Interplay between light and heavy electron bands in magic-angle twisted bilayer graphene

Abstract

Recent studies have suggested that the strongly correlated flat bands of magic-angle twisted bilayer graphene may host coexisting light and heavy carriers. Although transport and spectroscopic measurements have hinted at this behaviour, distinct signatures of incoherent heavy carriers have not been reported. Here we provide evidence of this by performing thermoelectric transport measurements of magic-angle twisted bilayer graphene using the photo-thermoelectric effect in gate-defined p–n junctions. At low temperatures, we observe sign-preserving, filling-dependent oscillations of the Seebeck coefficient at non-zero integer fillings of the moiré superlattice. This suggests the preponderance of one carrier type even when the Fermi level is tuned through the charge neutrality point of the correlated states. At higher temperatures, the thermoelectric response provides evidence of strong electron correlations in the unordered, normal state. Our observations are explained by the interplay between light, long-lived electron states and heavy, short-lived hole excitations near the Fermi level of the symmetry-broken ground states. These findings are in qualitative agreement with the topological heavy fermion model.