  Recently, magic-angle twisted bilayer graphene (MATBLG) has shown the
emergence of various interaction-driven novel quantum phases at the
commensurate fillings of the moir'e superlattice, while the charge neutrality
point (CNP) remains mostly a vanilla insulator. Here, we show an emerging phase
of nearly compensated semimetallicity at the CNP of twisted double bilayer
graphene (TDBLG), a close cousin of MATBLG, with signatures of electronic
correlation. Using electrical and thermal transport, we find almost two orders
of magnitude enhancement of the thermopower in magnetic fields much smaller
than the extreme quantum limit, accompanied by a large magnetoresistance($\sim
2500\%$) at CNP. This provides indisputable experimental evidence that TDBLG
near CNP is a compensated semimetal. Moreover, at low temperatures, we observe
an unusual sublinear temperature dependence of resistance. A recent theory
predicts the formation of an excitonic metal near CNP, where small electron and
hole pockets coexist. We understand the sublinear temperature dependence in
terms of critical fluctuations in this theory.
