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

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.

4.2 pages, 4 figures, submitted. v2: references format corrected, no change to text

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• Thin Film Growth and Device Fabrication of Iron-Based Superconductors
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• Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe_{1.84}Co_{0.16}As_2 ultrathin film: an analysis of the optical gaps in the Co-doped BaFe_2As_2 pnictide
• Correlation between unconventional superconductivity and strange metallicity revealed by operando superfluid density measurements
• Direct measurement of the temperature dependence of the magnetic penetration depth in Ba(Fe${}_{1-x}$Ni${}_x$)${}_2$As${}_2$ superconductors
• Studies on DC transport and terahertz conductivity of granular molybdenum thin films for microwave radiation detector applications
• On the determination of the quasiparticle scattering rate in unconventional superconductors by microwave surface impedance
• Divergence of Dynamical Conductivity at Certain Percolative Superconductor-Insulator Transitions
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