Enhanced surface metallic density of states in icosahedral quasicrystals

Valence and core-level photoemission spectra show that cleaved $i$-Al-Pd-Mn quasicrystals have a pronounced metallic signature. Spectra from states above $E_F,$ populated by increasing the sample temperature, indicate the presence of a shallow pseudogap 0.09 eV above $E_F.$ The Al $2p$ line asymmetry, on the other hand, which indicates metallic behavior, decreases with increasing photoelectron escape depth. This implies a decreasing density of states at $E_F$ with increasing distance from the surface, consistent with indications that the density of states near $E_F$ in the bulk is reduced. Our results thus help to resolve the apparent contradiction between the theoretical predictions for a bulk pseudogap, and the clear metallic Fermi edges in photoemission.