The Study Of Charge Dynamics In Iron-based Superconductors With New Type Of Magnetic Order

Iron-based superconductors?FeSCs?have been attracting intensive attention since its discovery in 2008.Even though,up to now,the highest superconducting temperature discovered in FeSCs is 55 K,which is far below that in cuprates?164K?,yet the large magnetic moment and the the strong charge-spin interacting benefit us to get further investigation to the relation between the magnetism and superconductivity.Most parent compounds of FeSCs are bad metals with antiferromagnetic?AFM?order,by pressure or charge doping,magnetic order could be suppressed with the emergence of superconductivity,therefore magnetism has been intimately related to superconductivity.Up to now,many types of magnetic order have been discovered in FeSCs,understanding their origin and relation to superconductivity is crucial in understanding the unconventional pairing mechanism.In this dissertation,I reported the investigation of charge dynamics in recent-ly discovered new type of magnetic order in FeSCs with the method of infrared optical spectroscopy.In Sr_0.67Na_0.33Fe₂As₂with tetragonal?C4?spin-charge densi-ty wave state,we observed two different kinds of density-wave gaps.The opening of the two gaps greatly suppresses the density of state?DOS?near the Fermi level,so does to the superconductivity.However,after the magnetic transition in electron-overdoped Ca_0.77Nd_0.23FeAs₂,we have not observed any density-wave gap in optical conductivity,the spectral weight at low-energy range has been transferred to high energy range??0.5 eV?,indicating a localized origin.Further investigation of the Hall parameter points out that instead of the Fermi surface topology,the hybridization of Fe-As bonding,which is affected by lattice distor-tion determines the superconductivity.Besides,during the collapsed-tetragonal phase transition in Ca_0.86Pr_0.14Fe₂As₂and the incoherent-coherent crossover in KFe₂As₂,we further recognised the important role of the Fe-As hybridization in determining the correlation,magnetism and related superconductivity.Finally,in highly two-dimensional(CaFe_1-xPt_xAs)₁₀Pt₃As₈,we find that while interlayer correlation is weak,strong intralayer AFM fluctuation may great-ly enhance electron-phonon coupling,trapping the electrons and forming large polaron,which might be the origin for the semiconducting-like behavior in the normal state.However,when entering the superconducting state,we find that the trapped electrons also contribute to the superfluid.Our discovery may provide important clue for further investigation of the unconventional pairing mechanism in iron-based superconductors.