Experimental signatures of spicy local density of states in quasicrystals

Our recent investigations on the valence electronic structure of the icosahedral Al70Pd21Mn9 quasicrystals by means of scanning tunneling microscopy and spectroscopy at low temperatures (5 K) have revealed signatures of a spiky local density of states near the Fermi energy. The local tunneling spectra are characterized by narrow peaks and pseudogaps exhibiting typical energy width of 20-50 meV. The spectroscopic features are of very localized nature, with a typical spatial extent of 0.5-1 nm. This observation reanimates the discussion of the nature of the valence electronic states in aperiodic crystals, where Bloch's theorem-fundamental to the band picture in periodic crystals-cannot be applied. We observe that the spiky aspect of the density of states is only present on the nanometer scale. If series of local point spectra recorded over surface areas larger than typically 10x10 nm(2) are averaged, the multitude of local features is smeared out and lead to a rather smooth total spectrum