Electronic structure of decagonal Al65Co15Cu20 and Al70Co15Ni15.

Abstract

Photoemission spectroscopy measurements in the photon-energy range 35--120 eV have been used to make determinations of the valence bands of high-quality decagonal alloys ${\mathrm{Al}}_{65}$${\mathrm{Co}}_{15}$${\mathrm{Cu}}_{20}$ and ${\mathrm{Al}}_{70}$${\mathrm{Co}}_{15}$${\mathrm{Ni}}_{15}$. Resonance photoemission near the Co 3p\ensuremath{\rightarrow}3d transition has been employed to show that the feature in the valence bands of both alloys with a maximum intensity at 0.7(2) eV below the Fermi level is predominantly of the Co 3p character. The feature at 3.7(1) eV in the valence band of ${\mathrm{Al}}_{65}$${\mathrm{Co}}_{15}$${\mathrm{Cu}}_{20}$ has been identified as being mainly of the Cu 3d character, whereas the feature due to the Ni 3d states overlaps with the Co 3d-like feature in the valence band of ${\mathrm{Al}}_{70}$${\mathrm{Co}}_{15}$${\mathrm{Ni}}_{15}$. Within the energy resolution, our results reveal that, contrary to the prediction of recent band-structure calculations and to the widespread qualitative interpretation of various electronic transport data, there is no pseudogap in the density of states at the Fermi level in both decagonal alloys. This shows that decagonal alloys differ significantly in their electronic structure from icosahedral alloys and that a Hume-Rothery mechanism cannot be invoked to explain their stability and electronic transport properties. The need of high-energy-resolution photoemission experiments, which were shown to be essential to observe the theoretically predicted fine structure in the density of states, has been emphasized. The possible influence of chemical and topological disorder on the electronic structure of high-quality stable quasicrystals was indicated. A review of published experimental data on the electronic structure of decagonal alloys has also been presented.