  We carried out combined transport and optical measurements for BaFe2As2 and
five isostructural transition-metal (TM) pnictides. The low-energy optical
conductivity spectra of these compounds are, to a good approximation,
decomposed into a narrow Drude (coherent) component and an incoherent
component. The iron arsenides, BaFe2As2 and KFe2As2, are distinct from other
pnictides in their highly incoherent charge dynamics or bad metallic behavior
with the coherent Drude component occupying a tiny fraction of the low-energy
spectral weight. The fraction of the coherent spectral weight or the degree of
coherence is shown to be well correlated with the TM-pnictogen bond angle and
the electron filling of TM 3d orbitals, which are measures of the strength of
electronic correlations. The iron arsenides are thus strongly correlated
systems, and the doping into BaFe2As2 controls the strength of electronic
correlations. This naturally explains a remarkable asymmetry in the charge
dynamics of electron- and hole-doped systems, and the unconventional
superconductivity appears to emerge when the correlations are fairly strong.
