Electronic structure of icosahedral ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{20}$${\mathrm{Fe}}_{15}$ and crystalline ${\mathrm{Al}}_7$${\mathrm{Cu}}_2$Fe studied by photoemission spectroscopy

The electronic structure properties of stable icosahedral ${\mathrm{Al}}_{65}$ ${\mathrm{Cu}}_{20}$ ${\mathrm{Fe}}_{15}$ and crystalline ${\mathrm{Al}}_7$ ${\mathrm{Cu}}_2$Fe are studied with synchrotron-radiation-based photoemission spectroscopy for photon energies between 40 and 160 eV. The valence-band spectra of both alloys do not change significantly with photon energy and their structure is essentially identical. The feature at 0.7 eV below the Fermi level is shown, by employing resonant photoemission near the Fe 3p threshold, to be due mainly to Fe 3d states, and the feature at 4.0 eV originates primarily from Cu 3d states. No unusual features in the valence band of the icosahedral ${\mathrm{Al}}_{65}$ ${\mathrm{Cu}}_{20}$ ${\mathrm{Fe}}_{15}$ alloy, which could be associated with its quasicrystalline nature, are observed within the energy resolution of the experiment. A rapid decrease of the intensity towards the Fermi level is interpreted to be indicative of the existence of a minimum in the density of states at the Fermi level. It is argued that the similar electronic structure of the alloys supports a recently proposed structural model of icosahedral ${\mathrm{Al}}_{65}$ ${\mathrm{Cu}}_{20}$ ${\mathrm{Fe}}_{15}$. A review of published experimental data on the electronic structure of quasicrystals and a survey of various theoretical analyses are also presented.