Mossbauer Study Of Iron-Based High-Tc Superconductors

The magnetism, superconductivity, lattice instability usually closely related to each other in the iron-based superconductors. It’s of fundamental importance to clarify the relationship mong them in exploreing the high-temperature superconductivity mechanism.In this thesis, we prepared single crystals of Ca1-x,NdxFe2As2(x=0.05,0.15), Ca1-xPrxFe2As2(x=0.075,0.13) and Ca1-xCexFe2As2(x=0.09,0.2) with high quality by FeAs self-solvent method, and investigated the hyperfine interactions of these compounds systematically by Mossbauer spectrometer.It is found that the single crystals of Ca1-xNdxFe2As2(x=0.05,0.15) have two phases at low temperatures, paramagnetic phase and antiferrmatic spin density wave phase (SDW). Superconducting phase transition occurs at Tc=38K for Ca0.85Nd0.15Fe2As2 single crystal. The magnetic phase and superconducting phase coexistence at the temperatures below Tc. The harmonic amplitude h3 behaves negative and the shape of the SDW becomes quasi-triangle then. We found the index β increased to 0.16 from the 0.07 with with increasing Nd doping by Ising model, indicating the strengthening of the 2D features of the magnetic interaction between Fe moments and the suppressing of interlayer magnetic exchange by Nd doping. In addition, with decreasing temperature, the paramagnetic phase experiences collapsed phase transition in Ca0.85Nd0.15Fe2As2 single crystal.According to the theoretical calculations of the electronic structures and the electric field gradient (EFG) of Ca1-xRexFe2As2, it is confirmed that there exist two local environments of Fe ions in Ca0.85Nd0.15Fe2As2 single crystal at low temperature, one is composed of high spin Fe atoms, the other is non-magnetic. And the two kinds of Fe ions coexist with each other with the ratio of 3:7. Collapsed phase transition occurres at some temperature between 60K and 77K in non-magnetic phase. While it appeared the magnetic fluctuations emerges with the decreasing temperature in the Fe ions with high spin. And superconducting phase coexist with magnetic fluctuations below Tc.Finally, there are two Fe environments in Ca0.8Ce0.2Fe2As2 single crystals at low temperatures, non-magnetic phase and the phase with Fe ions at high spin state. Magnetic fluctuations happens in the high spin Fe atoms, and is enhanced with decreasing temperature. And superconductivity coexists with magnetic fluctuations under the superconducting transition temperature in this comound.