Quasicrystalline valence bands in decagonal AlNiCo

Quasicrystals are metallic alloys that possess perfect long-range structura l order, in spite of the fact that their rotational symmetries are incompat ible with long-range periodicity. The exotic structural properties of this class of materials(1) are accompanied by physical properties that are unexp ected for metallic alloys. Considerable progress in resolving the geometric structures of quasicrystals has been made using X-ray and neutron diffract ion, and concepts such as the quasi-unit-cell model(2) have provided theore tical insights. But the basic properties of the valence electronic states-w hether they are extended as in periodic crystals or localized as in amorpho us materials-are still largely unresolved(3). Here we investigate the elect ronic bandstructure of quasicrystals through angle-resolved photoemission e xperiments on decagonal Al71.8Ni14.8Co13.4. We rnd that the s-p and d state s exhibit band-like behaviour with the symmetry of the quasiperiodic lattic e, and that the Fermi level is crossed by dispersing d-bands. The observati on of free-electron-like bands, distributed in momentum space according to the surface diffraction pattern, suggests that the electronic states are no t dominated by localization.