Remote projection and quantum mirages in STS and STM

The phenomena of remote projection and quantum mirages are investigated using standard quantum mechanics. The information inherent in delocalized wave functions in the vicinity of the Fermi level, including contributions from localized states, is available wherever the waves propagate coherently and have a non-vanishing amplitude and therefore can be probed remotely. This can explain the observation of a “quantum mirage” by Manoharan et al.: Nature 403, 512 (2000), i.e., the Kondo antiresonance due to a single adsorbed Co atom on Cu (111) far from the location of the cobalt atom. Similar quantum effects can give rise to “mirage” features in the scanning tunnelling spectrum (STS) both on clean and adsorbate-covered metal surfaces, features which are not resolved in other surface-sensitive spectroscopies (IPES, 2PPE). Within a theory based on a many-particle treatment of the tunnelling phenomena in STS and in scanning tunnelling microscopy (STM), the unexpected features in the scanning tunnelling spectra are associated with the spectral weight of transient ion-resonance states generated in the process of electron injection. They transport in a coherent way the information from the tip towards the sample and vice versa over distances of the order of 10 A or more, generating spectroscopic structures. These “mirage” states are important for the tunnelling current and the imaging properties.