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

TL;DR

This study uses optical spectroscopy to reveal a temperature-linear scattering rate in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$, indicating possible proximity to an antiferromagnetic quantum critical point affecting its electronic properties.

Contribution

The paper identifies a distinct temperature-linear scattering rate in a Fe-based superconductor, linking it to spin fluctuations and quantum criticality, which was not previously demonstrated in this material.

Findings

Two Drude components describe carrier scattering rates.

The narrow Drude component exhibits T-linear scattering rate.

Evidence suggests spin fluctuations cause the T-linear behavior.

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 ($1/\tau$), 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 $\rho_n \equiv 1/\sigma_{1n}(\omega\rightarrow 0)$ also linear in temperature. An arctan($T$) low-frequency spectral weight is also a strong evidence for a $T$-linear 1/$\tau$. 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$_{2}$ originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.