Condensed Matter Seminar Series

Annica Black-Schaffer

Uppsala University, Sweden

Flat bands and superconductivity in graphene-based systems

Materials with flat energy bands close to the Fermi level often exhibit extraordinarily high critical ordering temperatures for symmetry breaking orders. The currently most studied example is likely magic-angle twisted bilayer graphene, where both superconductivity and other correlated phases appear, but also simpler graphene-bases systems, such as ABC-multilayer graphene hosts flat bands.

In this talk I will start by showing generic results on how a flat band close to the Fermi level gives rise to a universal phase diagram with doping. Notably, we find superconductivity surviving to decisively higher doping, and thus, even if a magnetic or charge order initially dominates, superconducting domes are still likely to exist on the flanks of flat bands. I will then show results for the superconducting state in magic-angle twisted bilayer graphene. Using full-scale atomistic modelling and self-consistently solving for electron-driven superconductivity, we find a d-wave nematic order on both the atomic and moiré lattice length scales. These results show that the superconducting state in twisted bilayer graphene is distinctly different from that of both the cuprate superconductors and monolayer graphene. Finally, if time permits, I will show how the surface flat bands in ABC-stacked multilayer graphene can host both enhanced spin-triplet f-wave superconductivity and ferrimagnetism. Favoring of a fully gapped spin-triplet f-wave state is in contrast to other graphene systems, a behavior we trace back to a strong sublattice polarization.

Together these results illustrate how graphene-based systems with flat bands can easily host superconductivity, but they also highlight the importance of incorporating the detailed properties of the normal state when studying superconductivity.