We report on a $c$-axis polarized optical measurement on a Ba${}_{0.67}$K${}_{0.33}$Fe${}_2$As${}_2$ single crystal. We find that the $c$-axis optical response is significantly different from that of high-$T_c$ cuprates. The experiments reveal an anisotropic three-dimensional (3D) optical response with the absence of the Josephson plasma edge in $R(\omega )$ in the superconducting state. Furthermore, different from the $\mathit{ab}$-plane optical response, a large residual quasiparticle population down to $T~\frac{1}{5}{T}_c$ was observed in the $c$-axis polarized reflectance measurement. We elaborate that there exist horizontal nodes for the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the $c$-axis optical conductivity.

• Received 6 March 2011

DOI:https://doi.org/10.1103/PhysRevB.83.144522