| Since superconductivity was discovered in mercury by H.K.Onnees in1911, researches on superconducting materials and microscopic mechanism have been one of the primary aspects in condensed matter physics. During the past100years, superconductivity was found in elementary metals, alloies and especially in cuperates which spured the searching for more high temperature superconductors and the comprehensive understanding of this macroscopic quantum behavior. However, scientists encountered problems both on the microscopic mechanism and the material technique. At the beginning of2008, superconductivity was discovered in LaO1-xFxFeAs compounds with a superconducting transition temperature,Tc=26K by Hosono’s group. In the following several years, physicist leaded by Chinese discovered superconductivity in many compounds including RO1-xFxFeAs,(Ba,K)Fe2As2, LiFeAs, Sr4V2O6Fe2As2, Fe(Te,Se,S) and (Tl,K,Rb,Cs)FexSe2.From then on, understanding the mechanism of Iron-based superconductors became the important issue in condensed matter physics.In this thesis, based on the background of pervious investigations, we studied two systems including KFe2As2with ThCr2Si2structure and Fe(Te,Se) with PbO structure. The results were presented as follows:(1) based on the successfully synthesizing KFe2(As1-xPx)2polycrystalline samples, we measured their structure, resistivity, susceptibility. It was found that,(a)As the concentration of P is increasing,the lattice parameter a value changes hardly,while the c value decreases obviously, indicating that P can partially substitutes for As and the substitution may cause inner stress.(b)For0≤x≤0.2samples, the superconducting transition temperature Tc decrease monotonously, while for0.2≤x≤0.6samples, their Tc shows a peak at x=0.35. And the x=0.6sample does not show superconductivity above2K.(c)Substitution by P for As leads to the decreasing of superconducting volume fraction, which may be related to the different paring-symmetry in this system.(2)Based on the previous work in our group, the author carefully investigated the effect of excess Fe atoms on superconductivity in the FeTe0.8Se0.2single crystal. These results enrich the phase diagram of Fe(Te,Se) system.In summary, we studied the chemical pressure effect in KFe2(As1-xPx)2and the suppression effect by excess Fe atom of superconductivity in Fe(Te,Se)system, which help to understand of Fe-based superconductivity. |
