Speaker: Tero Heikkilä, University of Jyväskylä, Finland
Title: Asymptotic flat band superconductivity in twisted bilayer graphene and other systems
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 . 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 . In addition, I will discuss other model systems possibly exhibiting flat-band superconductivity.
 R. Ojajärvi, T. Hyart, M.A. Silaev, and T.T. Heikkilä, arXiv:1801.01794.  N.B. Kopnin, T.T. Heikkilä, and G.E. Volovik, Phys. Rev. B 83, 220503(R) (2011).  T.T. Heikkilä and G.E. Volovik, arXiv:1504.05824.  T. Peltonen, R. Ojajärvi, and T.T. Heikkilä, arXiv:1805.01309