Exploratory Study On New Fe-based Chalcogenides Superconductors

Since the discovery of the superconductivity in LaO1-xFxFeAs in the early of 2008, more superconductors were obtained by replacing La by other lanthanides, partly substituting F for O (electron doped) or Sr for La (hole doped), or applying high pressure in the synthesis or measurement process. In these superconductors, the highest phase transition temperature reached 56K, which terminated the monopoly position of the copper oxides in the high temperature superconductors (HTSC) and heralded a new era in superconductivity research. After nearly a year of hard, people have found four new superconducting systems which contain the similar Fe-based tetragonal layers:(Ba,K)Fe2As2, LixFeAs and Fe(Se,Te)1-x. In the last year, a large number of theoretical and experimental research were carried out in the crystal structure, electronic structure, magnetic properties, transport properties, superconductivity properties and so on. The understanding of the intrinsic of the physical properties was deepening. In these compounds, the comparison with the Cu-O superconductors aroused much interesting. However, the common features and the implied physical meaning are still not clear. Such as:why high temperature superconductivity exists only in two-dimensional systems?Why the superconductivity phase in the Cu-0 and Fe-based superconductors is close to the antiferromagnetic ordered phase? Is the antiferromagnetic fluctuation beneficial to the high temperature superconductivity or they just compete with each other? If the electron-phonon interaction is important in the Fe-based superconductors, then what's the relationship between the antiferromagnetic fluctuation and the superconductivity? Compared with the common features of the Fe-based and Cu-0 superconductors, the difference of the physical properties may seem more important. In a sense, the Fe-based superconductors are simpler than the Cu-O superconductors, in spite of its multi-band Fermi surface. Because, the nature of the normal state of the Fe-based superconductors is not so hard to understand like in Cu-0 superconductors. So, the physical properties should be more easily understood.In this paper, the FeSeo.85 and Re2O3Fe2Se2 (Re=La, Ce) were synthesized successfully. The substitution of Cu for Fe and F for O was carried out in these compounds, respectively. On this basis, we have a systematic research on the structure, transport properties and magnetic properties.The paper is divided into three chapters.The first chapter introduces the basic nature of the superconductivity. Then we have a review of the Fe-based superconductors and introduce the motivation of this work.The second chapter introduces the experiment methods used in this paper. The samples were synthesized by a normal solid state reaction. The structure was characterized by powder x-ray diffractions (XRD). Electrical resistance was measured by employing four-probe technique. Magnetic susceptibility as a function of temperature was performed on a Quantum-Design SQUID system.The third chapter reports the experiment results of the Fe1-xCuxSe0.85, La2O3-xFxFe2Se2 and Ce2O3-xFxFe2Se2, including the study on their structure, transport properties, magnetic properties and superconductivity. In the Fe1-xCuxSe0.85 system, we found that the partial substitution of Cu for Fe not only suppresses its superconductivity, but also induces a metal-insulator transition. In the antiferromagnetic insulator La2O3Fe2Se2, the partial substitution of F for O leads to the metal behavior in the whole measuring temperature, and a superconducting transition is observed around 10K. We have successfully synthesized the Ce2O3Fe2Se2 compound for the first time. It's found that this compound is an anti ferromagnetic insulator, which has the same structure of La2O3Fe2Se2 compound. The F doping effect on physical properties was investigated systematically.