Strong electron-phonon coupling, electron-hole asymmetry, and nonadiabaticity in magic-angle twisted bilayer graphene

TL;DR

This study reveals strong, nearly isotropic electron-phonon coupling in magic-angle twisted bilayer graphene, with significant electron-hole asymmetry and nonadiabatic effects, potentially influencing its superconducting properties.

Contribution

It provides the first atomistic calculations of electron-phonon coupling in MA-TBG, showing large lambda values and nonadiabatic effects, advancing understanding of its superconductivity mechanisms.

Findings

Lambda exceeds 1 near flat bands at half-filling

Electron-hole asymmetry significantly affects coupling strength

Phonon energies are much larger than electronic energies, indicating nonadiabaticity

Abstract

We report strong electron-phonon coupling in magic-angle twisted bilayer graphene (MA-TBG) obtained from atomistic description of the system including more than 10000 atoms in the moire supercell. Electronic structure, phonon spectrum, and electron-phonon coupling strength lambda are obtained before and after atomic-position relaxation both in and out of plane. Obtained lambda is very large for MA-TBG, with lambda > 1 near the half-filling energies of the flat bands, while it is small (lambda ~ 0.1) for monolayer and unrotated bilayer graphene. Significant electron-hole asymmetry occurs in the electronic structure after atomic-structure relaxation, so lambda is much stronger with hole doping than electron doping. Obtained electron-phonon coupling is nearly isotropic and depends very weakly on electronic band and momentum, indicating that electron-phonon coupling prefers single-gap s-wave superconductivity. Relevant phonon energies are much larger than electron energy scale, going far beyond adiabatic limit. Our results provide a fundamental understanding of the electron-phonon interaction in MA-TBG, highlighting that it can contribute to rich physics of the system.