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Flat-band superconductivity in twisted bilayer graphene and other systems

Date and Time: Tuesday, January 26, 2021, 01:30pm -
Location: Zoom

Speaker: Tero Heikkilä (University of Jyväskylä)

Our traditional theories of interacting states in electronic systems are usually based on Fermi surface systems, where the interaction induced phases are governed by energy scales far below the Fermi energy. In the case of superconductivity, this approach has lead to the celebrated MacMillan formula that helps in estimating the size of the critical temperature for superconductivity. It also shows how superconductivity usually comes with an exponentially suppressed critical temperature compared to the microscopic energy scales associated with the mechanisms for superconductivity. I will show in my talk how this approach fails in semimetals where the Fermi energy is formally zero. An example semimetal energy dispersion is that with flat bands, where the MacMillan formula has to be replaced by one containing a linear scaling between the critical temperature and the coupling constant. Hence such systems could be expected to exhibit much higher critical temperatures than ordinary metal-based systems - as long as another critical parameter, superfluid weight, can be non-vanishing. In particular, I will show how this approach helps understanding the recent experimental findings of superconductivity in twisted bilayer graphene in terms of the ordinary electron-phonon mediated mechanism, and how quantum geometry of the twisted bilayer graphene helps realizing a large superfluid weight. Using the parameters describing the superconducting state in twisted bilayer graphene, we can also make predictions for other systems with flat bands, such as graphene with periodic corrugations, recently explored in the group of Eva Andrei.

Host:  Eva Andrei

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