Superconductivity Study Of FeTe_1-xS_x System And MgB₂ With FeSe Doping

Superconductivity is one of the most important phenomena in condensed matter physics.Since the discovery of superconductivity by Dutch scientist Onnes in 1911,this macroscopic quantum phenomenon has been a research hotspot in the field of condensed matter.In the history of superconducting research for nearly 100 years,new superconductors have been discovered and the transition temperature has been refreshed.The application of superconductors has been limited to alloy superconductors such as Ni₃Sn and superconducting materials such as YBCO.The discovery of MgB₂superconductivity has attracted the attention of scientists.The transition temperature is significantly higher than that of metal alloy superconductors and there is no weak connection problem of high temperature superconductors.It has good application prospects.We try to introduce magnetic flux pinning into the parent by doped FeSe.It is expected to further increase the critical current density without lowering its transition temperature,and reasonably explain the change of superconducting performance after doping.At the same time,the FsTeS system has aroused our interesting.As the most basic and simple system in iron-based superconductors,it is more important to explore its superconductivity.We doped different elements of the FeTeS system,which increase the transition temperature of the parent.We establish a suitable model to explain the changes in electrical transport and magnetic properties.The discovery of new superconducting systems and doping of existing superconducting systems to improve the superconducting properties of parent materials to obtain more clues and evidence of unconventional superconducting mechanism will play an important role in research of superconducting.This paper is divided into six chapters:The first chapter:the discovery of superconductivity and its basic properties are summarized.Several properties,important parameters and several important theoretical explanations of superconductivity are introduced.The history of superconducting materials in the past 100 years is introduced,and the feasibility of exploring unconventional superconductors is discussed.The second chapter:introduces the experimental methods,characterization instruments and their principles used in this paper.Including the determination of crystal structure,electrical transport properties test,magnetic test and morphology characterization.In the third chapter,the phase formation and doping mechanism of MgB₂superconducting materials are introduced.The research progress of a small amount of FeSe-doped MgB₂ is introduced in detail.We introduce FeSe as magnetic flux pinning into MgB₂,and we hope to improve the superconductivity in MgB₂,especially superconducting critical current density.However,the experimental results show that FeSe reacts with MgB₂,and some Fe enters the lattice to replace the position of Mg ions,which causes the superconducting transition temperature and critical current density to decrease.The impurity phase MgSe is also used as Flux pinning changes the superconductivity of the matrix.The two effects compete with each other,eventually affecting the change in superconductivity.In the fourth chapter:the superconductivity of FeTeS system in Fe-based superconductor 11 system is introduced.The influence of K element doping on the FeTeS is introduced in detail.We perform X-ray diffraction test analysis,magnetic test analysis,and electrical test analysis on samples.The experimental results show that some K ions enter the interlayer of FeTeS and cause the change of lattice constant.The electrical test showed that the superconducting transition temperature was significantly improved.The superconducting transition temperature of K_0.2FeTe_0.9S_0.1 reached 11.4K.It was about 1.5 K higher than FeTe_0.9S_0.1.Due to the influence of excessive magnetic Fe ions,there is no obvious Meissner effect in the M-T data by the magnetic measurement,but the results obtained from the dM/dT data consistent with the electrical properties.The fifth chapter:The effect of Sn element doping on the superconductivity of the parent FeTe_0.8S_0.2 is described.It can be seen from the analysis of XRD and SEM that part of the Sn enters the lattice of FeTe_0.8S_0.2,which resulting in the shift of the diffraction peak and the change of the lattice constant.In addition,the doping of Sn makes the particles contact more tightly,and increases the growth of the superconducting tetragonal phase.The superconducting performance of this system is significantly improved.The T_C^zero of the best doped sample is about 3 K higher than the parent.In the sixth chapter:we explored the physical properties of the antiferromagnetic compound MPX₃ system.We dope and intercalate this high Neel temperature compound to explore its possible superconductivity.The seventh chapter:we have summarized the work of this paper and discussed the research directions and possible problems of the subject.