Untying the insulating and superconducting orders in magic-angle graphene

• Published in: Nature (London) Vol. 583; no. 7816; pp. 375 - 378
• Main Authors: Stepanov, Petr, Das, Ipsita, Lu, Xiaobo, Fahimniya, Ali, Watanabe, Kenji, Taniguchi, Takashi, Koppens, Frank H. L., Lischner, Johannes, Levitov, Leonid, Efetov, Dmitri K.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.07.2020; Nature Publishing Group
• Subjects: 639/766/119/1003; 639/766/119/2795; 639/925/918/1052; Bilayers; Coexistence; Correlation; Dielectrics; Electric properties; Graphene; Graphite; Humanities and Social Sciences; Insulators; Magnetic fields; Magnetic properties; multidisciplinary; Science; Science (multidisciplinary); Screening; Superconductivity; Spain
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-020-2459-6

The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene 1 – 11 prompts fascinating questions about their relationship. Independent control of the microscopic mechanisms that govern these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing the separation distance between the graphene and a metallic screening layer 12 , 13 . We observe quenching of correlated insulators in devices with screening layer separations that are smaller than the typical Wannier orbital size of 15 nanometres and with twist angles that deviate slightly from the magic angle of 1.10 ± 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 tesla, giving rise to quantized Hall states with a Chern number of 2. Our study suggests re-examination of the often-assumed ‘parent-and-child’ relation between the insulating and superconducting phases in moiré graphene, and suggests a way of directly probing the microscopic mechanisms of superconductivity in strongly correlated systems. Tuning the electronic interactions by changing the dielectric environment of twisted bilayer graphene reveals the disappearance of the insulating states and their replacement by superconducting phases, suggesting a competition between the two phases.