$T$-linear scattering rate in optimally doped K- and P-Ba122 iron-pnictides

The optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ reveal two groups of carriers with different scattering rates ($1/\tau$), described by two Drude components in the optical conductivity. A ``broad'' Drude component results in an incoherent background with a $T$-independent $1/\tau_b$, while a ``narrow'' Drude component reveals a $T$-linear scattering rate $1/\tau_n$ resulting in a resistivity $\rho_n \equiv 1/\sigma_{1n}(\omega \rightarrow 0)$ also linear in temperature. This fact explains the $T$-linear $\rho$ at low temperatures and the tendency to saturation at room temperature observed by transport measurements in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. The low frequency spectral weight increases with decreasing $T$, following an arctan($T$) dependence, which is also strong evidence for a $T$-linear scattering rate. A comparison to other materials with similar behavior suggests that the $T$-linear $1/\tau_n$ and $\rho_n$ in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ may arise out of scattering from spin fluctuations due to the proximity to a quantum critical point (QCP). Similar behaviors are found in the optimally doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_{2}$.