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Abstract

Quasicrystals are metallic alloys that possess perfect long-range structural order, in spite of the fact that their rotational symmetries are incompatible with long-range periodicity. The exotic structural properties of this class of materials¹ are accompanied
by physical properties that are unexpected for metallic alloys. Considerable progress in resolving the geometric structures of
quasicrystals has been made using X-ray and neutron diffraction, and concepts such as the quasi-unit-cell model² have provided theoretical insights. But the basic properties of the valence electronic states - whether they are extended as in periodic crystals or localized as in amorphous materials - are still largely unresolved³. Here we investigate the electronic bandstructure of
quasicrystals through angle-resolved photoemission experiments on decagonal Al_71.8Ni_14.8Co_13.4. We find that the s-p and d states exhibit band-like behaviour with the symmetry of the quasiperiodic
lattice, and that the Fermi level is crossed by dispersing d-bands. The observation of free-electron-like bands, distributed in momentum space according to the surface diffraction
pattern, suggests that the electronic states are not dominated by localization.