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Title: Phonon or electronic pairing in rhombohedral trilayer and Bernal bilayer graphene
Plain: Graphene with three layers in staircase stacking, and ordinary two-layer graphene, superconduct at very low electron densities without any moire pattern, and it is disputed whether lattice vibrations or electron interactions cause the pairing.
Precise: Rhombohedral trilayer graphene superconducts with $T_{c}$ up to about 100 mK next to isospin-symmetry-breaking transitions (Zhou et al., Nature, 2021, doi:10.1038/s41586-021-03926-0), and Bernal bilayer graphene superconducts under in-plane magnetic field or with spin-orbit coupling induced by an adjacent WSe2 layer. Determine whether pairing is mediated by acoustic phonons (Chou et al., PRL 127, 187001, 2021) or by isospin fluctuations or Kohn-Luttinger repulsion (e.g., Chatterjee et al., Nature Communications, 2022, doi:10.1038/s41467-022-33561-w). An answer reproduces $T_{c}(n,D)$ in both systems and predicts the response to gate screening.
Settled by: $T_{c}$ measured in devices with different gate-to-sample distances (screening suppresses electronic but not phonon pairing), compared with parameter-free calculations of both mechanisms.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: Phonon and electronic theories both remain in use as of 2026.
Posed since: 2021