Condensed Matter Virtual Seminar - https://bluejeans.com/868758165

Abstract

Graphene, an atom thick layer of carbon atoms, was first isolated in 2004. By now its electronic properties are very well known, in particular it is well known that graphene does not have any tendency to be a superconductor. However, in the last two years experimentalists have shown that a system formed by two layers of graphene, when stacked with a specific --"magic"-- relative twist angle, can become superconducting. One of the key features of twisted bilayer graphene is the fact that its electronic bands are extremely flat. The flat nature of the bands favors the formation of collective ground states. However, standard results suggest that for a system with flat bands the superconducting current should be very small and therefore that superconductivity could be observed only at vanishingly small temperatures. This is not the case in magic-angle twisted bilayer graphene. In this talk I will first introduce the key properties of "magic-angle twisted bilayer graphene". I will then discuss how it is possible for such a system to show all the hallmarks of superconductivity despite the fact that, due to the extreme flatness of the bands, results valid for standard superconductors would lead to conclude that the macroscopic signatures of superconductivity should be absent.

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References:

"Geometric and conventional contribution to superfluid weight in twisted bilayer graphene"

Xiang Hu, Timo Hyart, Dmitry I. Pikulin, Enrico Rossi

Phys. Rev. Lett, 123, 237002 (2019)