| The discovery of iron-based superconductors in 2008 has brought new vigor and vitality to the study of high-temperature superconductivity. The multi-band effect and "abnormal" normal state in iron-based superconductor have brought rich physics to us. The gap symmetry is very much crucial for understanding the superconducting mech-anism, while its form in the iron-based superconductors is still under debate. It seems that the origin of nematic electronic state is closely related to the superconducting mechanism.In chapter 1, we give a brief introduction to the history and basic knowledge of superconductivity. Then we give a two band model analysis of Hall effect and magnetoresistance. Last part we briefly introduce vortex dynamics and the observation of vortex.In chapter 2, we briefly introduce the basic experiment method, including single crystal growth of iron-based superconductors, magnetization and electrical transport measurements. In particular, Montgomery method and single crystal detwinning tech-nique were introduced.In chapter 3, we studied the impurity effect in Na(Fe0.97-xCo0.03Tx)As (T=Cu, Mn) single crystals. Analysis of the DC magnetization based on the Curie-Weiss law indicates that Mn doping gives magnetic impurities, whereas Cu dopants behave as nonmagnetic or very weak magnetic impurities. However, it is found that both doping with Cu and doping with Mn can enhance the residual resistivity and suppress the su-perconductivity, at the same rate, in the low doping region, which is consistent with the prediction of the S± model. For the Cu-doped system, the superconductivity is sup-pressed completely at a residual resistivity of 0.87 mQ cm, when a strong localization effect is observed. However, in the case of Mn doping, the suppression of Tc gets much weaker beyond x=0.03 and superconductivity survives even up to a residual resistivity of 2.86mΩ cm. Clearly the magnetic Mn impurities are even not as detrimental as the nonmagnetic Cu impurities to the superconductivity in the Na(Fe0.97Co0.03)As system in the high doping regime.In chapter 4, we measured the in-plane resistive of in NaFe1-x-CoxAs single crys-tal under a uniaxial pressure, and we successfully resolved the nematicity, structural and AFM temperatures. We depict the phase diagram based on our in-plane resistive data together with earlier published data with non-resistive measurements. It is found that the structural transition and nematicity vanish simultaneously at the doping level of about x=0.025±0.002, while the AFM order disappears at a lower doping level (x≤0.02). We propose that both the magnetic fluctuation and lifting the degeneracy of the dxz and dyz orbitals entangle each other and contribute to the nematicity. Our results also point to a close relationship between nematicity and superconductivity in electron doped systems.In chapter 5, Hall effect and magnetoresistance have been measured on single crystals of the parent phase NaFeAs under a uniaxial pressure, with the configurations: I || a-axis and I || b-axis. The temperature dependence of longitudinal resistivity is very different in the two configurations below the structural transition temperature, however the transverse resistivity and Hall coefficient show almost an isotropic behav-ior. Large magneto-resistance and non-linear Hall effect are also observed below struc-tural transition temperature and the Kohler’s rule is severely violated, which suggests the multiband nature in the nematic state. Two-band model with different charge carrier density and mobilities is used to analyze the non-linear Hall effect and the magnetore-sistance between the two configurations. Detailed analysis indicates that the moving charge carrier densities n1 and n2 should be isotropic whatever the current direction is, however, the anisotropic in-plane resistivity in the nematic state is closely related to the distinct quasiparticle mobilities when they are moving parallel or perpendicular to the direction of the uniaxial pressure.A summary was presented in the end. |
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