  We have investigated the charge dynamics and the nature of many-body
interactions in La- and Pr- doped CaFe2As2. From the infrared part of the
optical conductivity, we discover that the scattering rate of mobile carriers
above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic
transport. However, the dc resistivity continues to increase with temperature
above 200 K due to the loss of Drude spectral weight. The loss of Drude
spectral weight with increasing temperature is seen in a wide temperature range
in the uncollapsed tetragonal phase, and this spectral weight is recovered at
energy scales about one order of magnitude larger than the Fermi energy scale
in these semimetals. The phenomena noted above have been observed previously in
other correlated metals in which the dominant interactions are electronic in
origin. Further evidence of significant electron-electron interactions is
obtained from the presence of quadratic temperature and frequency-dependent
terms in the scattering rate at low temperatures and frequencies in the
uncollapsed tetragonal structures of La-doped and Pr-doped CaFe2As2. For
temperatures below the structure collapse transition in Pr-doped CaFe2As2 at 70
K, the scattering rate decreases due to weakening of electronic correlations,
and the Drude spectral weight decreases due to modification of the low-energy
electronic structure.
