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 performed by the full-potential Korringa-Kohn-Rostoker Green's-function method, and also partly by the full-potential linearized augmented-plane-wave method. For the clean Fe(001) surface we demonstrate that the correct tunneling spectrum is only obtained in a full potential treatment, while the atomic-sphere approximation yields incorrect results. For $3d$ impurities in the surface layer, peaks appear in the spectra due to surfacelike states localized 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-metal impurities.