paper

Dichotomy of Electron-Phonon Coupling in Graphene Moiré Flat Bands

arXiv:2103.16132 · doi:10.1103/PhysRevLett.127.167001

Abstract

Graphene moire superlattices are outstanding platforms to study correlated electron physics and superconductivity with exceptional tunability. However, robust superconductivity has been measured only in magic-angle twisted bilayer graphene (MA-TBG) and magic-angle twisted trilayer graphene (MA-TTG). The absence of a superconducting phase in certain moire flat bands raises a question on the superconducting mechanism. In this work, we investigate electronic structure and electron-phonon coupling in graphene moire superlattices based on atomistic calculations. We show that electron-phonon coupling strength lambda is dramatically different among graphene moire flat bands. The total strength lambda is very large (lambda>1) for MA-TBG and MA-TTG, both of which display robust superconductivity in experiments. However, lambda is an order of magnitude smaller in twisted double bilayer graphene (TDBG) and twisted monolayer-bilayer graphene (TMBG) where superconductivity is reportedly rather weak or absent. We find that the Bernal-stacked layers in TDBG and TMBG induce sublattice polarization in the flat-band states, suppressing intersublattice electron-phonon matrix elements. We also obtain the nonadiabatic superconducting Tc that matches well with the experimental results. Our results clearly show a correlation between strong electron-phonon coupling and experimental observations of robust superconductivity.

arXiv comment6 pages for the main text (including 4 figures and 1 table) and 3 pages for Supplemental Material

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