Physical Properties Of Electron Doped Iron-based 122 System Superconductors

It has been nearly 10 years since the discovery of iron-based superconduc-tors,the rich physical properties still attract us to conduct a lot of research nowadays.In the traditional concept,the magnetic ions are very detrimental to the formation of superconductivity.However,the iron ion in iron-based su-perconductor is magnetic,so its emergence has subvert this concept and caught much attention.Iron-based superconductors have a common property,which is a layered structure similar to the copper oxide high-temperature superconductors,it can also be dopant to suppress the antiferromagnetic order and induce super-conductivity.There is a strong interaction between electron and spin fluctuation,so the strong antiferromagnetic fluctuations in the system may be closely related to the formation of superconductivity.The study of iron-based superconduc-tors is of great significance in understanding the mechanism of high-temperature superconductors."122" system superconductor is one of the most studied systems in Fe-based superconductors,and it is also the main object of this thesis.Compared to other systems of iron-based superconductors,it can be grown into large pieces of high quality single crystal samples,which is very advantageous for neutron scattering experiment.Moreover,large single crystal samples can be cut into a variety of shapes,so that we can apply an external force on it to measure the anisotropy.In chapter 1,the history of the development of superconducting physics is introduced first,and then the physical properties of iron-based superconductors,related research methods and their theoretical basis are emphatically introduced.At last the development of neutron scattering and scattering theory are intro-duced.In chapter 2,the influence of spin fluctuation on electronic specific heat is introduced in the electron-based doped BaFe2-xNixAs2 iron-based superconduc-tors.We find that the normal-state electronic specific heats in the superconduct-ing samples between optimal doping and over-doped region may have a nonlinear temperature dependence,which challenges previous results in the electron-doped Ba-122 iron-based superconductors.The theory of ferromagnetism spin fluctua-tion can describe this nonlinear behavior very well.The origin of ferromagnetic fluctuations may be the Stoner instability of the hole pocket at ? points or a tiny amount of magnetic impurities.We point out that in a highly fluctuating system,specific heat research needs to be treated more cautiously.In Chapter 3,the detailed phase diagram for BaFe1.9-xNi0.1CrxAs2 is mapped out by neutron scattering measurements.The superconductivity is rapidly sup-pressed with x = 0.05 Cr doping level,and the antiferromagnetic order develop from the original short-range incommensurate state into a long-range commen-surate state.Further doping,the magnetic moments of Fe ions show obvious nonlinear behavior with Cr doping,which reaching the maximum at x = 0.5.While the magnetic transition temperature and structural transition tempera-ture does not seem to have any effect.When the Cr doping level exceeds x=0.8,G-type antiferromagnetic order emergence,and the original C-type anti-ferromagnetic order is slight suppressed,that the magnetic transition decreases slowly and the magnetic moment becomes very small.There are two kinds of antiferromagnetic properties in the sample at this region.In conjunction with X-ray results,we believe that the rapid increase in the magnetic moment of Fe ions is due to the increase of the distance from the As ion to the Fe layer,and the further decrease in the magnetic moment is due to the formation of the G-type antiferromagnetism.The in-plane(2D)distortion of the a-b plane associated with nematic phase is almost constant with the doping,resulting in the magnetic transition temperature almost does not change with the doping due to the strong magnetoelastic coupling.In Chapter 4,the detailed phase diagram for BaxSr1-xFe1.9-7Ni0.03As2 is mapped out by the resistance measurement,magnetization measurement and nematic susceptibility measurement.With the increase of Ba doping level,the antiferromagnetic phase transition and the nematic phase transition change from first-order to second-order,respectively.In the middle part of the phase diagram,there is a new region where the magnetic phase transition is second order but the nematic phase transition is first order.Moreover,the magnetic phase transition and the nematic phase transition temperature become different,and the jump of nematic susceptibility becomes small.These observations agree with the mag-netic scenario for an itinerant fermionic model,which indicate that the electronic nematic order is spin driven.The last chapter is a summary of this thesis,we introduce the significance of this work in the field of research,as well as the prospects for future work.