Observation of a coherence peak and pair-breaking effects in THz conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$

TL;DR

This study uses THz spectroscopy to investigate BaFe$_{2-2x}$Co$_{2x}$As$_2$ thin films, revealing a coherence peak near T$_c$, pair-breaking effects at low temperatures, and supporting an extended s$ extpm$ symmetry in the superconductor.

Contribution

First direct observation of a coherence peak and pair-breaking effects in the THz conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$, providing evidence for an extended s$ extpm$ symmetry.

Findings

Evidence of a coherence peak near T$_c$

Significant pair-breaking effects at low temperature

Power-law temperature dependence of penetration depth

Abstract

We report a study of high quality pnictide superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ thin films using time-domain THz spectroscopy. Near T$_c$ we find evidence for a coherence peak and qualitative agreement with the weak-coupling Mattis-Bardeen form of the conductivity. At low temperature, we find that the real part of the THz conductivity is not fully suppressed and $\sigma_2$ is significantly smaller than the Matthis-Bardeen expectation. The temperature dependence of the penetration depth $\lambda$ follows a power law with an unusually high exponent of 3.1. We interpret these results as consistent with impurity scattering induced pair-breaking. Taken together our results are strong evidence for an extended s$\pm$ symmetry order parameter.