| The electrical resistivity of a pure metallic conductor decreases gradually as tempera-ture is lowered. However, the resistance saturates at low temperatures due to impurities and other defects in the material. Nevertheless, this behavior changes dramatically in some materials. For instance, the resistance drops abruptly to zero when the material is cooled below its critical temperature in a conventional superconductor. Moreover, due to the magnetic impurities, the resistance increases rather than saturating as the temperature is lowered further in a Kondo type material. The common character of these two materials is that the original Fermi liquid state is damaged at low tempera-tures while the spin singlet state is formed. Compared to the Kondo effect which is totally understood in 1970s, the mechanism of high-temperature superconductors is still not clear after more than twenty years. In 2008, the discovery of iron-based super-conductors has brought new vigor to the study of unconventional superconductivity.In Chapter Ⅰ, we briefly introduced the basic knowledge of conventional super-conductivity and the theoretical description. Then we reviewed the history and current situation of the research on cuprate superconductors as well as the iron-based super-conductors. The issues on the magnetoresistance and Hall effect of the two band model in the weak-field limit were also addressed.In Chapter Ⅱ, we reported the single crystal growth of iron-based superconductors, including the FeSe1-xTex system, the KxFe2-ySe2 system, the Ba1-xKxFe2As2 system and the NaFe1-xCoxAs system. Based on these high quality single crystals, we have carried out effective international cooperation. In addition, the related experimental instruments and techniques were described.In Chapter Ⅲ, we performed electrical and magnetization measurements, scan-ning electron microscopy and microanalysis, X-ray diffraction and scanning tunnelling microscopy on KxFe2-ySe2 single crystals prepared under different thermal treatments to understand the relationship between its microstructure and its superconducting phase. We identified a three-dimensional network of superconducting filaments within this material and presented evidence for the existence of K2Fe7Se8 which may be the pos-sible parent phase for superconductivity. This 278 phase has a Fe structure of a single Fe vacancy out of every eight Fe-sites arranged in a (?)×(?)10 parallelogram structure.In Chapter IV, motivated by the observation of an orbital-selective Mott phase at a high temperature in the recent angle-resolved photoemission spectroscopy measure-ment. We studied the normal-state transport properties of the quenched KxFe2-ySe2 s-ingle crystals, including magnetoresistance and Hall effect measurements. It was found that Kohler’s rule is only obeyed below 80 K. With the two-band model analysis on the Hall coefficient, we concluded that a gap may open below 65 K. The data above 65 K are interpreted as a temperature induced crossover from a metallic state at a low temperature to an orbital-selective Mott phase at a high temperature.In Chapter V, We briefly introduced the basic conception of Kondo effect. The VTe2 single crystals were grown by using KCl as the flux. Then we investigated the transport properties of it. We found that the resistivity diplays a logarithmic increase at low temperatures which could be expected by the Kondo effect. To further identi-fy the mechanism, we conducted the measurements of MR under the magnetic field along different axes and excluded the weak localization explanation. By analyzing the negative magnetoresistance, it is shown that the Kondo impurity may be given by the localized V moment. Further supporting evidence of the Kondo picture could be found in the temperature dependence of resistivity under the applied magnetic fields. Under high magnetic fields, the system shows the Fermi liquid behavior. Thus, we found the filed-tuned Feimi liquid in the Kondo type VTe2.A summary was presented in the end. |
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