Thermopower Probing Emergent Local Moments in Magic-Angle Twisted Bilayer Graphene

Recent experiments on magic-angle twisted bilayer graphene (MATBLG) have revealed the formation of flatbands, suggesting that correlation effects are likely to dominate in this system. Yet, a global transport measurement showing distinct signatures of strong correlations like local moments arising from the flatbands is missing. Utilizing thermopower as a sensitive global transport probe for measuring entropy, we unveil the presence of emergent local moments through their impact on entropy. Remarkably, in addition to sign changes at the Dirac point (ν = 0) and full band filling (ν = ±4), the thermopower of MATBLG demonstrates additional sign changes at the location, νcross ∼ ±1, which do not vary with temperature from 5K to ∼ 60K. This is in contrast to sensitive temperature-dependent crossing points seen in our study of twisted bilayer graphene devices with weaker correlations. Further, we have investigated the effect of magnetic field (B) on the thermopower, both B∥ and B⊥. Our results show a 30% and 50% reduction, respectively, that is consistent with suppression seen in the layered oxide due to the partial polarization of the spin entropy. The observed robust crossing points, together with suppression in a magnetic field, cannot be explained solely from the contributions of band fermions; instead, our data is consistent with the dominant contribution arising from the entropy of the emergent localized moments of a strongly correlated flatband.