Transport Behaviors And Ground-state Properties Of La_1.8Sr_0.2Cu_1-zM_zO₄ (M = Zn, Mg) Single Crystals

Superconducting material is characterized as a zero resistance and Meissner effect below a certain temperature. As the superconducting materials have many unique properties, they are widely used in many fields. A lot of experiments show that Fermi liquid theory can not explain the ground-state properties of high-temperature superconductors well. So we need to know whether Fermi liquid theory can be used to describe the ground-state properties of high-temperature superconductors or not. In this thesis, we focus on this issue.Chapter 1: The history and progresses of research on high-temperature superconductors are reviewed. Details of the structure and phase diagram of La2–xSrxCuO4 (LSCO) are introduced. The main properties of LSCO system, such as the electronic structure, Fermi surface, the band dispersion, the energy gap nodes, the universal nodal Fermi velocity, the magnetic shape-memory effects, the magnetic susceptibility anisotropy, one-dimensional static spin stripe and the resistivity anisotropy are introduced. Then, Fermi liquid theory and Wiedemann-Franz (WF) law which is used to determine whether a material is the Fermi liquid is described. Methods and advantages of very low-temperature thermal conductivity measurement is introduced, which can be used to detect metal-insulator transition of the ground state of high-temperature superconductors and verify Wiedemann-Franz Law. Finally, the elements substitution effect and the impact of transport of high-temperature superconducting materials is described. In addition to using the very high magnetic field to obtain the properties of the ground state of high-temperature superconductors, we can also replace Cu ions in the CuO2 plane to destroy the superconductivity, and then obtain the properties of the ground state.Chapter 2: In this chapter we study the transport behaviors and the ground-state properties of La1.8Sr0.2Cu1–zMzO4 (M = Zn or Mg) (LSCMO) single crystals, in which the superconductivity is destroyed by Zn- or Mg-doping. Moderately Zn-doped (z = 0.049, 0.082) samples have a very metallic behavior, for heavily Zn-doped (z = 0.13) or Mg-doped (z = 0.15) samples, low temperature resistivities show a weak upturn, but are still metallic. Thermal conductivity is also measured at very low temperature. The residual resistivity and residual thermal conductivity are obtained by extrapolating the experimental data to T = 0. These data show that moderately Zn-doped (z = 0.049, 0.082) samples satisfy the WF law, but a negative departure from WF law is found in heavily Zn-doped (z = 0.13) and Mg-doped (z = 0.15) samples, which is opposite to the theoretical expectation on disordered correlated electron systems.Chapter 3: In this chapter, we study the growth of Nd2CuO4 single crystal using flux-method. The obtained crystals are characterized by X-ray diffraction, magnetic susceptibility and low-temperature thermal conductivity. These data are briefly discussed.