  Ever since the initial experimental observation of correlated insulators and
superconductivity in the flat Dirac bands of magic angle twisted bilayer
graphene, a search for the microscopic description that explains its strong
electronic interactions has begun. While the seemingly disagreeing electronic
transport and scanning tunneling microscopy experiments suggest a dichotomy
between local and extended electronic orbitals, definitive experimental
evidence merging the two patterns together has been much sought after. Here, we
report on the local photothermoelectric measurements in the flat electronic
bands of twisted symmetric trilayer graphene (TSTG). We use a cryogenic
scanning near-field optical microscope with an oscillating atomic force
microscopy (AFM) tip irradiated by the infrared photons to create a nanoscopic
hot spot in the planar samples, which generates a photocurrent that we probe
globally. We observe a breakdown of the non-interacting Mott formalism at low
temperatures (10K), signaling the importance of the electronic interactions.
Our measurements reveal an overall negative offset of the Seebeck coefficient
and significant peaks of the local photovoltage values at all positive integer
fillings of the TSTG's moir\'e superlattice, further indicating a substantial
deviation from the classical two-band semiconductor Seebeck response. We
explain these observations using the interacting topological heavy-fermion
model. In addition, our data reveal a spatial variation of the relative
interaction strength dependent on the measured local twist angle (1.2{\deg} -
1.6{\deg}). Our findings provide experimental evidence of heavy fermion
behaviour in the topological flat bands of moir\'e graphene and epitomize an
avenue to apply local thermoelectric measurements to other strongly correlated
materials in the disorder-free limit.
