Theory of Pseudogap Effects in Magic Angle Twisted Bilayer Graphene

In this talk we discuss the signatures of the pseudogap effects seen in transport studies of superconducting magic angle twisted bilayer graphene (MATBG). They should be viewed as rather natural; quite generally pseudogap phenomena are widely observed in very thin superconducting films of conventional materials such as Pb and TiN, high $T_c$ cuprate superconductors and in interfacial superconductors. The pseudogap regime is characterized by two distinct temperature scales corresponding to the onset of phase coherence, i.e. $T_{BKT}$ and the onset of pairing, i.e. $T^{\ast}$. Here we develop the pairing fluctuation theory of the Berezinskii-Kosterlitz-Thouless (BKT) transition for the MATBG, demonstrating how pseudogap effects are enhanced in two dimensional superconductors. Moreover, they do not require anomalously strong attractive pairing mechanisms as proposed for the 3D cuprates. Using our theory along with the transport estimates for the pairing onset temperature $T^{\ast}$, we quantify the expected strength of the pairing and fluctuations for the MATBG superconductors. Finally, we propose new tunneling based experiments from one two dimensional system to another, which can further probe the pseudogap phenomena.