Electronic structure of decagonal Al-Ni-Co quasicrystal studied by photoelectron spectroscopy and density functional theory

The stability of the decagonal quasicrystal $d$-Al-Ni-Co that exhibits two-dimensional quasiperiodicity is unresolved to date. Here, employing photoelectron spectroscopy performed with hard x-rays and density functional theory (DFT), we demonstrate existence of the pseudogap in the bulk electronic structure, which is associated with strong Al-transition metal bonding. However, the pseudogap in $d$-Al-Ni-Co is shallower compared to other icosahedral quasicrystals, which explains the lower resistivity of the former. The $W$-Al-Ni-Co approximant used to perform the DFT calculation represents $d$-Al-Ni-Co well, as demonstrated by the existence of a calculated pseudogap and good agreement with the overall shape of the experimental valence band. The surface electronic structure of $d$-Al-Ni-Co studied using surface-sensitive ultraviolet photoemission spectroscopy and slab model of the $W$-Al-Ni-Co approximant also show satisfactory agreement and indicate that the pseudogap is suppressed at the surface.