Quantum critical point in SmO_1−xF_xFeAs and oxygen vacancy induced by high fluorine dopant

The local lattice and electronic structure of the high-T_c superconductor SmO_1–xF_xFeAs as a function of F-doping have been investigated by Sm L₃-edge X-ray absorption near-edge structure and multiple-scattering calculations. Experiments performed at the L₃-edge show that the white line (WL) is very sensitive to F-doping. In the under-doped region (x ≤ 0.12) the WL intensity increases with doping and then it suddenly starts decreasing at x = 0.15. Meanwhile, the trend of the WL linewidth versus F-doping levels is just contrary to that of the intensity. The phenomenon is almost coincident with the quantum critical point occurring in SmO_1–xF_xFeAs at x ≃ 0.14. In the under-doped region the increase of the intensity is related to the localization of Sm-5d states, while theoretical calculations show that both the decreasing intensity and the consequent broadening of linewidth at high F-doping are associated with the content and distribution of oxygen vacancies.