Fermi-liquid like normal state electrodynamics in Co-doped BaFe$_2$As$_2$

Elucidating the origin of high temperature superconductivity requires two equally important ingredients: a framework for the normal state electron dynamics and a pairing interaction. In iron-pnictide high T$_{c}$ superconductors the electron doped compounds, such as BaFe$_{2-x}$Co$_{x}$As$_{2}$, are predicted to be weakly correlated Fermi liquids [1,2], but clear evidence has thus far been lacking. In this contribution we unveil the true nature of the normal state dynamics by carefully annealing BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ single crystals. We show that optical spectroscopy experiments on such annealed crystals display a characteristic Fermi liquid scaling of frequency and temperature over a large energy range [3]. A comparison with as-grown single crystals shows that magnetic impurity scattering has thus far masked this behavior. A further analysis shows that a Fermi-liquid like single-particle self-energy can well describe both the mass renormalization and optical scattering rate, leaving little room for additional contributions.\\[4pt] [1] P. Werner et al., Nature Phys. 8, 1 (2012).\\[0pt] [2] L. De' Medici et al., Phys. Rev. B 83, 205112 (2011).\\[0pt] [3] C. Berthod et al., Phys. Rev. B 87, 115109 (2013).