AQP Seminar: Superfluid weight and Berezinskii-Kosterlitz-Thouless transition temperature of twisted bilayer graphene

Aalto Quantum Physics Seminar (Nanotalo). Speaker: Aleksi Julku (QD, Aalto)

Twisted bilayer graphene (TBG) has recently gained widespread attention due to its intriguing phase diagram which consists of strongly correlated insulating and superfluid phases when two graphene sheets are twisted relative to each other by the so-called magic angles. At magic angles nearly dispersionless Bloch bands appear and, due to reduced kinetic energy, interaction processes play more prominent role than in case of dispersive bands. In this work we study superconducting properties of TBG  by utilizing Fermi-Hubbard model in the mean-field level. We consider local and non-local attractive Hubbard interactions and, by using linear response theory, compute the superfluid weight and Berezinskii-Kosterlitz-Thouless temperature for both the interaction schemes. We point out qualitative differences between local and non-local interactions and argue how these findings can be of use to experimentally characterize the pairing mechanism leading to superconductivity in TBG systems.