Competing Zero-Field Chern Insulators in Superconducting Twisted Bilayer Graphene
Petr Stepanov, Ming Xie, Takashi Taniguchi, Kenji Watanabe, Xiaobo Lu, Allan H. MacDonald, B. Andrei Bernevig, Dmitri K. Efetov
Abstract
The discovery of magic angle twisted bilayer graphene has unveiled a rich variety of superconducting, magnetic, and topologically nontrivial phases. Here, we show that the zero-field states at odd integer filling factors in h-BN nonaligned devices are consistent with symmetry broken Chern insulators, as is evidenced by the observation of the anomalous Hall effect near moiré cell filling factor ν=+1. The corresponding Chern insulator has a Chern number C=±1 and a relatively high Curie temperature of T_{c}≈4.5 K. In a perpendicular magnetic field above B>0.5 T we observe a transition of the ν=+1 Chern insulator from Chern number C=±1 to C=3, characterized by a quantized Hall plateau with R_{yx}=h/3e^{2}. These observations demonstrate that interaction-induced symmetry breaking leads to zero-field ground states that include almost degenerate and closely competing Chern insulators, and that states with larger Chern numbers couple most strongly to the B field. In addition, the device reveals strong superconducting phases with critical temperatures of up to T_{c}≈3.5 K. By providing the first demonstration of a system that allows gate-induced transitions between magnetic and superconducting phases, our observations mark a major milestone in the creation of a new generation of quantum electronics.