Double-edged Role of Interactions in Superconducting Twisted Bilayer Graphene

TL;DR

This study demonstrates that electronic interactions are crucial for superconductivity in twisted bilayer graphene, with tunable dielectric environments affecting the superconducting dome and revealing complex interaction roles.

Contribution

It provides experimental evidence that electronic interactions strongly influence superconductivity in tBLG and introduces a model involving Coulomb interaction screening mechanisms.

Findings

Superconductivity in tBLG is suppressed by increased dielectric screening.

A superconducting pocket appears at large twist angles with high dielectric constant.

The results support a Coulomb interaction-based pairing mechanism.

Abstract

For the unconventional superconducting phases in moire 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 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. By raising the dielectric constant 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 angle>1.3 deg, we observe a superconducting pocket in a large-angle (angle=1.4 deg) tBLG/STO device while the correlated insulating states are 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.