Phonon-mediated superconductivity in magic-strain bilayer graphene

Extensive investigations on the moiré magic angle have been conducted in twisted bilayer graphene, unlocking the mystery of unconventional superconductivity and insulating states. In analogy to the magic angle, here we demonstrate the concept of magic strain in graphene systems by judiciously tailoring mechanical relaxation (stretch and compression) which is easier to implement in practice. We elucidate the interplay of strain-induced effects and delve into the resulting unconventional superconductivity or semimetal-insulator transition in relaxation-strained graphene, going beyond the traditional twisting approach. Our findings reveal how relaxation strain can trigger superconducting transitions (with an ultraflat band at the Fermi level) or a semimetal-insulator transition (with a gap opening at the K point of 0.39 eV) in both monolayer and bilayer graphene. These discoveries open up another branch for correlated phenomena and provide deeper insights into the underlying physics of superconductors, which positions graphene as a highly tunable platform for different electronic applications.