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IQIM Postdoctoral and Graduate Student Seminar

Friday, October 23, 2020
11:00am to 12:00pm
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Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp
Cyprian Lewandowski, Moore Postdoctoral Scholar in Theoretical Physics,

Abstract: 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. In this talk, I will discuss a 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 will 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, I will 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. Time permitting, I will also consider 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. Finally, I will propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance and also discuss several experimental implications of a pairing mechanisms relying on plasmons.

The talk will follow a joint work with Debanjan Chowdhury and Jonathan Ruhman, available at arXiv:2007.15002.

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Talks will also be posted on IQIM's YouTube page

For more information, please contact Marcia Brown by phone at 626-395-4013 or by email at