Spin Excitations And Neutron Spin Resonance Studies In New-type Iron-based Superconductors

Since the copper-oxide high temperature superconductors?HTSC?were dis-covered,the mechanism of HTSC has been a great mystery in condensed mat-ter physics.The HTSC were called unconventional superconductors along with heavy-fermion superconductors because their superconducting mechanism can not be explained by the conventional BCS theory.Until 2008,the discovery of iron-based superconductors?another HTSC family?opens a new era of the study of HTSC.Then,scientists begin to summarize the characteristics of these two HTSC families to obtain some inspiration about the mechanism of HTSC.Indeed,peo-ple found these two HTSC families have several obvious characteristics that their parent compound are all antiferromagnet,and superconductivity can be achieved through methods like chemical doping to suppress the magnetic order,and the op-timal superconductivity always appears around the critical doping level where the antiferromagnetism disappears.A direct idea is that the antiferromagnetic inter-actions have a very close relationship with superconductivity.Neutron scattering is a very important probe to study the magnetism in condensed matter.Physicists have acquired a series of progress in the study of HTSC by neutron scattering.The most representative one should be the neutron spin resonance mode.The neutron spin resonance mode is a collective mode of spin excitations that appears in the superconducting state around an certain energy at the antiferromagnetic wave vector.The temperature dependence behavior of the resonance intensity looks like a superconducting order parameter.What's more,the resonance en-ergy has been found can linearly scale with T_cand superconducting gap.Thus,the neutron spin resonance can be a strong evidence of the magnetic excitation mediated pairing mechanisms.This thesis will be about the magnetic excitaitons and spin resonance in iron-based superconductors.We studied two new iron-based superconductors Ca_1-yLa_yFe_1-xNi_xAs₂?112 type?and CaKFe₄As₄?1144 type?by neutron scattering,our results improve our understanding about the supercon-ductivity,magnetism,and their correlation in iron-based HTSC.The contents of this thesis will contain five parts as follows:Firstly,we have mapped out an 3D phase diagram of the new 112-type iron-based superconductor Ca_1-yLa_yFe_1-xNi_xAs₂by transport and neutron diffraction.We have grown sizeable crystals of 112-type iron pnictide with 100%diamagnetism using CaAs as flux.Then,transport and neutron diffraction ware used to map the phase diagram.The results show that Ni doping at Fe site can suppress the AFM order gradually,and then induce the optimal superconductivity.These behaviors indicate that the magnetism and the superconductivity compete and coexist with each other.Secondly,we have studied the neutron spin resonance mode systematically in Ca_0.82La_0.18Fe_0.96Ni_0.04As₂by inelastic neutron scattering.We confirm the linear scaling behavior of E_Rand T_c.Through the neutron polarization analysis,we found the spin excitations in Ca_0.82La_0.18Fe_0.96Ni_0.04As₂are almost isotropic.This is consistent with the theoretical prediction of a singlet to triplet exciton of the neutron spin resonance.Thirdly,we have measured the spin excitations in a new 1144-type iron pnic-tide CaKFe₄As₄,and found three spin resonance modes around 9.5 meV,13 meV,and 18 meV.These multiple modes behavior correspond to the multiple Fermi surfaces nesting.And here the three resonance energies can linearly scale with the corresponding total superconducting gaps on the nesting fermi surfaces.More importantly,the resonance mode around 18 meV shows an even L modulation behavior.This is the first discovery of a resonance mode with even L modulation in iron-based superconductors.After comparing with the even mode in cuprates,we conclude that the appearance of even mode can be explained by the strong magnetic coupling between the FeAs layers within the bilayer structure.This means that the iron-based and cuprates HTSC may have the same mechanism about the neutron spin resonance and superconductivity.Fourthly,we have studied the anisotropy of the spin excitations in CaKFe₄As₄by polarized neutron scattering.We found the spin resonance mode around 9meV is a c-polarized mode,the mode around 12 meV is dominated by in plane components,and the even mode around 18 meV is totally isotropic.The spin anisotropy is found to couple with T_c,this indicates that spin-orbit coupling has close relationship with superconductivity.Fifthly,we have studied the spin excitations spectrum of CaKFe₄As₄and Ca_0.82La_0.18Fe_0.96Ni_0.04As₂.The dispersion behavior of the spin excitations in these two compound are very similar with that in doped BaFe₂As₂system.The band top of the spin excitation around the Brillouin zone boundary is about 220 meV.Our results indicates iron pnictides share an universal magnetic coupling.