Surface states of d character imaged by scanning tunneling microscopy

The electronic structure of NiAl(I 1 0) is measured with scanning tunneling spectroscopy at a temperature of 4 K. Reciprocal space images of surface s tates 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 wi th strong s-p character exhibit contours distributed uniformly in all direc tions over most of their energy range, while states with d character exhibi t strongly directional patterns. For these states, the dispersion is measur ed and compare favorably with results from photoemission experiments and wi th theoretical calculations. An indication of a strong interaction between sp- and d-surface states was found from both the Fourier transformed conduc tance images and their dispersion behaviors. We put forward some arguments to assign one of the FT image features to the 12 (d(xy) character) surface state, which is dipole forbidden in photoemission. (C) 2001 Elsevier Scienc e B.V. All rights reserved.