Hidden T-Linear Scattering Rate in Ba0.6K0.4Fe2As2 Revealed by Optical Spectroscopy

The optical properties of Ba0.6K0.4Fe2As2 have been determined in the normal state for a number of temperatures over a wide frequency range. Two Drude terms, representing two groups of carriers with different scattering rates (1/tau), well describe the real part of the optical conductivity sigma(1)(omega). 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 1/tau(n) resulting in a resistivity rho(n) equivalent to 1/sigma(1n)(omega -> 0) also linear in temperature. An arctan(T) low-frequency spectral weight is also strong evidence for a T-linear 1/tau. A comparison to other materials with similar behavior suggests that the T-linear 1/tau(n) and rho(n) in Ba0.6K0.4Fe2As2 originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.