Record-High <i>T</i> _c and Multiple Superconducting Phase Diagrams Driven by Flat-Band Engineering in Quasi-One-Dimensional Mo ₄ S ₂ I ₆

Exploring flat-band superconductivity is of paramount importance, as it addresses key challenges in condensed matter physics, including the interplay between electronic topology, correlation effects, and superconductivity. In this work, we have unambiguously established the superconducting phase diagram in quasi-one-dimensional Mo₄S₂I₆ through the band adjustments driven by pressure and iodine vacancies. When iodine vacancies in the Mo₄S₂I_6-δ reach δ = 0.48, the superconducting pressure is significantly reduced and a record-high superconducting critical temperature (<i>T</i>_c) is achieved with 13.1 K at 2 GPa among quasi-one-dimensional materials. Theoretical calculations reveal that iodine vacancies introduce band gaps narrowing and the emergence of flat bands derived from intrachain interactions, with higher concentrations strengthening this effect, while pressure suppresses flat bands. The flat band boosts electron-electron interactions, allowing superconductivity at pressures much lower than those of defect-free crystals. Defect engineering and pressure tuning jointly advance high-temperature superconductivity in low-dimensional materials and provide critical insights into superconductivity mechanisms.