Experimental Studies On Electronic Phase Diagrams Of Some Iron-arsenide Superconductors

As an important branch in condensed matter physics, superconductivity was firstly discovered by Heike Kamerlingh Onnes in 1911. However, advances in superconductivity continued to proceed slowly. at the same time, many theoretical physicists began to unlock the mysteries of superconductivity by various theories such as the famous BCS theory. In 1986, Georg Bednorz and Alex Muller, working at IBM in Zurich Switzerland, historically found superconductivity at 35 K in copper-oxide ceramics called perovskites beyond the BCS theory. By now, all the high temperature superconductors consist of layered structures, and are obtained by doping methods. Copper oxides, as antiferromagntic Mott insulators, have achieved the highest superconducting transition temperature by doping, which bring electron/hole type carriers. The crystal structure and chemical composition of layers are crucial and sensitive for the emerge of superconductivity. Containing the same layer struc-ture, iron arsenide superconductors have been researched by doping at the beginning of discovery, and many important results have been reported.My dissertation focuses on the doping effects on the system of iron arsenides, includ-ing so-called "1111" and "122" compounds. LaFeAsO with the ZrCuSiAs-type structure, shows the magnetic and structural transitions at approximate 150 K. The resistivity exhibits an upturn below 50 K. EuFe2As2 and BaFe2As2 possess the same ThCr2Si2-type structure, however, the magnetic and structural transitions happen at different temperature. By dop-ing methods, superconductivity was realized in the above three compounds, and electronic or magnetic phase diagrams are obtained. The anisotropic properties of single crystals are also investigated by us. A brief introduction of my works during Ph. D. is shown below:(1) Superconductivity was realized in LaFeAsO system by nickel doping. We found a narrow superconducting window of 0.03≤x≤0.06 in LaFe1-xNixAsO compounds with a maximum superconducting transition temperature of 6.5 K. The normal-state resistivity strikingly exhibits a semiconducting like behavior. And then, the electronic phase diagram has been established by systematical investigation on LaFe1-xNixAsO (0≤x≤1) in the whole range of x.We found that in the other superconducting range, Fe doping killed the superconductivity of LaNiAsO. And the upturn behavior of resistivity exhibited in the large-scale doping range.(2) We have proposed the A-type antiferromagnetic structure for Eu2+ spins in EuFe2As2 system, which has been recently proved by another group using single crystal neutron diffraction. At the same time, magnetic phase diagram of EuFe2As2 were established by us. And then, we have independently found coexistence of superconductivity and local moment ferromagnetism in cobalt doped EuFe2As2.(3) Bulk superconductivity induced by an isovalent doping of phosphorus in BaFe2As2 was first reported by us, the maximum transition temperature was 30 K. In the whole phase diagram, Superconductivity emerges at x= 0.32, coinciding with a magnetic quantum crit-ical point which is shown by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. we argued that there existed a quantum critical point at x～1/3 in the BaFe2(As1-xPx)2 system.