Doping dependence of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin films by the THz conductivity measurement

We investigated the THz conductivity for thin films of Fe-based superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_{2}$ with different Co concentrations. For the optimally doped sample, we found a structure corresponding to superconductivity gap, 2 $\Delta$, whose magnitude is 2.8 meV at lowtemperatures, leading to 2 $\Delta/k_BT_c=$ 4.1[1]. This value is in good agreement with the smaller gap found in an ARPES measurement[2]. For the underdoped sample in which the coexistence of antiferromagnetic ordering with superconductivity was observed, we found the strong suppression of the carrier lifetime around the antiferromagnetic phase transition temperature ($T \sim$ 40 K). However, the real part of the complex conductivity did not be clearly suppressed in this temparature regigon. This behavior may be related to the responce of carriers at the Dirac cone, which observed in BaFe$_2$As$_2$[3]. Details will be discussed in the presentation.\\[4pt] [1] D. Nakamura {\it et al.}, arXiv: 0912.4351.\\[0pt] [2] K. Terashima {\it et al.}, PNAS 106 (2009) 7330.\\[0pt] [3] P. Richard {\it et al.}, Phys. Rev. Lett. 104 (2010) 137001.