Nuclear Magnetic Resonance Study On Iron-based Superconductors NaFe_1-xCo_xAs

In cuprates and Iron-based high-temperature superconductor,both antiferomagnetism and nematic order are adjacent to superconductivity.With the introducing of control parameter such as doping and press,the transition temperature of both the two ordered phases are suppressed and the extrapolation of the points which the transition temperatures are suppressed to absolute zero are in close proximity to or directly inside the superconducting dome.Besides,the normal states above the critical point show nor-Fermi liquid behavior.These all suggest that the emergency of superconductivity may be associated to the antiferomagnetism or nematic quantum critical point.However,whether there exists a quantum critical point beneath the superconducting dome is under debate and is the key to understand the normal states and even the superconductivity mechanism.Firstly,this thesis briefly summarizes the history of the development of the material and the theory of superconductivity.Also summarized is the physical characteristics of iron-based superconductors and the ‘122' and the ‘111' system are specially introduced.Among them,the researches on the antiferromagnetic quantum point beneath the superconducting dome and the reason why we choose NaFe_1-xCo_xAs as the object of the study are mainly introduced.Then,I introduce the basic principles and the experimental methods of the nuclear magnetic resonance?NMR?.In this part,I introduce how we can get the information of the electronics via the measurement the nuclear and how to obtain the signals of NMR.At last,the results of the study on NaFe_1-xCo_xAs is reported.According to the study of the spin fluctuation of the system,in the case of H//c,we find for the sample x=0.027,the spin-lattice relaxation rate of 75 As which comes from the contributions of antiferomagnetic fluctuation becomes unchanged below a certain temperature,indicating x=0.027 is an antiferromagnetic quantum critical point.In addition,we study the spin fluctuation of H//ab plain and confirm that x=0.027 is an antiferromagnetic quantum critical point.