Scanning tunneling spectra of impurities in the Fe(001) surface

We present ab initio calculations of scanning tunneling spectra for the Fe( 001) surface and for 3d impurities in this surface. The calculations are pe rformed by the full-potential Korringa-Kohn-Rostoker Green's-function metho d, and also partly by the full-potential linearized augmented-plane-wave me thod. For the clean Fe(001) surface we demonstrate that the correct tunneli ng spectrum is only obtained in a full potential treatment, while the atomi c-sphere approximation yields incorrect results. For 3d impurities in the s urface layer, peaks appear in the spectra due to surfacelike states localiz ed on the impurity site. Our results can explain recent scanning tunneling microscopy experiments on Cr impurities in the Fe(001) surface. and predict that chemical identification is also possible for many other transition-me tal impurities.