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Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp

Lewandowski, Cyprian and Chowdhury, Debanjan and Ruhman, Jonathan (2020) Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp. . (Unpublished)

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Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a non-trivial structure of the underlying Bloch wavefunctions. We perform a systematic theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, N≫1, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving mini-optical phonon modes, which arise physically as a result of the folding of the original acoustic branch of graphene due to the moiré superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. We propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance and also discuss experimental implications of a pairing mechanism relying on plasmons.

Item Type:Report or Paper (Discussion Paper)
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Additional Information:C.L. acknowledges support from the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319, and from the Gordon and Betty Moore Foundation through Grant GBMF8682. D.C. is supported by a faculty startup grant at Cornell University. J.R. acknowledges funding by the Israeli Science Foundation under grant number 994/19.
Funding AgencyGrant Number
Gordon and Betty Moore FoundationGBMF8682
Cornell UniversityUNSPECIFIED
Israeli Science Foundation994/19
Record Number:CaltechAUTHORS:20200824-134627257
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105079
Deposited By: Tony Diaz
Deposited On:24 Aug 2020 22:33
Last Modified:24 Aug 2020 22:33

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