  We study the interacting transport properties of twisted bilayer graphene
(TBG) using the topological heavy-fermion (THF) model. 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. We find that the TBG's
Seebeck coefficient shows unconventional (strongly-interacting) traits:
negative values with sawtooth oscillations at positive fillings, contrasting
typical band-theory expectations. This behavior is naturally 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 essential 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, and show the natural presence of strongly-correlated heavy
and light carriers in the system.
