  Many transition metal oxides (TMOs) are Mott insulators due to strong Coulomb
repulsion between electrons, and exhibit metal-insulator transitions (MITs)
whose mechanisms are not always fully understood. Unlike most TMOs, minute
doping in CaMnO3 induces a metallic state without any structural
transformations. This material is thus an ideal platform to explore band
formation through the MIT. Here, we use angle-resolved photoemission
spectroscopy to visualize how electrons delocalize and couple to phonons in
CaMnO3. We show the development of a Fermi surface where mobile electrons
coexist with heavier carriers, strongly coupled polarons. The latter originate
from a boost of the electron-phonon interaction (EPI). This finding brings to
light the role that the EPI can play in MITs even caused by purely electronic
mechanisms. Our discovery of the EPI-induced dichotomy of the charge carriers
explains the transport response of Ce-doped CaMnO3 and suggests strategies to
engineer quantum matter from TMOs.
