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Critical magnetic fields and electron-pairing in magic-angle twisted bilayer graphene

The velocities of the quasiparticles that form Cooper pairs in a superconductor are revealed by the upper critical magnetic field. Here we use this property to assess superconductivity in magic-angle twisted bilayer graphene (MATBG), which has been observed over a range of moiré band filling, twist angle, and screening environment conditions. We find that for pairing mechanisms that are unrelated to correlations within the MATBG flat bands, minima in an average Fermi velocity $v_F^* \equiv k_B T_c \ell_c /\hbar $, where $\ell_c$ is the magnetic length at the critical perpendicular magnetic field, are always coincident with transition temperature maxima. Both extrema occur near flat-band van Hove singularities. Since no such association is present in MATBG experimental data, we conclude that electronic correlations that yield a band-filling-dependent pairing glue must play a crucial role in MATBG superconductivity.