The Research Of New Superconductors And Its Properties With Doping

It has been a hundred years since superconductivity was discovered, which continues to bring us new surprises and it’s still one of the most popular research area in the world. The exploration of new superconductors and mechanism of unconventional superconductors play important roles in the field of condensed matter physics and material science, which has great impact on research of strong correlation physics. In 2008, the emergence of iron-based superconductors aroused the world and a lot of experiment facts of iron-based superconductors, such as crystal structure, electron structure, superconducting gap function, etc, were known to all, but there is still much work to do to get better aware of the mechanism of high-Tc superconductivity. In this work, we studied the pressure effect on the superconductivity of iron-based superconductors, which laid a solid foundation for the understanding of mechanism of superconductivity of iron-based superconductors. In additon, we did some research on the newly found superconductor SrPt3 P, in which we studied the impurity scattering. Meanwhile, the effect of electron correlation and spin-obit coupling is also studied to reveal the influence of these effects on the superconductivity. The dissertation is composed of several chapters as follows:The first chapter outlines the history of progress of superconductors, and then specified the research status of iron-based superconductors and SrPt3 P which are involved in the main part of this work. Finally, the theory of superconductivity is introduced at the end of first paragraph.The second chapter presents some methods of material preparation and test in detail, including the process of experiment and mechanism of sample analysis.The third chapter studied the pressure effect on the superconductivity of iron-based superconductors. In order to clarify the actual mechanism of the chemical pressure, here we show a detailed study and indepth analysis on the effects of P substitution induced chemical pressure on the superconductivity of the SrFe2-xCoxAs2 system over the entire doping range(x = 0.16, 0.25, 0.3, 0.4) in this work. Our results suggest that further P substitution induces the effect with the same direction as Co-doping on the superconductivity of the SrFe2-xCoxAs2 system, and a uniform phase diagram was obtained in this Co and P codoped system. Moreover, a comprehensive understanding for the pressure effect was discussed based on various kinds of systems of the FeSCs.The forth chapter studied the impurity scattering effect in Pd-doped superconductors of SrPt3 P. In this work, 4d element Pd was substituted to the site of Pt of SrPt3 P successfully, as revealed by the clear changes in the lattice constants. The electron transport in the normal states was studied by the resistivity and Hall effect measurements. A clear increase of the residual resistivity ρ0 and decrease of the residual resistivity ratio(RRR) were observed in the Pd-doped samples, which was attributed to the distinct scattering with impurities. Such an impurity scattering effect also results in the suppression of the nonlinear field dependent behavior of the Hall resistivity ρxy and the strong temperature dependence of the Hall coefficient RH in the low temperature region.The fifth chapter studied the effects of electron correlation and spin-orbit coupling in the isovalent Pd-doped superconductor SrPt3 P. Importantly, we find that the superconducting transition temperature Tc decreases with Pd doping, which could not be attributed to the physics of changes of the DOS at the Fermi level and the impurity scattering. Thus, it suggests that the system exhibits a competition between electron correlation and spin-orbit coupling in our system. Such a competing relation is also consistent with the first-principles calculation, which shows that the strength of electron susceptibility is suppressed gradually as increasing Pd dopants.The sixth chapter briefly introduces the work of exploring new superconductors conducted by the author during graduate study. The samples that have been tried to synthesized and according experiment results are given in this chapter, which we hope could be the reference of investigation of new superconductors.