ELECTRONIC-STRUCTURE OF ICOSAHEDRAL ALLOYS - THE CASE OF AL65CU20OS15

Measurements with photoemission spectroscopy in the photon-energy rang e 35-130 eV have been used to determine the valence band of the stable icosahedral Al65Cu20Os15. Resonant photoemission near the Os 5p --> 5 d and 4f --> 5d transitions has been employed to show that the feature in the valence band with the maximum intensity at 1.5 (1) eV below th e Fermi level is predominantly of the Os 5d character. This has been a dditionally verified by conducting the photoemission measurements in t he constant-initial-state mode and by using the effect of the Cooper m inimum in the photoionization cross section of the Os 5d orbitals. The valence band feature with the maximum intensity at 3.7 (1) eV below t he Fermi level has been shown as being due mainly to the states of the Cu 3d character. The Os 5d and Cu 3d empirical partial density of sta tes have been determined from the photoemission spectre. The decrease of intensity towards the Fermi level has been interpreted as being ind icative of the presence of a theoretically predicted pseudogap around the Fermi level. It has been indicated, however, that the Fermi cut-of f also contributes to the observed intensity decrease. It has been dem onstrated that the energy resolution of the spectroscopic measurements performed so far on quasicrystals was not high enough to unambiguousl y determine the presence of such a pseudogap. No unusual features in t he valence band of icosahedral Al65Cu20Os15, which could be ascribed t o its quasiperiodic nature, have been observed within the resolution o f the experiment. High energy-resolution spectroscopic measurements we re also shown to be essential to observe the theoretically predicted s pikiness of the density of states in quasicrystals. A critical review of published spectroscopic data on the electronic structure of quasicr ystals has also been presented.