Surface states of d character imaged by scanning tunneling microscopy

The electronic structure of NiAl(1 1 0) is measured with scanning tunneling spectroscopy at a temperature of 4 K. Reciprocal space images of surface states are obtained by Fourier transformation (FT) of wave patterns created by defects in differential conductance images. Each surface state exhibits a different wave vector pattern in 2-dimensional k-space. Surface states with strong s–p character exhibit contours distributed uniformly in all directions over most of their energy range, while states with d character exhibit strongly directional patterns. For these states, the dispersion is measured and compare favorably with results from photoemission experiments and with theoretical calculations. An indication of a strong interaction between sp- and d-surface states was found from both the Fourier transformed conductance images and their dispersion behaviors. We put forward some arguments to assign one of the FT image features to the Σ2 (dxy character) surface state, which is dipole forbidden in photoemission.