the spin-orbit coupling and thus is a secondary effect to the chiral OAM. Results from first principles calculations on a single Bi layer under an external electric field verify predictions of the new model, including the direction of the spin texture which is predicted to be in opposite direction to the conventional Rashba picture. We extend our understanding to general spin-orbit coupling case and apply it to the well-known surface states of Cu and Au. Evolution of the angular momentum structures as a function of the spin-orbit coupling is discussed.