  Recent experiments on magic-angle twisted bilayer graphene (MATBLG) indicate
an unusual coexistence of heavy and light electrons, characteristic of
Heavy-Fermion physics. Yet, these experiments lack direct evidence of the
existence of local moments associated with the heavy electrons, a key component
of Heavy-Fermion physics. Thermopower serves as a sensitive probe for measuring
entropy and can, therefore, unveil the presence of local moments by assessing
their impact on entropy. In this work, we have carried out comprehensive
thermopower studies on MATBLG by varying temperatures ($T$) and magnetic fields
($B$). While the resistance exhibits prominent resistance peaks at integer
fillings apart from the Dirac point (DP, $\nu = 0$) and full band filling ($\nu
= \pm 4$), the thermopower remains featureless and symmetric to the DP with
opposite signs, in complete violation of the Mott formula, even up to $\sim
100K$. This discrepancy indicates that resistance and thermopower are carried
by different kinds of carriers. With the expected sign changes at the DP and
full band filling, the thermopower demonstrates additional sign changes within
the conduction and valence bands around $\nu \sim \pm 1$, persisting from $5K$
to $50K$. These observations cannot be explained solely by thermopower arising
from an emergent Heavy-Fermion band picture, which is highly sensitive to
temperature. Instead, our data is consistent with the dominant contribution
coming from the entropy of the localized moments of heavy electrons. To
validate the claim, we studied the thermopower with $B_{\parallel}$ and
$B_{\perp}$, revealing a $30\%$ and $50\%$ reduction, respectively, and
attributed to the partial polarization of the local moments (spin/valley),
resulting in decreased entropy. Our results highlight the thermopower
contribution from local moments and establish the Heavy-Fermion physics in
MATBLG.
