Study Of The Iron-based Superconductors By High Resolution Mobility Spectrum Method

Iron-based superconductor are unconventional high temperature superconductor that is very similar to cuprate superconductor.It has(quasi)two-dimensional layered crystal structure,superconducting transition temperature exceeding the theoretical limit of BCS,antiferromagnetic order of parent phase,and dome type charge-doped superconducting phase diagram,etc.It is an important system for studying the mechanism of high temperature superconductivity.In addition,iron-based superconductor exhibits obvious multi-band effects,resulting in peculiar band structure and electron behavior.It has abundant quantum ordered states,so it becomes the frontier issue of condensed matter physics research.Mobility spectrum method is an effective method to study the band structure and related mechanism of multi-band superconductor.Because this method doesn't need to assume the type and number of carriers in advance,it has been successfully applied to the band structure analysis of semiconductor materials,superconducting materials and topological quantum materials.In this paper,we mainly use high resolution mobility spectrum method to study the pressure-regulated Fe Se single crystal,the doping regulated Fe Se_1-xS_x single crystal and the protonated Fe Se single crystal.The specific content of this paper is as follows:In chapter 1,we briefly introduce the discovery and development of superconductor,and describe the research progress of iron-based superconductor,especially the regulation of“11”iron-based superconductors.These regulation methods include pressure regulation,doping regulation and protonation.We also introduce the research progress and application of mobility spectrum method.In chapter 2,we introduce the theoretical background of mobility spectrum method.Then we deduce algorithm of high resolution mobility spectrum method in detail and verifies the effectiveness and accuracy of this method through the simulation data.In chapter 3,we use high resolution mobility spectrum method to study the pressure-regulated Fe Se single crystal systematically.The mobility spectrum of Fe Se at different temperatures in 0 GPa,in different pressures at 20 K or 30 K,and at different temperatures in 3.8 GPa are obtained.Symmetrical high mobility peaks are successfully observed in the mobility spectrum of the superconducting transition region,which was considered to be Cooper pairs.Meanwhile,the contribution of Cooper pair to the electrical conductivity was quantitatively observed.In chapter 4,we have studied doping regulated Fe Se_1-xS_x single crystals by high resolution mobility spectrum method and the first-principles calculations.The final results show that there is a transition from electron-hole compensation for Fe S to electron dominated for Fe Se.The mobility spectrum of Fe S shows the existence of electron-hole compensation at all temperatures.We also observe the random divergence relationship between T_c and carrier density and the V-shaped relationship between T_c and Se/S anion height.These relationships indicate that the superconductivity of Fe Se_1-xS_x is more likely to be directly related to bond length and bond angle due to the strength of exchange coupling.In chapter 5,we used high resolution mobility spectrum method to analyze protonated Fe Se single crystals.The results show that the carrier mobility decreases with the increase of protonation time.Meanwhile,it is found that the conductivity contribution rate of electron carriers in Fe Se is always greater than that of hole carriers.With the increase of protonation time,the conductivity contribution rate of electron carriers increases sharply,indicating that electron carriers are always dominant in Fe Se.Finally,we obtain the carrier density,and we find that there is a correlation between superconducting transition temperature and carrier density in protonated Fe Se.In chapter 6,I summarize the content of this paper and I put forward some idea for future work.