Conventional superconductivity in quasicrystals

Motivated by a recent experimental observation of superconductivity in the Al-Zn-Mg quasicrystal, we study the low-temperature behavior of electrons moving in the quasiperiodic potential of the Ammann--Beenker tiling in the presence of a local attraction. We employ the Bogoliubov--de Gennes approach for approximants of different sizes and determine the local pairing amplitude ${\mathrm{\ensuremath{\Delta}}}_{i}$ as well its spatial average, ${\mathrm{\ensuremath{\Delta}}}_{0}$, the superconducting order parameter. Due to the lack of periodicity of the octagonal tiling, the resulting superconducting state is inhomogeneous, but we find no evidence of the superconductivity islands, as observed in disordered systems, with ${\mathrm{\ensuremath{\Delta}}}_{i}\ensuremath{\rightarrow}0$ at ${T}_{c}$ for all sites. In the weak-coupling regime, we find that the superconducting order parameter depends appreciably on the approximant size only if the Fermi energy sits at a pseudogap in the noninteracting density of states, with ${\mathrm{\ensuremath{\Delta}}}_{0}$ decreasing as the system size increases. These results are in line with the experimental observations for the Al-Zn-Mg quasicrystal, and they suggest that, despite their electronic structure, quasicrystals are prone to display conventional BCS-like superconductivity.