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Asymptotic flat band superconductivity in twisted bilayer graphene and other systems

June 8, 2018 - 11:00am
Tero Heikkilä
University of Jyväskylä, Finland

Abstract: The effective attractive interaction between electrons, mediated by electron-phonon coupling, is a well-established mechanism of conventional superconductivity. In metals exhibiting a Fermi surface, the critical temperature of superconductivity is exponentially smaller than the characteristic phonon energy. Therefore such superconductors are found only at temperatures below a few Kelvin. In contrast, electron systems with exotic energy dispersion exhibit different functional dependence on electron-phonon coupling, and can in principle host a superconducting state at much higher temperatures. We have in particular studied electron-phonon mediated superconductivity in systems with asymptotically flat bands [1]. There, the critical temperature is a linear function of the effective attractive interaction [2,3]. A particularly interesting case is that of twisted bilayer graphene. It holds an approximate flat band at certain magic twisting angles, and recent experiments ! have demonstrated the presence of superconductivity in such systems. In my talk I will discuss these findings from the perspective of flat-band superconductivity mediated by electron-phonon interaction [4]. In addition, I will discuss other model systems possibly exhibiting flat-band superconductivity.

[1] R. Ojajärvi, T. Hyart, M.A. Silaev, and T.T. Heikkilä, arXiv:1801.01794.
[2] N.B. Kopnin, T.T. Heikkilä, and G.E. Volovik, Phys. Rev. B 83, 220503(R) (2011).
[3] T.T. Heikkilä and G.E. Volovik, arXiv:1504.05824.
[4] T. Peltonen, R. Ojajärvi, and T.T. Heikkilä, arXiv:1805.01309

CMTC Conference Room (2205 Physics Toll Building)