Emergent superconductivity upon disordering a topological insulator

Abstract

We study the emergence of superconductivity in a quantum spin Hall insulator and identify a disorder-driven enhancement of pairing arising from quantum geometry. Using sign-problem-free quantum Monte Carlo simulations of the attractive Bernevig-Hughes-Zhang (BHZ) Hubbard model, we obtain a quantum phase transition as a function of interaction strength for different impurity densities. In the clean limit, the system develops bulk superconductivity for Hubbard interaction $\vert U \vert$ above a finite critical strength. Interestingly, strong impurities significantly reduce such $\vert U \vert$ required for the onset of superconductivity. Our calculations indicate that Cooper pairing first nucleates in subgap ring states surrounding the impurities and then evolves into a globally coherent superconducting phase. Our results demonstrate that impurity-generated bound states can promote superconductivity in systems with strong quantum geometry. This mechanism is expected to be relevant in nearly flat-band systems like moir\'e materials where quantum geometry plays a dominant role.