Condensed Matter Seminar: Interacting electrons in twisted bilayer graphene

Twisted bilayer graphene (tBLG) with interlayer twist angles near the magic angle 1.08º hosts flat bands and exhibits correlated states including Mott-like insulators, superconductivity and magnetism. We will discuss our recent measurements on a tBLG device with a twist angle of 0.93º, below the magic angle. We find in addition to Mott-like insulator and superconducting states near half filling, evidence for a novel correlated state at five electrons/moiré unit cell. Our results reveal that the magic range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG. Moreover, we will discuss combined temperature-dependent transport measurements of the longitudinal and Hall resistivities in close to magic-angle tBLG. While the observed longitudinal resistivity follows linear temperature T dependence consistent with previous reports, the Hall resistance shows an anomalous T dependence with the cotangent of the Hall angle . Boltzmann theory for quasiparticle transport predicts that both the resistivity and should have the same T dependence, contradicting the observed behavior. This failure of quasiparticle-based theories is reminiscent of other correlated strange metals such as cuprates.