THz conductivity of BaCo$_{0.2}$Fe$_{1.8}$As$_{2}$ and Fe(Se,Te) superconductor

Ac complex conductivity is investigated for Fe-based superconductors, BaCo$_{0.2}$Fe$_{1.8}$As$_{2}$ (122) with $T_c=$17 K and Fe(Se,Te) (11) with $T_c=$10 K. For the 122 material, typical features of a superconductor were seen in the frequency dependence of conductivity, $\sigma$. In particular, we find a structure corresponding to superconductivity gap, 2$\Delta$, whose magnitude is 3meV at lowtemperatures, leading to 2 $\Delta/k_BT_c=$ 4.1. Althogh the material is known to be a multi-band superconductor, this value is in good agreement with the smaller gap found in an ARPES measurement[1]. For the 11 material, we have not suceeded in obtaining a clear gap structure, probablly because of lower $T_c$, at present. We are now trying to measure microwave conductivity by a broadnabd technique[2], the result of which will be presented. \\[4pt] [1] K. Terashima {\it et al.}, PNAS 106 (2009) 7330. \\[0pt] [2] H. Kitano {\it et al.}, RSI 79 (2008) 074071, {\it ibid}, PRB 73 (2006) 174522, T. Ohashi {\it et al.}, PRB79 (2009) 184507.