This is part of a virtual seminar series in theoretical condensed matter and atomic physics, with the aim to connect researchers working at different Nordic universities.
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 carbon structures such as ABC-stacked graphite exhibits flat bands.
In this talk I will first show how full-scale atomistic modelling of magic-angle twisted bilayer graphene generates an unexpected superconducting state. Specifically, solving self-consistently for superconductivity assuming local electronic interactions, mimicking closely those of the high-temperature cuprate superconductors, we find d-wave nematic ordering on both the atomic and moiré lattice length scales. Despite the d-wave nature, the superconducting state surprisingly has a full energy gap. These results show that the superconducting state in twisted bilayer graphene can be distinctly different from that of both monolayer graphene and the cuprate superconductors.
Then I will show how flat bands generically give rise to a universal phase diagram with doping. In particular, the critical ordering temperature follow one of two universal phase diagram curves with doping away from the flat band, depending on it being superconducting or magnetic/charge ordering. Notably, we find that superconductivity survives 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. This is consistent with the behavior of twisted bilayer graphene and, as an additional example, we illustrate how these results can be directly applied to the topological surface flat bands of rhombohedral or ABC-stacked graphite.
Zoom-link: https://nordita.org/zoom/nvcms