M(?)ssbauer Study Of Iron-based High-Tc Superconductors

The discovery of iron-based superconductors?FeSC?has provided a new kind of candidate to study the relationship between superconducting and other properties of the related materials,for which the reaserch interests focuse on discovering the pairing mechanism by analyzing the correlations among the crystal structure,electronic structure and magnetic properties.The 122 family compoundsare the most widely studied iron-based superconductors due to the feasibility of single crystals with high qualityand the the emergence of rich physical phenomena in doped compounds and those under pressure.In this thesis,Ca_1-xPr_xFe₂As₂?x=0.06,0.1?and Ca_0.9Pr_0.1Fe₂As_1.9P_0.1 single crystals have been prepared using FeAs self-flux method,the crystal structure were analyzed using XRD?EDX and Raman spectrum,the electromangtic properties were studied using SQUID.57 Fe M?ssbauer spectra were measured in the temperature range from 10 K to 300 K,and the temperature dependences of hyperfine parameters have been carefully investigated.The reasrch findings are as follows:The magnetism is suppressed by Pr dopants,and the paramagnetic phase coexists with spin density wave?SDW?phase at low temperatures in Ca_0.9Pr_0.1Fe₂As₂ compound.According to the evolution of the distribution of hyperfine field,the magnetic phase transition is continuous in character.The theory of spin density wave is used to analysis the M?ssbauer spectrum.The profile of SDW changes from quasi-triangle at 75 K to quasi-rectangle at 10 K.The harmonic amplitude ratio h3: h1 indicates that the localization of Fe-3d electrons is strong.The superconducting transition temperature of Ca_0.9Pr_0.1Fe₂As₂ alloy is about 35 K.It is found that the distributions of Pr atoms are heterogeneous and there are two kinds of local environments of Fe atoms,i.e.Pr-rich phase and Pr-poor phase,where the volume fraction of the latter is about 90%.According to the quadruple splitting distributions?QSD?and the first-principles calculations,the T-CT structural phase transition occurs at 115 K for La-rich phase and the superconducting effect appears at the boundary of the two phases.Information about electron-phonon correlation,instead of magnetic fluctuation,has been observed above Tc.The lattice parameter c in Ca_0.9Pr_0.1Fe₂As_1.9P_0.1is obviously smaller than that of Ca_0.9Pr_0.1Fe₂As₂,the distortion of FeAs tetrahedral and distribution of electron density differs from those of Ca_0.9Pr_0.1Fe₂As₂.The superconducting transition temperature TC?= 25 K?decreases in co-doping system.QSD provides evidence of heterogeneous distribution of the Pr atoms,Pr-rich phase and the Pr-poor phase is separated spatially.Due to the co-doping of P atoms,the temperature of T-CT structural phase transition is improved and the volume fraction of the Pr-rich phase increases in co-doping system.The evolutions of hyperfine parameters with temperatures confirm that the Pr-poor phase takes part in magnetic fluctuations,and electron-phonon coupling occurs at low temperature.On the basis of the lattice dynamics of the compounds,it is considered that superconductivity is closely related to the effect of phonons in rare-earth doped 122-type FeSCs.