Tunable electron interactions in moiré graphene: extinction of Mott order in the presence of screening

Magic-angle twisted bilayer graphene (TBG) is a strongly-correlated system that hosts different ordered states, notably Mott-insulating and superconducting, stabilized by the electron-electron (e-e) and electron-phonon (el-ph) interactions. Understanding the individual roles of these interactions, as well as their interplay, is a central question of the moiré graphene physics. To that end, we explore the robustness of the Mott order in the presence of a screened e-e interaction, as is the case in currently studied heterostructures comprised of TBG, hexagonal boron nitride (hBN), and graphite layers. Screening of electron charges in TBG by image charges in graphite weakens e-e interactions as the hBN spacer width decreases. The suppression of the Mott order due to enhanced screening sets in at the hBN widths on the order of the Wannier orbital radius (~10nm) and is followed by complete extinction of the Mott order. At the same time, el-ph coupling strength remains unchanged. The experiments, too, show a drastic change of the Mott insulator phase upon varying hBN width but find that the superconducting phase survives in the absence of long-range e-e interactions. Comparison of theory and experiment sheds new light on the origin of Mott order and superconductivity in the TBG system.