Study Of The Se-site Doping Effects In TI_0.4K_0.4Fe_2-ySe₂Superconductor

The research on superconductivity has been going on for over100years. To date, it is still one of the most intriguing subjects in physics community. Because the resistance in superconducting materials is zero below the Tc, it can transfer current without any energy loss, so it has a broad application prospect. In1911, superconductivity is found in mercury below4.2K. The discovery leads to a qualitative leap in the understanding of the physical world, and opens an era for the study of superconducting materials. In1986, the superconductivity with Tonset=35K is found in the La-Ba-Cu-O compound, leading a boom in the physics community on study of high-Tc superconductors. In2008, the superconductivity with rc≈26K is found in the poly crystal compound LaO1-xFxFeAs, opening the era of research on high-Tc superconductivity based on iron.It is known that the high Tc in cuprate oxide superconductors is due to the suppression of antiferromagnetic Mott insulating phase, and the superconductivity of iron-based superconductors originates from the antiferromagnetic half-metal and the superconductivity can coexist with the antiferromagnetic order. However, the discovery of iron-chalcogenide superconductors AxFe2-ySe2puts a challenge to the weak coupling scenarion in iron-pnictide superconductors. Because only the electrons can be observed in the corner of the Brillouin zone, the scattering between electronic pockets cannot explain the superconducting mechanism of AxFe2-ySe2superconductors. There are a lot of new features in the new materials:Firstly, the resistance of higher temperature is eight orders of magnitude lower than that of lower temperature in antiferromagnetic insulating phase; Secondly, the TN of the long-range antiferromagnetic order in AxFe2-ySe2materials, compared with the iron-pnictide superconductors, is up to559K, and the magnetic moment is more than3μB; Finally, AxFe2-ySe2materials also display iron vacancy order at the temperature Ts=578K, higher than TN. The insulating phase and the large magnetic moment imply that the physics in AxFe2-ySe2is rather local. The dissertation is mainly composed of four chapters as follows:The first chapter outlines the development of the superconducting materials. We make a brief introduction of the development of conventional single-element superconductors, alloy and compound superconductors, heavy fermion superconductors, copper-oxide superconductors, C60superconductors, iron-based high temperature superconductivity, and so on.The second chapter gives a more detailed introduction to the study of iron-based superconductors. It includes the research on iron-pnictide superconductors and iron-chalcogenide superconductors. The former mainly contains the research on the superconductors with1111phase,122phase, and111phase. The later is made of the discovery of11phase superconductors and its preparing conditions, the physics of FeSe1-x, FeSe1-x,FeSe1-xTex, Fe1+d/Te1-xSex, FeTe1-xSx and FeSe1-xSx, the discovery of superconductors, the influence of Fe to AxFe2-ySe2superconductivity, the phase separation of AxFe2-ySe2superconductors, the pressure effect on AxFe2-ySe2superconductors, the element-doping effect in AxFe2-ySe2superconductors, and the contrast between iron-pnictide superconductors and iron-chalcogenide superconductors.In the third chapter, we have studied the influence of As-doping on the superconductivity in Tlo.4Ko.4Fe2-ySe2superconductor. In the samples, the introduction of As dopants leads to the suppression of the superconductivity. We find that the staggered magnetic moments increase with increasing As-doping content, which could be responsible for the total destruction of the superconductivity in Tlo.4K0.4Fe2-ySe2-xAsx superconductors at last.In the fourth chapter, we report the phase transition in the Tlo.4Ko.4Fe2-ySe2-.xSx system and the chemical phase separation in heavily S-doping Tl0.4Ko.4Fe2-ySe2-xSx single crystals. We find that the phase separation into an undistorted1×1phase and a (?)×(?) charge ordering phase is an intrinsic phenomenon in the AxFe2-ySe(S)2systems, while the (?)×(?) Fe-vacancy ordering phase is responsible for the semiconducting-like behavior at higher temperature. In heavily S-doping samples, the (?)×(?) charge ordering phase dominates their transport behavior and the samples also exhibit typical metallic-like conductivity.