Nuclear Magnetic Resonance Studies On Quantum Criticality Of (Sr_1-xCa_x)₃Ir₄Sn₁₃ And A₂Cr₃As₃

In the phase diagram of unconventional superconductors,such as cuprate oxide superconductors,iron-based superconductors,heavy fermions superconduc-tors,superconductivity is always accompanied by structural transition,magnetic transition,charge density waves or other phase transition.In some case,the tran-sition temperature of structural transition or magnetic transition can be tune to zero by pressure,doping,magnetic field or uniaxial stress,the quantum critical fluctuations accompanied by zero temperature transition are considered as key factors for the occurrence of superconductivity.Firstly,this thesis explains the basic principle and method of nuclear mag-netic resonance.In second part,I report the NMR results on(Sr_1-xCa_x)₃Ir₄Sn₁₃and isostructure Ca₃Rh₄Sn₁₃.For(Sr_1-xCa_x)₃Ir₄Sn₁₃,structural phase transition will occur with decreasing temperature,which can be seen obviously by NMR data.I find a characteristic temperature T^*above structural phase transition temperature T_s,FWHM of NMR spectra,Knight shift K and spin-lattice relax-ation rate?1/T₁T?become abnormal below this characteristic temperature.By contrast,Ca₃Rh₄Sn₁₃doesn't occur structural phase transition,also doesn't exist T^*.By analysing Korringa coefficient of four samples,I found antiferromagnetic fluctuation begin to appear below T^*and become more obviously at low temper-ature.Based on these data,I construct the phase diagram of(Sr_1-xCa_x)₃Ir₄Sn₁₃and Ca₃Rh₄Sn₁₃.My results provide new insight into the relationship between structural phase transition and superconductivity.In third part,I introduce re-sults of ferromagnetic?FM?quantum criticality in A₂Cr₃As₃(A=Na,Na_0.75K_0.25,K,Rb)by ⁷⁵As nuclear quadrupole resonance.Based on four sample's 1/T₁T,we conclude the system gradually close to FM quantum critical point?QCP?from Na to Rb.This may be ascribed to the Cr2-As2-Cr2 bond angle that decreases towards 90^?,which enhances the FM interaction via the Cr2-As2-Cr2 path.Upon moving away from the QCP,the superconducting transition temperature T_cin-creases progressively up to 8.0 K in Na₂Cr₃As₃,which is in sharp contrast to the AFM case where T_cusually shows a maximum around a QCP.Using Moriya's 3D ferromagnetic fluctuation theory fitting my 1/T₁T data,I get parameter?which characterizes the magnitude of FM fluctuation.?or R characterizes the exchange interaction.Based on these,we construct phase diagram of A₂Cr₃As₃.Under su-perconducting state,1/T₁decreases rapidly below T_cwith no Hebel-Slichter peak for four samples,and ubiquitously follows a T⁵variation below a characteristic temperature T^*?0.6T_c,which indicates the existence of point nodes in the su-perconducting gap function commonly in the family.These results suggest that the A₂Cr₃As₃family is a possible solid-state analog of superfluid³He.