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Interaction-driven Band Flattening and Correlated Phases in Twisted Bilayer Graphene

Choi, Youngjoon and Kim, Hyunjin and Lewandowski, Cyprian and Peng, Yang and Thomson, Alex and Polski, Robert and Zhang, Yiran and Watanabe, Kenji and Taniguchi, Takashi and Alicea, Jason and Nadj-Perge, Stevan (2021) Interaction-driven Band Flattening and Correlated Phases in Twisted Bilayer Graphene. . (Unpublished) https://resolver.caltech.edu/CaltechAUTHORS:20210302-094400352

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Abstract

Flat electronic bands, characteristic of magic-angle twisted bilayer graphene (TBG), host a wealth of correlated phenomena. Early theoretical considerations suggested that, at the magic angle, the Dirac velocity vanishes and the entire width of the moiré bands becomes extremely narrow. Yet, this scenario contradicts experimental studies that reveal a finite Dirac velocity as well as bandwidths significantly larger than predicted. Here we use spatially resolved spectroscopy in finite and zero magnetic fields to examine the electronic structure of moiré bands and their intricate connection to correlated phases. By following the relative shifts of Landau levels in finite fields, we detect filling-dependent band flattening, that unexpectedly starts already at ~1.3 degrees, well above the magic angle and hence nominally in the weakly correlated regime. We further show that, as the twist angle is reduced, the moiré bands become maximally flat at progressively lower doping levels. Surprisingly, when the twist angles reach values for which the maximal flattening occurs at approximate filling of −2, +1,+2,+3 electrons per moiré unit cell, the corresponding zero-field correlated phases start to emerge. Our observations are corroborated by calculations that incorporate an interplay between the Coulomb charging energy and exchange interactions; together these effects produce band flattening and hence a significant density-of-states enhancement that facilitates the observed symmetry-breaking cascade transitions. Besides emerging phases pinned to integer fillings, we also experimentally identify a series of pronounced correlation-driven band deformations and soft gaps in a wider doping range around ±2 filling where superconductivity is expected. Our results highlight the role of interaction-driven band-flattening in forming robust correlated phases in TBG.


Item Type:Report or Paper (Discussion Paper)
Related URLs:
URLURL TypeDescription
http://arxiv.org/abs/2102.02209arXivDiscussion Paper
ORCID:
AuthorORCID
Kim, Hyunjin0000-0001-9886-0487
Lewandowski, Cyprian0000-0002-6944-9805
Peng, Yang0000-0002-8868-2928
Thomson, Alex0000-0002-9938-5048
Polski, Robert0000-0003-0887-8099
Zhang, Yiran0000-0002-8477-0074
Watanabe, Kenji0000-0003-3701-8119
Taniguchi, Takashi0000-0002-1467-3105
Alicea, Jason0000-0001-9979-3423
Nadj-Perge, Stevan0000-0002-2916-360X
Additional Information:Attribution 4.0 International (CC BY 4.0). We acknowledge discussions with Francisco Guinea, Felix von Oppen, and Gil Refael. This work has been primarily supported by NSF grants DMR-2005129 and DMR-172336; and Army Research Office under Grant Award W911NF17-1-0323. Part of the STM characterization has been supported by NSF CAREER program (DMR-1753306). Nanofabrication efforts have been in part supported by DOE-QIS program (DE-SC0019166). S.N-P. acknowledges support from the Sloan Foundation. J.A. and S.N.-P. also acknowledge support of the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation through Grant GBMF1250; C.L. acknowledges support from the Gordon and Betty Moore Foundation’s EPiQS Initiative, Grant GBMF8682. A.T. and J.A. are grateful for the support of the Walter Burke Institute for Theoretical Physics at Caltech. Y.P. acknowledges support from the startup fund from California State University, Northridge. Y.C. and H.K. acknowledge support from the Kwanjeong fellowship. Author Contribution: Y.C. and H.K. fabricated samples with the help of R.P. and Y.Z., and performed STM measurements. Y.C., H.K., and S.N.-P. analyzed the data. C.L. and Y.P. implemented TBG models. C.L., Y.P., and A.T. provided theoretical analysis of the model results supervised by J.A. S.N.-P. supervised the project. Y.C., H.K., C.L., Y.P., A.T., J.A., and S.N.-P. wrote the manuscript with input from other authors. Data availability: The data that support the findings of this study are available from the corresponding authors on reasonable request.
Group:Institute for Quantum Information and Matter, Walter Burke Institute for Theoretical Physics
Funders:
Funding AgencyGrant Number
NSFDMR-2005129
NSFDMR-172336
Army Research Office (ARO)W911NF17-1-0323
NSFDMR-1753306
Department of Energy (DOE)DE-SC0019166
Alfred P. Sloan FoundationUNSPECIFIED
Institute for Quantum Information and Matter (IQIM)UNSPECIFIED
Gordon and Betty Moore FoundationGBMF1250
Gordon and Betty Moore FoundationGBMF8682
Walter Burke Institute for Theoretical Physics, CaltechUNSPECIFIED
California State University, NorthridgeUNSPECIFIED
Kwanjeong Graduate FellowshipUNSPECIFIED
Record Number:CaltechAUTHORS:20210302-094400352
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20210302-094400352
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108266
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:02 Mar 2021 18:25
Last Modified:02 Mar 2021 18:25

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