Iron-based Superconductors Studied By X-ray Absorption Spectroscopy

In2010, for the first time, the smart grid construction has been included into the "Government Work Report of China" as a national strategy initiative. After that, in March2011, the smart grid program appeared in the national "12th Five-Year plan". The large-scale smart grid facility is based on key technology such as the superconductivity, key issue to achieve the competitiveness of this national initiative. According to the United States Department of Energy claiming, the superconductivity is the only high-technology reserve for the21st-century power industry. Similarly, the Japanese New Energy Development Organization claimed that superconductivity is a key technology for cutting edge researches in the21st century. Superconductivity really may offer a wide range of applications in the21st century and will play a role in the research and in industrial applications in the new century.Just a century ago, in1911K. Onnes a Profess of the Hebrew University of Jerusalem found the first superconducting element:the mercury. Then in1986the discovery of high-temperature cuprate superconductors leads the superconductive transition temperature up to more than100K. Nowadays, the discovery of iron-based superconductors in2008is offering new research opportunities and a better understanding of pairing mechanism in high-temperature superconductors triggering new researches and applications in the broad condensed matter community.As well known, the material properties mainly depend on the crystalline structure. In these new iron-based superconductors doping is a key parameter to modify the lattice structure, and then all the electronic structure/magnetic structure and superconductive properties of the system can be modified. In this study of iron-based superconductors the main effort was devoted to better understand the relationship between the lattice structure and the superconductive mechanism. A powerful conventional method to investigate the structure:the XRD technique can only return average long-range-order parameter, while no detailed local information (e.g. the doping site, vacancy, local structural distortion) can be obtained. On the contrary, synchrotron radiation-based X-ray absorption spectroscopy (XAS) is a unique element-selective tool capable to probe both the local lattice and electronic structure of a material. Therefore XAS combined with the first-principle calculation is no doubt the best technical method to study the doping role in these materials.In this thesis, we focus on the doping effects on the local structure, the electronic structure and the superconducting behavior of new iron-based high-temperature superconductors. The most valuable results are briefly summarized below:1. The simple model material of iron-based superconductors:BaFe2As2system. Potassium doping induces a superconductive transition at38K and by the means of XAS, we found that potassium doping finely modulates the local structure. An increasing disorder in the iron layer versus potassium doping level occurs which is intimately related to the softening of Fe-Fe bonds. This work is the first direct experimental proof of Fe-Fe bond softening providing detailed structure information for our future theoretical and experimental studies on superconducting materials.2. The influence of fluorine doping on the carrier SmO layer in the SmO1-xFxFeAs compound has been investigated by Sm L3-edge XAS and multiple scattering calculations. The work points out that oxygen vacancies occur at high F-doping level. Moreover the presence of oxygen vacancy enhances the hybridization between Sm and As atoms and this mechanism could be beneficial to the superconductivity. Actually, the analysis establishes a relationship between oxygen vacancy at high F-doping and the superconductive property. It is an important first step for better understanding the iron-based superconductors.3. We investigate the microscopic origin of a complicated effect of Zn substitution in the Fe site on the Tc of the LaFeAsO1-xFx superconductor. The system shows an increasing Tc with Zn substitution in the F-underdoped region while superconductivity is suppressed with Zn substitution in the F-overdoped regime. Fe and As K-edge XAS combined with first-principle calculation identifies a synergistic effect of F and Zn dopants to finely modulate the charge redistribution and the regularity of FeAs4tetrahedra. As a consequence, the crystal structure especially the regularity of FeAs4tetrahedra is an essential issue to determine the Tc of iron-based superconductors. Local structural information may indeed offer a great insight toward understanding the role of NMI (non-magnetic impurities) substitution and the underlying superconducting mechanism in iron-based superconductors.4. Probed by As and Fe K-edge temperature-dependent XAS, we describe the different doping effect between Mn and Zn dopants on the optimally doped Ba0.5K0.5Fe2As2superconductor in terms of local lattice structure (i.e. the regularity of FeAs4tetrahera and stiffness of Fe-As bond). The new scenario may complement the previous magnetic scattering description providing new insights on the interplay among atom, spin and charge in iron-based superconductors. 5. The isotope effect is the key point and also the difficult point for understanding the high-temperature superconductors. Previous results on the isotope effect of the new iron-based superconductors are highly controversial. In this work we present an investigation of the Fe isotope effect in (Ba, K)Fe2As2system by temperature-dependent XAS. Data point out that the vibration contributions of both Fe-As and Fe-Fe bonds are strongly associated with the Fe isotope substitution. The analysis supports the hypothesis that electron-phonon interaction plays an important role in the superconductive mechanism of iron-based superconductors.