  The ability to control the strength of interaction is essential for studying
quantum phenomena emerging from a system of correlated fermions. For example,
the isotope effect illustrates the effect of electron-phonon coupling on
superconductivity, providing an important experimental support for the BCS
theory. In this work, we report a new device geometry where the magic-angle
twisted bilayer graphene (tBLG) is placed in close proximity to a Bernal
bilayer graphene (BLG) separated by a 3 nm thick barrier. 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 effect on the
insulating and superconducting states: as Coulomb interaction is weakened by
screening, the insulating states become less robust, whereas the stability of
superconductivity is enhanced. Out results demonstrate the ability to directly
probe the role of Coulomb interaction in magic-angle twisted bilayer graphene.
Most importantly, the effect of Coulomb screening points toward electron-phonon
coupling as the dominant mechanism for Cooper pair formation, and therefore
superconductivity, in magic-angle twisted bilayer graphene.
