  Controlling and understanding electron correlations in quantum matter is one
of the most challenging tasks in materials engineering. In the past years a
plethora of new puzzling correlated states have been found by carefully
stacking and twisting two-dimensional van der Waals materials of different
kind. Unique to these stacked structures is the emergence of correlated phases
not foreseeable from the single layers alone. In Ta-dichalcogenide
heterostructures made of a good metallic 1H- and a Mott-insulating 1T-layer,
recent reports have evidenced a cross-breed itinerant and localized nature of
the electronic excitations, similar to what is typically found in heavy fermion
systems. Here, we put forward a new interpretation based on first-principles
calculations which indicates a sizeable charge transfer of electrons (0.4-0.6
e) from 1T to 1H layers at an elevated interlayer distance. We accurately
quantify the strength of the interlayer hybridization which allows us to
unambiguously determine that the system is much closer to a doped Mott
insulator than to a heavy fermion scenario. Ta-based heterolayers provide
therefore a new ground for quantum-materials engineering in the regime of
heavily doped Mott insulators hybridized with metallic states at a van der
Waals distance.
