Thermopower probing emergent local moments in magic-angle twisted bilayer graphene

Abstract

Recent experiments on magic-angle twisted bilayer graphene (MATBLG) indicate an unusual coexistence of heavy 1-6 and light 7-10 fermions. Yet, these experiments lack direct evidence of the existence of local moments arising from the flatband, a key component of heavy-fermion physics 11-16. Utilizing thermopower as a sensitive probe for measuring entropy 17, we unveil the presence of 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 demonstrates additional sign changes at the location, νcross ∼ ±1, which does not vary with temperature from 5K to ∼ 60K. This observation 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 localized moments. To validate our claim, 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, and can be attributed to the partial polarization of the local moments (spin/valley), resulting in decreased entropy. Our thermopower results highlight the contribution from local moments arising from flatbands, distinct from the band contributions to the resistivity in MATBLG.