Mossbauer Study Of Iron-based High-tc Superconductors

Iron-based superconductors belong to unconventional superconductor. The superconductivity mechanism of unconventional superconductor differs with conventional one. Magnetism of iron-based superconductor is relative to its superconductivity, which play an important role in unconventional superconductivity. The magnetism of iron-based superconductor arises from Fe-3d near the Fermi surfaces. Fermi surfaces will influence the magnetism and superconductivity. Doping including electron doping and hole doping will change the structure of Fermi surfaces. The doping acts to suppress antiferro-magnetic order, being beneficial to the superconductor.In this thesis, we report a detail of Mossbauer study of trivalent La3+ions and Nd3+instead of divalent Ca2+ions in CaFe2As2superconductors. As a useful tool, we can get hyperfine parameters and microscopic properties of superconductors from Mossbauer spectrum.Firstly, for the single crystal of Ca1-xLaxFe2As2(x=0.2,0.3), we fine phase separations at low temperatures in both samples. The La-rich phase undergoes superconducting transition, while the La-short phase takes part in magnetic fluctuations. The temperature dependences of the line width and spectral area exhibit anomalies above the superconducting transmission point may be due to the appearance of spin-phonon coupling. From the result of QSD method, we can deduce the heterogeneous distribution of the La atom. According to the theoretical calculations of the electron structure and electric field gradient of Ca1-xLaxFe2(x=0.125,0.25,0.5) model systems, we find that the Vzz produced by iron nucleus changing from positive to negative, which is ascribed to the change spatial distribution of Fe-3d electrons induced by the distortion of the FeAs tetrahedron with doping level.Secondly, Doping Nd atoms are different from La atoms. Superconductivity appears when doping level arrives0.15. From the result of Mossbauer spectrum, we find that coexistence of superconductivity and magnetism, the superconductor transition temperature achieve at38K in the sample of x=0.15. Superconductivity isn’t found in other sample. By fitting the temperature dependence of average of hyperfine field using Ising model, we can get theoretical temperature TN and exponent β. The value of TN decrease from135.1K to94.1K at x=0.05and x=0.15, respectively. The value of β increases from0.08(1) to0.16(4). The theory of spin density wave (SDW) is used to analysis the Mossbauer spectrum. The shape of SDW at20K become quasi-rectangle at x=0.05compared to the parent compound, while more triangle at x=0.15. The ratio of two line intensities was found to be2.02and2.08at x=0.05and x=0.15, respectively, which reveals a positive Vzz.