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Title: What binds electron pairs in magic-angle graphene superconductors
Plain: Superconductivity needs electrons to pair up despite repelling each other, and in magic-angle graphene it is unknown whether lattice vibrations (phonons) or the electrons' own interactions supply the attraction.
Precise: Twisted bilayer graphene at $\theta \sim 1.05-1.15\,\mathrm{deg}$ superconducts with $T_c$ up to $\sim 3\,\mathrm{K}$ near filling $\nu = -2 - \delta$ ($\nu$ = electrons per moire cell measured from charge neutrality). Determine the dominant pairing interaction (intervalley K-point optical phonons, other phonons, spin or valley fluctuations, Coulomb-driven Kohn-Luttinger pairing, or a combination) and the order-parameter symmetry. An answer is a microscopic model that reproduces the measured $T_c(\nu, \theta)$, tunnelling gap spectrum and superfluid stiffness without retuned parameters.
Settled by: A phase-sensitive gap-symmetry measurement plus an isotope or phonon-engineering test, combined with a parameter-free calculation reproducing $T_{c}(\nu, \theta)$.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: 2024 photoemission (Chen et al., Nature 636, 342) found flat-band replicas from strong coupling to a $\sim 150\,\mathrm{meV}$ K-point phonon, while 2025 superfluid-stiffness data show power-law temperature dependence pointing to unconventional pairing; no consensus as of 2026.
Posed since: 2018