  Optical conductivity measurements on a BaCoS2 single crystal show an unusual
linear behavior over a broad spectral range. In the paramagnetic phase above
300 K, the spectrum shows no gap, which contradicts the previously proposed
scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field
theory calculations including a retarded Hubbard interaction explain the data
in terms of an incipient opening of a Co(3d)-S(3p) charge-transfer gap
concomitant to incoherent charge transport driven by electronic correlations.
These results point to a non-Fermi liquid scenario with Hund's metal properties
in the paramagnetic state, which arises from an incipient Mott phase
destabilized by low-energy charge fluctuations across the vanishing 3d-3p
charge-transfer gap.
