Hidden $T$-Linear Scattering Rate in Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ Revealed by Optical Spectroscopy

Abstract

The optical properties of Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ 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 ($1/\ddot{I}„$), well describe the real part of the optical conductivity, $\ddot{I}{ƒ}_1(\ddot{I}‰)$. A "broad" Drude component results in an incoherent background with a $T$-independent $1/\ddot{I}{„}_b$, while a "narrow" Drude component reveals a $T$-linear $1/\ddot{I}{„}_n$ resulting in a resistivity $\ddot{I}{}_n\equiv 1/\ddot{I}{ƒ}_{1n}(\ddot{I}‰\to 0)$ also linear in temperature. An arctan($T$) low-frequency spectral weight is also a strong evidence for a $T$-linear 1/$\ddot{I}„$. Comparison to other materials with similar behavior suggests that the $T$-linear $1/\ddot{I}{„}_n$ and $\ddot{I}{}_n$ in Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.

6 pages, 4 figures