Coulomb-driven band unflattening suppresses $K$-phonon pairing in moir\'e graphene

TL;DR

This paper argues that Coulomb interactions in moiré graphene prevent $K$-phonon-driven superconductivity, challenging the idea that phonons alone explain the observed superconducting transition.

Contribution

It demonstrates that Coulomb-driven band unflattening suppresses $K$-phonon pairing, ruling out purely phonon-mediated mechanisms for superconductivity in moiré graphene.

Findings

Coulomb interactions weaken pairing susceptibility.

Coulomb screening reduces phonon attraction.

Pure $K$-phonon-mediated superconductivity is unlikely at 1 K.

Abstract

It is a matter of current debate whether the gate-tunable superconductivity in twisted bilayer graphene is phonon-mediated or arises from electron-electron interactions. The recent observation of the strong coupling of electrons to so-called $K$-phonon modes in angle-resolved photoemission spectroscopy experiments has resuscitated early proposals that $K$-phonons drive superconductivity. We show that the bandwidth-enhancing effect of interactions drastically weakens both the intrinsic susceptibility towards pairing as well as the screening of Coulomb repulsion that is essential for the phonon attraction to dominate at low temperature. This rules out purely $K$-phonon-mediated superconductivity with the observed transition temperature of $\sim 1$ K. We conclude that the unflattening of bands by Coulomb interactions challenges any purely phonon-driven pairing mechanism, and must be addressed by a successful theory of superconductivity in moir\'e graphene.