End superconductivity and three critical temperatures in Fibonacci quasicrystals

Abstract: Recently, the superconducting properties of Fibonacci quasicrystals have attracted considerable attention. By numerically solving the self-consistent Bogoliubov-de Gennes equations for a Fibonacci chain under superconducting proximity, we find that the system exhibits universal end superconductivity, where the pair potential at the chain ends can persist at higher temperatures compared to the bulk critical temperature ($T_{cb}$) of the condensate in the chain center. Furthermore, our study reveals two distinct critical temperatures at the left end ($T_{cL}$) and right end ($T_{cR}$), governing the superconducting condensate at the chain ends. This complex behavior arises from the competition between topological bound states and critical states, characteristic of quasicrystals. Due to the geometric configuration of Fibonacci chain approximants, $T_{cL}$ and $T_{cb}$ are independent of the Fibonacci sequence number $n$, while $T_{cR}$ significantly depends on the parity of $n$. With the chosen parameters, the maximal enhancement of $T_{cR}$ occurs for even $n$, reaching up to $50\%$ relative to $T_{cb}$, while $T_{cL}$ can increase by up to $23\%$. Our study sheds light on the phenomenon of end superconductivity in Fibonacci quasicrystals, pointing to alternative pathways for discovering materials with higher superconducting critical temperatures.