Cascade of electronic transitions in magic-angle twisted bilayer graphene

Journal Article · Thu Jun 11 00:00:00 EDT 2020 · Nature (London)

DOI:https://doi.org/10.1038/s41586-020-2339-0· OSTI ID:1780896

Wong, Dillon  ^[1]; Nuckolls, Kevin P.  ^[1]; Oh, Myungchul  ^[1]; Lian, Biao ^[2]; Xie, Yonglong  ^[3]; Jeon, Sangjun  ^[4]; Watanabe, Kenji  ^[5]; Taniguchi, Takashi ^[5]; Bernevig, B. Andrei  ^[2]; Yazdani, Ali  ^[1]

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1. Princeton Univ., NJ (United States). Joseph Henry Laboratories
2. Princeton Univ., NJ (United States). Princeton Center for Theoretical Science
3. Princeton Univ., NJ (United States). Joseph Henry Laboratories; Harvard Univ., Cambridge, MA (United States); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
4. Princeton Univ., NJ (United States). Joseph Henry Laboratories; Chung-Ang Univ., Seoul (Korea, Republic of)
5. National Institute for Material Science, Tsukuba (Japan)

Magic-angle twisted bilayer graphene exhibits a variety of electronic states, including correlated insulators, superconductors and topological phases. Understanding the microscopic mechanisms responsible for these phases requires determination of the interplay between electron–electron interactions and quantum degeneracy (the latter is due to spin and valley degrees of freedom). Signatures of strong electron–electron correlations have been observed at partial fillings of the flat electronic bands in recent spectroscopic measurements, and transport experiments have shown changes in the Landau level degeneracy at fillings corresponding to an integer number of electrons per moiré unit cell. However, the interplay between interaction effects and the degeneracy of the system is currently unclear. Here in this paper we report a cascade of transitions in the spectroscopic properties of magic-angle twisted bilayer graphene as a function of electron filling, determined using high-resolution scanning tunnelling microscopy. We find distinct changes in the chemical potential and a rearrangement of the low-energy excitations at each integer filling of the moiré flat bands. These spectroscopic features are a direct consequence of Coulomb interactions, which split the degenerate flat bands into Hubbard sub-bands. We find these interactions, the strength of which we can extract experimentally, to be surprisingly sensitive to the presence of a perpendicular magnetic field, which strongly modifies the spectroscopic transitions. The cascade of transitions that we report here characterizes the correlated high-temperature parent phase from which various insulating and superconducting ground-state phases emerge at low temperatures in magic-angle twisted bilayer graphene.

• Research Organization: Princeton Univ., NJ (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); Gordon and Betty Moore Foundation (GBMF); Princeton University; National Science Foundation (NSF)
• Grant/Contract Number: FG02-07ER46419; SC0016239; DMR12-1643312
• OSTI ID: 1780896
• Journal Information: Nature (London), Vol. 582, Issue 7811; ISSN 0028-0836
• Publisher: Nature Publishing Group
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
electronic properties and materials surfaces
interfaces and thin films