Tuning electron correlation in magic-angle twisted bilayer graphene using Coulomb screening

Tuning the interactions Elucidating the nature of the superconducting state in magic-angle twisted bilayer graphene (MATBG) has proven tricky. To study the role of electron-electron correlations in this state, Liu et al. placed another graphene bilayer, this one having a conventional arrangement of the graphene sheets, in the immediate vicinity of a sample of MATBG. By varying the carrier density in the conventional bilayer, the researchers controlled the strength of interactions in MATBG. Weakening the interactions strengthened superconductivity, consistent with scenarios in which the electron-phonon coupling competes against Coulomb interactions to stabilize the superconducting phase. Science, this issue p. 1261 A hybrid double-layer structure is used to probe the nature of superconductivity in twisted bilayer graphene. Controlling the strength of interactions is essential for studying quantum phenomena emerging in systems of correlated fermions. We introduce a device geometry whereby magic-angle twisted bilayer graphene is placed in close proximity to a Bernal bilayer graphene, separated by a 3-nanometer-thick barrier. By using charge screening from the Bernal bilayer, the strength of electron-electron Coulomb interaction within the twisted bilayer can be continuously tuned. Transport measurements show that tuning Coulomb screening has opposite effects on the insulating and superconducting states: As Coulomb interaction is weakened by screening, the insulating states become less robust, whereas the stability of superconductivity at the optimal doping is enhanced. The results provide important constraints on theoretical models for understanding the mechanism of superconductivity in magic-angle twisted bilayer graphene.