Electronic Raman Scattering Study Of "122"-type Iron Based Superconductors

Superconducting materials have great commercial potentials for applications in industry and human daily life.Below the superconducting transition temperature Tc,the electrons near Fermi energy in the superconductor become paired and form a Cooper pair,they gain phase coherence and enter into the superconducting ground state.It is of significance to deepen the understanding of the electron pairing mechanism for unconventional superconductors and the coupling between di?erent degrees of freedom in condensed matter physics.In this dissertation,we systematically study the normal state,superconducting gaps and in-gap collective mode in typical “122” system of iron-based superconductors by electronic Raman scattering.It contains three parts:1.Electronic Raman scattering study of c-axes As vibration mode across the structural phase transition TSand magnetic phase transition TNto investigate the magneto-elastic coupling.?1?We detected a forbidden Ag-symmetry As vibration mode in the XY scattering geometry below the structure phase transition temperature TS,demonstrating the existence of the electronic polarizability anisotropy along X and Y directions in the orthorhombic phase.?2?By measuring the intensity of the As vibrational mode in di?erent iron-based superconductors,we found that the intensity of A1g?As?in the XY scattering geometry is closely related to magnetism.In the tetragonal phase,the intensity of A1g?As?mode is zero in the XY scattering geometry.In the orthorhombic phase,the intensity of A1g?As?in the XY scattering geometry is very weak.In the magnetic phase,the intensity of A1g?As?in the XY scattering geometry is greatly enhanced.?3?We find that the ratio of the intensity of A1g?As?phonon in the XY scattering channel and the XX scattering channel is proportional to the square of the ordered magnetic moment at low temperature.?4?In the Au-doped Ba Fe₂As₂ system,we systematically studied the Fano lineshape of the A1g?As?phonon.This asymmetric phonon lineshape reveals that the As c-axes vibration mode is coupled to the magnetic B2g-like electronic continuum.Wedemonstrate that the anomalous behavior of the Ag?As?phonon mode in the XY scattering geometry can be consistently described by a Fano model involving the Ag?As?phonon mode interacting with the B2gsymmetry-like magnetic continuum in which the magneto-elastic coupling constant is proportional to the magnetic order parameter.2.In the Au-doped Ba Fe₂As₂ system,we systematically studied the temperature evolution of the nematic fluctuations and explored the origin of nematic fluctuations by electronic Raman scattering.?1?We found that the static Raman susceptibility increases gradually above the structural phase transition temperature TS,following the Curie-Weiss law.?2?There was only a slight decrease in static Raman susceptibility between TSand TN.?3?Below TN,the static Raman susceptibility decreased rapidly,which is closely related to the opening of a spin density wave gap below TN.?4?Our observation can be explained by the orbital fluctuations.3.In the K-doped Ba Fe₂As₂ system,we systematically studied the nematic fluctuations,the superconducting pair breaking peaks and the in-gap collective modes by electronic Raman scattering.?1?In the sample studied,we observed a quasi-elastic peak,which originates from nematic fluctuations,in the normal state in the B2 gchannel in a large doping range.?2?In the superconducting state,we observed two di?erent superconducting Cooper pairs breaking peaks which corresponds to the two di?erent superconducting gaps.The peak with larger energy corresponds to a larger superconducting gap while the one with smaller energy corresponds to a smaller superconducting gap.?3?In the B2 gchannel,below Tc,we detected a collective in-gap mode whose energy lie in between the larger SC gap and the smaller SC gap.We carefully studied the temperature evolution and doping evolution of this mode and presented di?erent scenarios to the origin of the collective in-gap mode.The structure of the dissertation is as follows: The first chapter is an introduction,the second chapter and the third chapter are introductions to the Raman theory and Raman experiment,respectively.The fourth chapter,the fifth chapter and the sixth chapter introduce the research of phonon,nematicfluctuations and superconducting collective in-gap mode by Raman scattering in iron-based superconductor,respectively.The seventh chapter is a summary and prospect.