Electronic structure of quasicrystals studied by ultrahigh-energy-resolution photoemission spectroscopy

The results of low-temperature, ultrahigh-resolution ultraviolet photoemission studies of the electronic structure of stable icosahedral Al₆₅{Cu}₂₀Fe₁₅, Al₆₄{Cu}₂₄Fe₁₂, Al₆₅{Cu}₂₀Ru₁₅, Al₆₅{Cu}₂₀Ru7.5{Fe}7.5$, ${Al}₆₅Cu₂₀{Os}₁₅$, ${Al}₇₀Pd₂₀{Mn}₁₀$, ${Al}₇₀Pd₂₀{Cr}₅Fe₅, Al70.5{Pd}₂₁Re8.5, and Zn₆₀{Mg}₃₂Y₈, decagonal Al₆₅{Co}₁₅C₂₀ and Al₇₀{Co}₁₅Ni₁₅, and crystalline Al₇{Cu}₂$Ru and ${Ni}₂$MnAl alloys are presented. It is shown that these alloys have a clearly developed Fermi edge, and are thus metallic down to the temperature of measurement (12--45 K). A marked decrease of the spectral intensity toward the Fermi level in quasicrystals is demonstrated to be consistent with the existence of the theoretically predicted pseudogap. With an experimental resolution of 5 meV, no evidence of the theoretically predicted spikiness of the density of states could be observed. A close similarity between the values and unusual dependencies of various physical parameters observed in quasicrystals and in their approximants suggests that they are not the consequence of the long-range quasiperiodicity, but rather result from a complex local atomic order. A review of the electronic properties of quasicrystals is also presented.