Electronic structure of icosahedral Al65Cu20Ru15 studied by photoemission spectroscopy.

Abstract

Measurements with photoemission spectroscopy in the photon-energy range 40--140 eV have been used to determine the valence band of the stable icosahedral ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{20}$${\mathrm{Ru}}_{15}$. Resonant photoemission near the Ru 4p\ensuremath{\rightarrow}4d transition has been employed to show that the feature in the valence band with the maximum intensity at 1.3(1) eV below the Fermi level is predominantly of Ru 4d character. This has been additionally verified by conducting the photoemission measurements in the constant-initial-state mode and by using the effect of the Cooper minimum in the photoionization cross section of the Ru 4d orbitals. The valence-band feature with the maximum intensity at 3.8(1) eV below the Fermi level has been shown as being due mainly to the states of Cu 3d character. The Ru 4d and Cu 3d empirical partial density of states have been determined from the photoemission spectra. The decrease of intensity towards the Fermi level has been interpreted as being indicative of the presence of a theoretically predicted pseudogap around the Fermi level. It has been indicated, however, that the Fermi cutoff also contributes to the observed intensity decrease. It has been demonstrated that the energy resolution of the spectroscopic measurements performed so far on quasicrystals was not high enough to unambiguously determine the presence of such a pseudogap. No unusual features in the valence band of icosahedral ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{20}$${\mathrm{Ru}}_{15}$, which could be ascribed to its quasiperiodic nature, have been observed within the resolution of the experiment. The need of high-energy-resolution spectroscopic measurements, which are essential in order to observe the theoretically predicted spikiness of the density of states in quasicrystals, has been emphasized. A review of published experimental data on the electronic structure of quasicrystals has also been presented.