“Revival of Quadratic Dirac fermions and the competition of ordered states in twisted bilayer graphe

https://arxiv.org/abs/2308.00748

Authors: Julian Ingham, Tommy Li, Mathias Scheurer, Harley Scammell

Twisted bilayer graphene (TBG) exhibits an interesting phase diagram as a function of electron density — when the number of electrons per moir\'e unit cell $\nu$ is an integer, insulating and nematic states are observed, with superconductivity appearing as $\nu$ is increased past integer values. The fourfold degenerate bands near charge neutrality feature Dirac fermions, and experiments have observed an effect known as the “Dirac revival”: at integer values of $\ nu$ the density ‘resets’ to the charge neutrality point, so the system is described by a Dirac theory even away from charge neutrality. In this talk I will describe a theoretical proposal in which these revived Dirac fermions have a small velocity, so that their dispersion is approximately quadratic in a range of energies. The quadratic Dirac fermions result in a logarithmic enhancement of interaction effects, which does not