Double-edged Role of Interactions in Superconducting Twisted Bilayer Graphene

For the unconventional superconducting phases in moiré materials, a critical question is the role played by electronic interactions in the formation of Cooper pairs. In twisted bilayer graphene (tBLG), the strength of electronic interactions can be reduced by increasing the twist angle q or screening provided by the dielectric medium. In this work, we place tBLG at 3-4 nm above bulk SrTiO₃ substrates, which have a large yet tunable dielectric constant ε. By raising ε in situ in a magic angle device, we observe suppression of both the height and the width of the entire superconducting dome, thus demonstrating that, unlike conventional superconductors, the pairing mechanism in tBLG is strongly dependent on electronic interactions. Interestingly, in contrast to the absence of superconductivity in devices on SiO₂ with θ > 1.25°, we observe a small superconducting pocket in a large-angle (θ = 1.4°) tBLG/STO device while the correlated insulating states are completely absent. These experimental results are in qualitative agreement with a theoretical model in which the pairing mechanism arises from Coulomb interactions that are screened by plasmons, electron-hole pairs, and longitudinal acoustic phonons. Our results highlight the unconventional nature of the superconductivity in tBLG, the double-edged role played by electronic interactions in its formation, as well as their complex interplay with the correlated insulating states.