I will present our optical spectroscopic measurements on different Fe-based superconducting systems. The experimental data indicate that the available Fe-based superconductors could be clarified into three different categories: the FeAs-based systems (including 122, 1111, 111, … systems), Fe(Te,Se), and A2Fe4Se5 (A=K, Rb, Cs, Tl). Based on the analysis of the spectral features and spectral weight transfers at low and high energies, we conclude that both itinerant and localized electrons from Fe 3d orbitals are present in those systems. However, the coupling between the itinerant and localized electrons changes in different systems. The coupling is relatively strong in FeAs-based compounds, becomes weak in FeTe system, and almost decoupled in A2Fe4Se5 (A=K, Rb, Cs, Tl) systems. The measurement results would help to reach a more comprehensive understanding about the nature of magnetism and superconductivity in the materials.
Work done with Z. G. Chen, R. H. Yuan, W. Z. Hu, G. Li, B. Cheng, J. Dong, T. Dong, P. Zheng, G. F. Chen, J. L. Luo, Z. Fang, X. Dai, C. L. Zhang and P. Dai