Independent superconductors and correlated insulators in twisted bilayer graphene

Here, it is shown that superconductivity can exist without correlated insulating states in twisted bilayer graphene devices a little away from the magic angle. This indicates the two phases compete with each other, in contrast to previous claims. When two sheets of graphene are stacked on top of each other with a small twist of angle theta approximate to 1.1 degrees between them, theory predicts the formation of a flat electronic band(1,2). Experiments have shown correlated insulating, superconducting and ferromagnetic states when the flat band is partially filled(3-8). The proximity of superconductivity to correlated insulators suggested a close relationship between these states, reminiscent of the cuprates where superconductivity arises by doping a Mott insulator. Here, we show that superconductivity can appear far away from the correlated insulating states. Although both superconductivity and correlated insulating behaviour are strongest near the flat-band condition, superconductivity survives to larger detuning of the angle. Our observations are consistent with a 'competing phases' picture in which insulators and superconductivity arise from different mechanisms.