Vortex Dynamics And Electornic Transport Property Of Series LSCO Samples

One of the significant content in the field of high temperature superconductors is the enrichment and modification of the electronic phase diagram and its theoretical explanations. This doctoral thesis mainly focuses on the vortex dynamic of LSCO films and the transport property of Zn doped LSCO polycrystals. The BEC-BCS theory can be employed to explain our data.In chapter 1, a brief history of superconductivity and the details of several theoretical explanation about HTSc, such as the RVB theory, prepairing theory (BEC-BCS), quantum critical point, Gossamer superconductor and DDW picture, will be introduced. Several methods for preparing cuprate superconductors are described in chapter 2. For the preparation of poly-crystal, there are solid state reaction, sol-gel, coprecipitation-filtration and spray-drying method, etc. For the preparation of single crystal, the methods mainly include the self-flux and Traveling Solvent Floating Zone method.The vortex dynamic of several LSCO films are studied in chapter 3 and chapter 4. It is found that the magnetic field can strongly suppress the superconducting parameters (such as J, T_irr, etc.) in the underdoped region, while weakly in the overdoped region. For the first time, the hole doping dependence of the coherence length or vortex core size has been determined directly from the low temperature data in wide doping regime. It is found that the coherence length drops in the underdoped region and increases in the overdoped side with the increase of hole concentration. In chapter 5, the preparation and measurement of Zn-doped La_2-xSr_xCu_1-yZn_yO₄ bulk samples will be introduced. It is found that the maximum T_c shifts to about x = 0.19 when y = 0.025. In addition, the low temperature resistive data show an insulating-like ground state when x ≤ 0.19 but a metallic ground state when x > 0.19. These can be understood with the picture of BEC in the underdoped side and BCS in the overdoped side.In chapter 6, possible superconductivity in sulfur intercalated graphite will be introduced. Sulfur intercalated graphite with diamagnetic transitions at 7 K and 37 K are prepared and measured. From the magnetization hysteresis loops we consider it more like a type-â…¡ superconductor.A detailed summary is given in the end.