Superconductivity in the Fibonacci Chain

Superconductivity was recently reported in several quasicrystalline systems. These are materials which are structurally ordered, but since they are not translationally invariant, the usual BCS theory does not apply. At the present time, the underlying mechanism and the properties of the superconducting phase are insufficiently understood. To gain a better understanding of quasiperiodic superconductors, we consider the attractive Hubbard model on the Fibonacci chain, and examine its low-temperature superconducting phase in detail using the Bogoliubov-de Gennes mean-field approach. We obtain superconducting solutions as a function of the parameters controlling the physical properties of the system: the strength of the Hubbard attraction U, the chemical potential μ, and the strength of the modulation of the Fibonacci Hamiltonian, w. We find that there is a bulk transition at a critical temperature that obeys a power law in U, where all sites become superconducting at once. All sites with the Fibonacci chain have the same superconducting gap width. The local superconducting order parameter is self-similar both in real and perpendicular space. The interplay between the Hubbard attraction and the intrinsic gaps of the Fibonacci chain results in a complex zero-temperature μ-U phase diagram with insulating domes surrounded by superconducting regions. Finally, we show that tuning w from weak to strong quasicrystalline modulation gives rise to qualitatively different thermodynamic behaviors as could be observed by measuring the specific heat.