Nuclear Magnetic Resonance Studies On (Li_0.8Fe_0.2)OHFeSe And AFe₂As₂ （A=K,Rb,Cs）

Iron-based superconductors attract wide attention of the condensed matter physics community in recent years because of its rich structures, phase diagrams and uncon-ventional mechanism which beyond the traditional framework of BCS theory descrip-tion. As the second high temperature superconductors family besides the copper oxide superconductors, iron-based superconductors and copper oxide superconductors show a great similarity, such as quasi two-dimensional layered structure, superconductivity is achieved from doping magnetic parent compound, and the magnetic fluctuation play a key role for superconducting pairing. Nuclear magnetic resonance (NMR)is a powerful tool to detect the spin fluctuation, which can help us better understand the relationship between the magnetism and superconductivity in iron-based supercon-ductors.This dissertation presents the nuclear magnetic resonance (NMR) study on (Li0.8Fe0.2)OHFeSe. Through the studying on the nuclear magnetic resonance (N-MR) spectrum of (Lio.sFeo.2)OHFeSe, we have demonstrated the random occupation between Li and Fe, also a field induced ferromagnetism. For heavily hole doped AFe2As2(A= K, Rb, Cs), we found the Knight shift (K) and the spin lattice relax-ation rate (1/Ti) show a scaling behavior with a characteristic temperature (T*). Sim-ilar characteristic temperature (T*) is also found in the transport results,the "Knight shift anomaly" is observed below (T*).This thesis is divided into three chapters as follows:1. IntroductionThis chapter firstly briefly reviews the history of superconducting material, the structure and the phase diagram of iron-based superconductors. Then we introduce the recent developments of the FeSe-derived superconductor and the background of the orbital selective Mott transition. Finally, we introduce the basic principle of nuclear magnetic resonance (NMR) and its application in condensed matter physics.2. Nuclear magnetic resonance study on (Li0.8Fe0.2)OHFeSe:field-induced ferromagnetismWe report on nuclear magnetic resonance (NMR)measurements in the iron-based superconductor (Li0.8Fe0.2)OHFeSe.Both remarkable change in spectrum and diver- gence of spin-spin relaxation(T2) are observed with decreasing temperature, suggest-ing a static magnetic ordering in this material.Comparison of linewidth between’ Li and 77Se spectra indicates that there is a ferromagnetic component along the external field direction and it happens on Fe sites in the (Lio.8Feo.2)OH layer, which is also responsible for the three-peak structure of the 7Li spectrum with (Li0.8Fe0.2) random occupation.The field-dependent spectrum and T2 suggest that the above ferromag-netic component is induced by external magnetic field.Our present results indicate that the superconductivity in (Li0.8Fe0.2)OHFeSe is very robust against the observed field-induced ferromagnetism and both of them could coexist under external magnetic-field.3. The heavy fermion behavior of heavily hole doped AFe2As2(A= K, Rb, Cs)We report on nuclear magnetic resonance (NMR)measurements in the iron-based superconductor AFe2As2(A= K, Rb, Cs).Unconventional superconductivity from heavy fermion (HF) is always observed in f-electron systems.We experimentally found the d-electron iron-based superconductors (FeSCs)AFe2As2(A=K, Rb, Cs) also show HF behavior. Nuclear magnetic resonance on 75As reveals a universal coherent-incoherent crossover with a characteristic temperature T*.Below T*, the K-χ plot departure the linear relationship is first observed in FeSCs. which exhibits a scaling behavior similar to f-electron HF materials. Below T*. the spin lattice relaxation rate 1/T1 of AFe2As2 (A= K, Rb, Cs) show a scaling law of -T0.75,which suggest the proximity to a specific quantum criticality.These results undoubtedly support an emergent Kondo lattice scenario for the d-electron HF behavior, which qualifies the AFe2As2(A= K, Rb, Cs) as d-electron HF superconductors.