We study the transport properties of twisted bilayer graphene (TBG) using the topological heavy-fermion (THF) model in an interacting framework. In the THF model, TBG comprises localized, correlated f-electrons and itinerant, dispersive c-electrons. We focus on the Seebeck coefficient, which quantifies the voltage difference arising from a temperature gradient. TBG's Seebeck coefficient shows unconventional traits: negative values with sawtooth oscillations at positive fillings, contrasting typical band-theory expectations. This behavior can be attributed to the presence of heavy (correlated, short-lived f-electrons) and light (dispersive, long-lived c-electrons) electronic bands. Their longer lifetime and stronger dispersion lead to a dominant transport contribution from the c-electrons. At positive integer fillings, the correlated TBG insulators feature c- (f-)electron bands on the electron (hole) doping side, leading to an overall negative Seebeck coefficient. Additionally, sawtooth oscillations occur around each integer filling due to gap openings. Our results highlight the importance of electron correlations in understanding the transport properties of TBG and, in particular, of the lifetime asymmetry between the two fermionic species (naturally captured by the THF model). Our findings are corroborated by new experiments in both twisted bilayer and trilayer graphene.
