Theory of STM and STS on p(2x2)Na/Al(111)

Scattering theory and Green function theory is used to study scanning tunneling microscopy (STM) and spectroscopy (STS) on p(2 x 2)Na/Al(111). The surface unit cell has two different Na and four Al atoms, however, in STM experiments in the constant current mode a single protrusion is observed per surface unit cell. The theory demonstrates that at relevant rip-sample separations it is due to me Na atoms in the fee position of the topmost intermixed Na-Al layer. The density of states induced by the sodium atoms near the Fermi level is the reason for the enhanced tunneling current in the vicinity of Na-fcc. At smaller tunneling distances image inversion is observed due to the tip-sample interaction. STS has been studied in the voltage range from -5 V to +1 V. Features on the clean Al(111) surface are observed at similar bias voltages as in the STS experiments, however, they are attributed to bulk band structure features and not to surface states. Within the voltage range studied STS on p(2 x 2)Na/Al(111) yields a sodium induced peak at the Fermi edge.